As the Paleozoic Era gave way to the Mesozoic, some 225 million years ago (MYA), an ovoid upwarping of Earth's crust developed in what is now northern Arizona. Reinforced during the Laramide Orogeny (the formation of the Rockies, 70 MYA), this broad ridge was eventually covered by layers of Mesozoic and Tertiary sediments (both erosional and volcanic). Late in the Tertiary Period, the Miocene-Pliocene Uplift (stretching from about 15-5 MYA) lifted the entire Colorado Plateau and its rim of mountain ranges another 5000 feet, increasing stream erosion across the Province. Rising on the west side of the Continental Divide, the Colorado River flowed westward and gradually southward to enter the Sea of Cortez; en route, it crossed northern Arizona, entrenched in the younger sediments that covered the Kaibab-Coconino ridge. As the Colorado Plateau rose beneath it, the river was forced to cut down through this ridge of Paleozoic rock (and the upper layer of the ancient Precambrian basement that lies beneath it). Augmented by the wet climate of the Pleistocene (2 to 0.01 MYA), the Colorado thereby sculpted the Grand Canyon, the most spectacular chasm on our planet.
The ridge itself, oriented NNW to SSE, has since been uncovered by erosion. Streams from its eastern edge drain directly into the Colorado (or into the Little Colorado south of the Grand Canyon), while its northwest flank drains to the Colorado via Kaibab Creek and its southwest flank feeds the Cataract River, another tributary of the Colorado. The exposed ridge is composed primarily of Kaibab limestone overlying Coconino sandstone; that portion north of the Grand Canyon is known as the Kaibab Plateau while its segment south of the Canyon is referred to as the Coconino Plateau. The rock strata of the plateau, deposited during the Permian Period, form the upper layers of the Grand Canyon
The Kaibab Plateau rises to elevations that exceed 9200 feet, supporting a rich forest of fir, spruce and aspen, giving way to ponderosa pine and then pinon-juniper woodlands at lower elevations. The Coconino Plateau is 7400 feet above sea level at the south rim of the Grand Canyon and gradually lowers toward the west, south and east; it is covered primarily by ponderosa pine parklands. South of the Coconino Plateau, the landscape is dominated by the high peaks and scattered cones of the San Francisco Volcanic Field, including Humphreys Peak (12,633 feet), the highest point in Arizona.
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Kamis, 12 Juli 2012
Jumat, 06 Juli 2012
Colorado's Black Canyon
Near the end of the Mesozoic Era, as the Cretaceous Sea retreated to the southeast, Colorado was a relatively flat landscape of wetlands, sandhills and primitive forest. Then, about 70 million years ago (MYA), pressure within the North American craton crumpled up the Rocky Mountains, pushing ancient Precambrian rock up through the overlying Paleozoic and Mesozoic sediments.
As soon as they formed, the forces of erosion began to act on these new mountains, filling the intervening valleys with debris. About 35 MYA, volcanism developed in central and southwestern Colorado, lifting the West Elk and San Juan Mountains; the copious ash, pumice and lava from these eruptions also coated the valleys and basins of that region. By 10 MYA, the Gunnison River had formed; rising on the west side of the Continental Divide, in the Sawatch Range, and receiving large tributaries from the West Elk Mountains, to its north, and the San Juans, to its south, this river flowed west to join the Colorado. Entrenched within the erosional and volcanic debris that had settled across the broad, intervening basin, the Gunnison was forced to cut into a ridge of Precambrian gneiss and schist, east of present day Montrose, that was buried within the sediments. Since that time, the river has sculpted the Black Canyon of the Gunnison from that ancient rock, a process that was augmented during the cooler, wetter climate of the Pleistocene.
Almost 50 miles in length, the Black Canyon is up to 2720 feet deep and 1100 feet across at its rim; at river level, it is much narrower, only 40 feet wide in one area. The Painted Wall, on the north flank of Black Canyon, is named for the light-colored lava rock that laces its surface and is the tallest cliff in Colorado, dropping 2250 feet. Within the canyon, the Gunnison River drops 43 feet per mile, a grade that is almost six times steeper than the Colorado River's course within the Grand Canyon. Named for its shaded walls, hidden from the sun by its deep and narrow topography, the Black Canyon of the Gunnison is protected within a National Park that stretches along its middle (and most spectacular) 14 miles. Farther west, the Gunnison is thought to have carved Unaweep Canyon through the Uncompahgre Plateau, later diverted northward by a landslide to join the Colorado west of Grand Junction (see my blog on 12-27-10).
As soon as they formed, the forces of erosion began to act on these new mountains, filling the intervening valleys with debris. About 35 MYA, volcanism developed in central and southwestern Colorado, lifting the West Elk and San Juan Mountains; the copious ash, pumice and lava from these eruptions also coated the valleys and basins of that region. By 10 MYA, the Gunnison River had formed; rising on the west side of the Continental Divide, in the Sawatch Range, and receiving large tributaries from the West Elk Mountains, to its north, and the San Juans, to its south, this river flowed west to join the Colorado. Entrenched within the erosional and volcanic debris that had settled across the broad, intervening basin, the Gunnison was forced to cut into a ridge of Precambrian gneiss and schist, east of present day Montrose, that was buried within the sediments. Since that time, the river has sculpted the Black Canyon of the Gunnison from that ancient rock, a process that was augmented during the cooler, wetter climate of the Pleistocene.
Almost 50 miles in length, the Black Canyon is up to 2720 feet deep and 1100 feet across at its rim; at river level, it is much narrower, only 40 feet wide in one area. The Painted Wall, on the north flank of Black Canyon, is named for the light-colored lava rock that laces its surface and is the tallest cliff in Colorado, dropping 2250 feet. Within the canyon, the Gunnison River drops 43 feet per mile, a grade that is almost six times steeper than the Colorado River's course within the Grand Canyon. Named for its shaded walls, hidden from the sun by its deep and narrow topography, the Black Canyon of the Gunnison is protected within a National Park that stretches along its middle (and most spectacular) 14 miles. Farther west, the Gunnison is thought to have carved Unaweep Canyon through the Uncompahgre Plateau, later diverted northward by a landslide to join the Colorado west of Grand Junction (see my blog on 12-27-10).
Selasa, 26 Juni 2012
The Nature of Volcanism
Volcanoes develop in areas where the heat from Earth's mantle melts the overlying crust. This occurs along subduction zones, where one of the tectonic plates is dipping toward the mantle, in rift zones, where the crust is thinning (allowing the mantle to move upward) and at hotspots, where a mantle plume is rising into the crust.
Subduction volcanoes are the most widespread form of volcanism on Earth and are concentrated along the Pacific Rim where the Pacific Plate and its associated oceanic plates are dipping beneath the South American, North American, Eurasian and Australian Plates. The Andes, the volcanoes of western Central America and Mexico, the Cascades, the Aleutians and the volcanoes along the western edge of the Pacific (Japan, Taiwan, Philippines, New Zealand) result from this process. Other subduction volcanoes include those along the western and southern edge of Indonesia, the volcanic islands of the eastern Caribbean and Mt. Etna, on Sicily, one of the most active volcanoes on our planet.
Rift volcanoes include those above mid-oceanic ridges and those developing along rift valleys in continental crust. Most oceanic ridge volcanoes are well below the surface of the sea and thus not readily observed; the major exception is the island nation of Iceland, which formed (and continues to form) above the mid-Atlantic ridge. Continental rift volcanoes are seen along and within the East African Rift and the Rio Grande Rift of the American Southwest; they are also scattered throughout the Great Basin of the U.S. where the crust is being stretched (and thinned). Hotspot volcanism occurs across the globe, along the ocean floor and beneath/within continental crust; the Hawaiian Islands, the Galapagos Islands, the Canary Islands, Yellowstone, the San Francisco Peaks of northern Arizona and the volcanic field of northeastern New Mexico are but a few examples.
Subduction volcanoes are the most widespread form of volcanism on Earth and are concentrated along the Pacific Rim where the Pacific Plate and its associated oceanic plates are dipping beneath the South American, North American, Eurasian and Australian Plates. The Andes, the volcanoes of western Central America and Mexico, the Cascades, the Aleutians and the volcanoes along the western edge of the Pacific (Japan, Taiwan, Philippines, New Zealand) result from this process. Other subduction volcanoes include those along the western and southern edge of Indonesia, the volcanic islands of the eastern Caribbean and Mt. Etna, on Sicily, one of the most active volcanoes on our planet.
Rift volcanoes include those above mid-oceanic ridges and those developing along rift valleys in continental crust. Most oceanic ridge volcanoes are well below the surface of the sea and thus not readily observed; the major exception is the island nation of Iceland, which formed (and continues to form) above the mid-Atlantic ridge. Continental rift volcanoes are seen along and within the East African Rift and the Rio Grande Rift of the American Southwest; they are also scattered throughout the Great Basin of the U.S. where the crust is being stretched (and thinned). Hotspot volcanism occurs across the globe, along the ocean floor and beneath/within continental crust; the Hawaiian Islands, the Galapagos Islands, the Canary Islands, Yellowstone, the San Francisco Peaks of northern Arizona and the volcanic field of northeastern New Mexico are but a few examples.
Sabtu, 23 Juni 2012
Avalonia
There are few terranes that illustrate the science of plate tectonics and continental drift better than Avalonia. This micro-continent formed as a volcanic island arc along a subduction zone off the African Coast; at that time, late in the Precambrian Era, Africa was attached to the other southern continents to form Gondwana, which stretched across the South Pole. During the Cambrian Period, some 530 million years ago (MYA), as shelled marine life was exploding in diversity, Avalonia rifted from the African Plate and drifted northward ahead of the Rheic Ocean, which opened between it and Gondwana.
Late in the Ordovician Period, some 450 MYA, Avalonia docked with Baltica, the craton that now underlies Scandinavia, Eastern Europe and western Russia. This combined continental mass then collided with Laurentia (proto-North America) during the Silurian Period (440 MYA) as plants and animals were first colonizing the land; the collision forced up the Northern Appalachians, an event known as the Acadian Orogeny. When the Earth's land masses merged into Pangea during the Permian Period, about 270 MYA, Avalonia was caught in the middle, compressed between the northern and southern continents.
As the Tethys Sea opened east to west, some 200 MYA, Avalonia remained with Laurasia (the combined northern continents). During the Jurassic (150 MYA), the Atlantic Ocean began to open, splitting Avalonia as it divided the North American and Eurasian Plates. Today, fragments of Avalonia form coastal New England, Nova Scotia and the Avalon Peninsula of Newfoundland on the North American Continent; across the Atlantic, it is represented by Wales, England and the northern portion of Western Europe. Small fragments of Avalonia have also been identified in South Carolina and along the western rim of the Iberian Peninsula.
Late in the Ordovician Period, some 450 MYA, Avalonia docked with Baltica, the craton that now underlies Scandinavia, Eastern Europe and western Russia. This combined continental mass then collided with Laurentia (proto-North America) during the Silurian Period (440 MYA) as plants and animals were first colonizing the land; the collision forced up the Northern Appalachians, an event known as the Acadian Orogeny. When the Earth's land masses merged into Pangea during the Permian Period, about 270 MYA, Avalonia was caught in the middle, compressed between the northern and southern continents.
As the Tethys Sea opened east to west, some 200 MYA, Avalonia remained with Laurasia (the combined northern continents). During the Jurassic (150 MYA), the Atlantic Ocean began to open, splitting Avalonia as it divided the North American and Eurasian Plates. Today, fragments of Avalonia form coastal New England, Nova Scotia and the Avalon Peninsula of Newfoundland on the North American Continent; across the Atlantic, it is represented by Wales, England and the northern portion of Western Europe. Small fragments of Avalonia have also been identified in South Carolina and along the western rim of the Iberian Peninsula.
Selasa, 12 Juni 2012
Siberian North America
Contrary to a popular assumption, the major tectonic plates of Planet Earth do not correspond directly to the contour of the continents and oceans for which they are named. The North American Plate, for example, extends westward from the mid-Atlantic Ridge, thereby including the western half of Iceland, the western half of the Atlantic Ocean, Greenland, Canada, the Continental U.S., Cuba, the Bahamas, the Gulf of Mexico, the northern Caribbean and the country of Mexico. Its western edge generally follows the west coast of Mexico, the U.S. and Canada (excluding the Baja and Southern California, which are on the Pacific Plate), curving westward below Alaska and its Aleutian Chain and then dipping southward to take in the northern islands of Japan; from there, the western edge of the North American Plate angles NNW, cutting across eastern Siberia.
The Chersky Range of eastern Siberia, trending NW to SE, is a swath of parallel ridges and deep gorges; geologically, these mountains represent a compression zone between the North American and Eurasian Plates and are thus prone to frequent earthquakes. Extending northwestward from the Chersky Range is the Laptev Sea Rift, cutting through the Laptev Shelf on the northern coast of Siberia; this rift is a continental extension of the Gakkel Ridge, the spreading zone of the Arctic Ocean which, across the globe, is continuous with the Mid-Atlantic Ridge. The North American and Eurasian Plates are thus diverging along the Mid-Atlantic/Gakkel Ridge and colliding at the Chersky Range.
Of more significance to politically-minded, nationalistic humans, eastern Siberia, including the Kamchatka Peninsula, is on the North American Plate while Southern California and Hawaii are on the Pacific Plate, tied tectonically to Tahiti, the Soloman Islands and Southern New Zealand. Perhaps, if we paid more attention to the geology of our planet and less to our cultural differences, we might be more devoted to our common welfare.
The Chersky Range of eastern Siberia, trending NW to SE, is a swath of parallel ridges and deep gorges; geologically, these mountains represent a compression zone between the North American and Eurasian Plates and are thus prone to frequent earthquakes. Extending northwestward from the Chersky Range is the Laptev Sea Rift, cutting through the Laptev Shelf on the northern coast of Siberia; this rift is a continental extension of the Gakkel Ridge, the spreading zone of the Arctic Ocean which, across the globe, is continuous with the Mid-Atlantic Ridge. The North American and Eurasian Plates are thus diverging along the Mid-Atlantic/Gakkel Ridge and colliding at the Chersky Range.
Of more significance to politically-minded, nationalistic humans, eastern Siberia, including the Kamchatka Peninsula, is on the North American Plate while Southern California and Hawaii are on the Pacific Plate, tied tectonically to Tahiti, the Soloman Islands and Southern New Zealand. Perhaps, if we paid more attention to the geology of our planet and less to our cultural differences, we might be more devoted to our common welfare.
Rabu, 30 Mei 2012
Italy, Earthquakes & Human Nature
After enduring two tragic earthquakes within a span of nine days, residents of northeastern Italy are both distraught and mystified. As emphasized in news reports, that region had not experienced a significant earthquake for hundreds of years. Yet, this industrial valley stretches between the Apennines and the Alps, mountain ranges that owe their very existence to the collision of the African and Eurasian Plates.
This tectonic collision, though too gradual to witness during our brief life span, has been going on for at least 40 million years, producing the varied landscape of southern Europe. A quiescent period of seismic activity in any given area, even lasting hundreds or thousands of years, is to be expected as pressure along the collision zone shifts from one region to another. While we may understand the geologic cause for the earthquakes, our inability to accurately predict the timing of such events has become all too clear over the past few decades; nevertheless, scientists in Italy are facing manslaughter charges for their failure to predict the 2009 quake in the Apennines, east of Rome, which killed more than 300 citizens.
We humans have a tendency to blame others for the misfortunes that we endure, even when they arise from the uncontrollable and, to date, unpredictable natural forces that mold our planet. We also tend to interpret our Universe, distant galaxies or local geography, from the narrow perspective of our human life span. Anyone who resides along the active plate margins of Planet Earth cannot afford to ignore the realities of its past and ongoing geologic evolution, however remote the risk of catastrophe might seem at the present time. After all, the Africa-Eurasian collision has been underway for 40 million years, 400 times longer than our own species has walked the planet.
This tectonic collision, though too gradual to witness during our brief life span, has been going on for at least 40 million years, producing the varied landscape of southern Europe. A quiescent period of seismic activity in any given area, even lasting hundreds or thousands of years, is to be expected as pressure along the collision zone shifts from one region to another. While we may understand the geologic cause for the earthquakes, our inability to accurately predict the timing of such events has become all too clear over the past few decades; nevertheless, scientists in Italy are facing manslaughter charges for their failure to predict the 2009 quake in the Apennines, east of Rome, which killed more than 300 citizens.
We humans have a tendency to blame others for the misfortunes that we endure, even when they arise from the uncontrollable and, to date, unpredictable natural forces that mold our planet. We also tend to interpret our Universe, distant galaxies or local geography, from the narrow perspective of our human life span. Anyone who resides along the active plate margins of Planet Earth cannot afford to ignore the realities of its past and ongoing geologic evolution, however remote the risk of catastrophe might seem at the present time. After all, the Africa-Eurasian collision has been underway for 40 million years, 400 times longer than our own species has walked the planet.
Rabu, 23 Mei 2012
Sediments, Fossils & Time
Amateur naturalists are often confused by the distribution of fossils across our globe, wondering why certain fossils (of dinosaurs, for example) are plentiful in some areas but absent in others. In general, the fossils of plants and animals result from their remains having been trapped within sediment (lake deposits, ocean floor sediment, river mud, volcanic debris, etc.) that, over millions of years, hardened into sandstone, mudstone, limestone or some other sedimentary rock. Of course, most animals and plants die under circumstances in which their remains are consumed, undergo decay or are scattered by predators and natural forces before such fossilization can occur.
The type of fossils present in any region of our planet depends upon the age of the exposed sedimentary rocks in that area; rocks that formed from sediments that accumulated during any given geologic era will harbor fossils of life from that era. The exposure of these sedimentary rocks is a product of regional uplift, erosion and the deposition of overlying sediments (not necessarily occuring in that order). For example, Jurassic sedimentary rock, which accumulated during the Age of Dinosaurs, may have been lifted to the surface (as in large parts of the American West), may be buried deep beneath younger sediments or may have long-since eroded from the surface due to action of streams or glaciers; in other areas, these sediments may have never accumulated in the first place due to regional topography throughout the Jurassic Period.
Millions of years from now, Holocene sedimentary rocks will be explored by our super-human decendants or, perhaps, by visitors from other solar systems. Encased in those rocks will be the fossils of human civilization, including our domestic livestock, our cultivated plants, our pet poodles and the plants and animals that comprise our natural ecosystems. These Holocene sediments may be found deep in canyons, atop mountain ranges or outcropping from desert plains; in many areas they will lie deep beneath the fossilized remnants of younger civilizations while, in others, they will have already washed away to the sea.
The type of fossils present in any region of our planet depends upon the age of the exposed sedimentary rocks in that area; rocks that formed from sediments that accumulated during any given geologic era will harbor fossils of life from that era. The exposure of these sedimentary rocks is a product of regional uplift, erosion and the deposition of overlying sediments (not necessarily occuring in that order). For example, Jurassic sedimentary rock, which accumulated during the Age of Dinosaurs, may have been lifted to the surface (as in large parts of the American West), may be buried deep beneath younger sediments or may have long-since eroded from the surface due to action of streams or glaciers; in other areas, these sediments may have never accumulated in the first place due to regional topography throughout the Jurassic Period.
Millions of years from now, Holocene sedimentary rocks will be explored by our super-human decendants or, perhaps, by visitors from other solar systems. Encased in those rocks will be the fossils of human civilization, including our domestic livestock, our cultivated plants, our pet poodles and the plants and animals that comprise our natural ecosystems. These Holocene sediments may be found deep in canyons, atop mountain ranges or outcropping from desert plains; in many areas they will lie deep beneath the fossilized remnants of younger civilizations while, in others, they will have already washed away to the sea.
Rabu, 09 Mei 2012
The Eurasian Mountain Arc
Looking at a map of Earth, one sees a complex of mountain ranges from Southeast Asia to Spain. Almost all of these ranges are relatively young, having crumpled skyward throughout the Tertiary Period; in fact, all are still rising today, a fact made evident by frequent earthquakes across this swath of landscape.
About 55 million years ago (MYA), soon after the Rocky Mountains formed in North America, the Indian Subcontinent began to collide with southern Asia, forcing up the Himalayas and its associated ranges, from southern China to Afghanistan. By 40 MYA, the Red Sea and Gulf of Aden began to open, splitting the Arabian Plate from Africa and pushing it northward into southwestern Asia; this compressed the crust of that region, lifting the ranges of Iran, eastern Turkey and the Middle East. About the same time, as the Tethys Sea was closing, Africa drifted northward to collide with southern Europe; this has crumpled up the Alps and its associated ranges, from western Turkey and Greece to the Pyrenees of Spain. In concert, regional subduction of the African Plate beneath the Eurasian Plate has produced a chain of volcanos along the western edge of Italy.
In some areas, such as the Pyrenees, older mountain ranges, having eroded to low hills, were renewed by these Tertiary orogenies. Today, as these tectonic forces persist and the "new" mountains continue to rise, the agents of erosion combat their uplift; molded by glaciers and incised by streams, their rock dust is carried off to the sea where, millions of years in the future, it may resurface as the core of another mountain range.
About 55 million years ago (MYA), soon after the Rocky Mountains formed in North America, the Indian Subcontinent began to collide with southern Asia, forcing up the Himalayas and its associated ranges, from southern China to Afghanistan. By 40 MYA, the Red Sea and Gulf of Aden began to open, splitting the Arabian Plate from Africa and pushing it northward into southwestern Asia; this compressed the crust of that region, lifting the ranges of Iran, eastern Turkey and the Middle East. About the same time, as the Tethys Sea was closing, Africa drifted northward to collide with southern Europe; this has crumpled up the Alps and its associated ranges, from western Turkey and Greece to the Pyrenees of Spain. In concert, regional subduction of the African Plate beneath the Eurasian Plate has produced a chain of volcanos along the western edge of Italy.
In some areas, such as the Pyrenees, older mountain ranges, having eroded to low hills, were renewed by these Tertiary orogenies. Today, as these tectonic forces persist and the "new" mountains continue to rise, the agents of erosion combat their uplift; molded by glaciers and incised by streams, their rock dust is carried off to the sea where, millions of years in the future, it may resurface as the core of another mountain range.
Sabtu, 28 April 2012
The Illinois Basin
Like the Michigan and Permian Basins, the Illinois Basin is a structural bowl of Precambrian basement rock within which younger layers of sedimentary rock have accumulated. This bowl, which covers the southern 75% of Illinois, the southwestern 40% of Indiana, western Kentucky and a small portion of northwest Tennessee, is surrounded by structural "arches," uplifts of the deep, ancient Precambrian rock; these include the Kankakee Arch to the northeast, the Cincinnati Arch to the southeast, the Wisconsin Arch to the north, the Mississippi River Arch to the northwest, the Ozark Uplift to the west and the Pascola Arch to the southwest.
Three miles deep at its center, this broad basin of Precambrian rock, 1.3 billion years old, accumulated layers of sediment throughout much of the Paleozoic Era (from 600 to 270 million years ago); the great majority of these deposits occured within shallow seas, which invaded and retreated from the basin at least 50 times during that period, while others were carried in by streams or deposited within vast wetlands. As the basin filled in from the Cambrian to the Pennsylvanian Periods, these layers of sediment dipped from the surrounding arches toward the center of the structural depression; at the surface, older sediments are thus found at the periphery of the basin while the youngest (Pennsylvanian) cover its center. Following this prolonged period of deposition, which was intermittently disrupted by uplift and erosion, the Illinois Basin has undergone surface molding by the Pleistocene Glaciers and numerous streams; the glaciers flattened northern portions of the basin and coated them with a thick layer of glacial till while streams have carved southern portions into a maze of hills and valleys.
Travelling across the Illinois Basin today, one sees no evidence of the Precambrian bowl that underlies the region; indeed, its edge only outcrops in limited areas of southern Wisconsin and southeastern Missouri. Glacial erosion and till have produced the flat, productive Corn Belt across much of Illinois and western Indiana while Carboniferous sediments of the basin have been mined for their coal and drilled for their oil. As in most regions of our globe, the surface topography of the Illinois Basin only hints at the miles of sediment and complex geologic formations that lie below.
Three miles deep at its center, this broad basin of Precambrian rock, 1.3 billion years old, accumulated layers of sediment throughout much of the Paleozoic Era (from 600 to 270 million years ago); the great majority of these deposits occured within shallow seas, which invaded and retreated from the basin at least 50 times during that period, while others were carried in by streams or deposited within vast wetlands. As the basin filled in from the Cambrian to the Pennsylvanian Periods, these layers of sediment dipped from the surrounding arches toward the center of the structural depression; at the surface, older sediments are thus found at the periphery of the basin while the youngest (Pennsylvanian) cover its center. Following this prolonged period of deposition, which was intermittently disrupted by uplift and erosion, the Illinois Basin has undergone surface molding by the Pleistocene Glaciers and numerous streams; the glaciers flattened northern portions of the basin and coated them with a thick layer of glacial till while streams have carved southern portions into a maze of hills and valleys.
Travelling across the Illinois Basin today, one sees no evidence of the Precambrian bowl that underlies the region; indeed, its edge only outcrops in limited areas of southern Wisconsin and southeastern Missouri. Glacial erosion and till have produced the flat, productive Corn Belt across much of Illinois and western Indiana while Carboniferous sediments of the basin have been mined for their coal and drilled for their oil. As in most regions of our globe, the surface topography of the Illinois Basin only hints at the miles of sediment and complex geologic formations that lie below.
Senin, 02 April 2012
Lake Missoula & the Channeled Scablands
Near the end of the Pleistocene Ice Age, about 13,000 years ago, a lobe of the Cordilleran Glacier that covered the Rocky Mountains blocked the flow of the Clark Fork River, in northwestern Montana. Behind this dam of ice, a massive lake developed; 200 miles long and 2000 feet deep, Glacial Lake Missoula contained 500 cubic miles of glacial meltwater.
Eventually, the ice dam failed and a torrent of water rushed across the Idaho Panhandle, eastern Washington and the Columbia River Valley. In fact, as the glacier advanced and retreated over 2500 years, the lake repeatedly formed and drained, eroding the Channeled Scablands of the Columbia Plateau. Characterized by broad, braided canyons, dry falls, rippled rock formations, massive gravel bars and countless erratic boulders, this unique topography attests to the power of the recurrent floods; indeed, the Columbia River Gorge was primarily carved by these torrents.
While similar glacial lake floods occured elsewhere across the Northern Hemisphere (including the Bonneville Flood of southern Idaho), none were as powerful as those arising from Lake Missoula. Geologists and hydrologists estimate that the flow of these deluge events (which numbered three dozen or more) exceeded the current total flow of all rivers on Earth and emptied the Lake within a few days.
Eventually, the ice dam failed and a torrent of water rushed across the Idaho Panhandle, eastern Washington and the Columbia River Valley. In fact, as the glacier advanced and retreated over 2500 years, the lake repeatedly formed and drained, eroding the Channeled Scablands of the Columbia Plateau. Characterized by broad, braided canyons, dry falls, rippled rock formations, massive gravel bars and countless erratic boulders, this unique topography attests to the power of the recurrent floods; indeed, the Columbia River Gorge was primarily carved by these torrents.
While similar glacial lake floods occured elsewhere across the Northern Hemisphere (including the Bonneville Flood of southern Idaho), none were as powerful as those arising from Lake Missoula. Geologists and hydrologists estimate that the flow of these deluge events (which numbered three dozen or more) exceeded the current total flow of all rivers on Earth and emptied the Lake within a few days.
Selasa, 27 Maret 2012
Future Exotic Terranes
The continents of planet Earth are composed of central, "stable" platforms to which smaller "exotic terranes" have accreted and from which other segments of crust have rifted away. In fact, the central platforms (or cratons) are, themselves, composed of ancient segments that fused together early in geologic history and possess aborted rift zones that could reactivate in the future.
Exotic terranes are segments of continental crust that either rifted from a larger land mass or formed above oceanic hotspots, mid-oceanic ridges or subduction zones as volcanic islands. Moving with the surrounding oceanic crust, they accrete to another continent where the intervening oceanic crust subducts into a trench. Looking at the current geologic map of our planet, we can thus identify land masses that will become exotic terranes in the future (as long as the current plate movements and subduction zones do not change).
Examples include Southern California and the Baja Peninsula, which are moving NNW with the Pacific Plate toward the Aleutian Chain of Alaska, the Hawaiian Islands, which are moving toward the Kamchatka Peninsula of Siberia, and the Galapagos Islands, which are moving eastward on the Nazca Plate toward Ecuador. If the East African Rift continues to develop, southeastern Africa will become a large exotic terrane that could swivel northeastward to join India or might drift eastward and attach to Southeast Asia. Of course, all of these projections will be wrong if the current oceanic spreading zones shut down or if new continental rifts develop; we should find out in 80 million years or so.
Exotic terranes are segments of continental crust that either rifted from a larger land mass or formed above oceanic hotspots, mid-oceanic ridges or subduction zones as volcanic islands. Moving with the surrounding oceanic crust, they accrete to another continent where the intervening oceanic crust subducts into a trench. Looking at the current geologic map of our planet, we can thus identify land masses that will become exotic terranes in the future (as long as the current plate movements and subduction zones do not change).
Examples include Southern California and the Baja Peninsula, which are moving NNW with the Pacific Plate toward the Aleutian Chain of Alaska, the Hawaiian Islands, which are moving toward the Kamchatka Peninsula of Siberia, and the Galapagos Islands, which are moving eastward on the Nazca Plate toward Ecuador. If the East African Rift continues to develop, southeastern Africa will become a large exotic terrane that could swivel northeastward to join India or might drift eastward and attach to Southeast Asia. Of course, all of these projections will be wrong if the current oceanic spreading zones shut down or if new continental rifts develop; we should find out in 80 million years or so.
Rabu, 14 Maret 2012
Geophysical Provinces of Ohio
Though known primarily for its rolling farmlands, Ohio is a mosaic of geophysical provinces, defined by underlying geology, tectonic uplift and the erosive power of Pleistocene Glaciers. From a broad perspective, the State overlaps two major provinces; the northwestern 60% of Ohio lies within the Central Lowlands of North America while the southeastern 40% is occupied by the Appalachian Plateau. The border between these two geophysical regions runs SSW, from the northeast corner of Ohio to the Ohio River Valley at the southern edge of the State.
The Appalachian Plateau, composed of Carboniferous sandstones, limestones and shales, was lifted with the Southern Appalachian Mountains as North America and Africa collided during the formation of Pangea, some 300 million years ago. While the Plateau's western edge has been obscured by glacial erosion in northeastern Ohio, it stands out as a prominent escarpment through the southern half of the State, rising 500 feet above the adjacent lowlands. The Central Lowlands within Ohio are comprised of four sub-provinces: the Lake Plain, the Glacial Till Plains, the Ohio Valley and a small segment of the Interior Low Plateaus; the latter, unglaciated province, characterized by glades and karst landscapes, extends from Adams County, in southwestern Ohio, southward and then westward through Kentucky, western Tennessee, southern Indiana and southern Illinois.
The Lake Plain of northern Ohio is a swath of flat terrain south of Lake Erie, representing the post-glacial extent of Lake Warren, Lake Erie's larger predecessor; it is underlain with Devonian sedimentary rocks which have since been covered by glacial till and sandy lake deposits (though these rocks are exposed along the Lake Erie shoreline, in major river valleys and on Ohio's Lake Erie islands). South of the Lake Plain, the gently rolling farmlands of central and west-central Ohio occupy the Till Plains, molded by a number of post-glacial streams that flow southward to the Ohio River; beneath the thick layer of till are Silurian (western Ohio) and Devonian (central Ohio) sedimentary rocks, deposited in shallow seas from 400-350 million years ago. Finally, in the southwest corner of Ohio, ancient Ordovician limestones and shales, 500 million years old, have been sculpted into the hilly terrain of Greater Cincinnati by the erosive force of glacial meltwater and the continued action of post-glacial streams; harboring fossils of trilobites, brachiopods and other early marine invertebrates, these are the oldest exposed rocks in the State.
The Appalachian Plateau, composed of Carboniferous sandstones, limestones and shales, was lifted with the Southern Appalachian Mountains as North America and Africa collided during the formation of Pangea, some 300 million years ago. While the Plateau's western edge has been obscured by glacial erosion in northeastern Ohio, it stands out as a prominent escarpment through the southern half of the State, rising 500 feet above the adjacent lowlands. The Central Lowlands within Ohio are comprised of four sub-provinces: the Lake Plain, the Glacial Till Plains, the Ohio Valley and a small segment of the Interior Low Plateaus; the latter, unglaciated province, characterized by glades and karst landscapes, extends from Adams County, in southwestern Ohio, southward and then westward through Kentucky, western Tennessee, southern Indiana and southern Illinois.
The Lake Plain of northern Ohio is a swath of flat terrain south of Lake Erie, representing the post-glacial extent of Lake Warren, Lake Erie's larger predecessor; it is underlain with Devonian sedimentary rocks which have since been covered by glacial till and sandy lake deposits (though these rocks are exposed along the Lake Erie shoreline, in major river valleys and on Ohio's Lake Erie islands). South of the Lake Plain, the gently rolling farmlands of central and west-central Ohio occupy the Till Plains, molded by a number of post-glacial streams that flow southward to the Ohio River; beneath the thick layer of till are Silurian (western Ohio) and Devonian (central Ohio) sedimentary rocks, deposited in shallow seas from 400-350 million years ago. Finally, in the southwest corner of Ohio, ancient Ordovician limestones and shales, 500 million years old, have been sculpted into the hilly terrain of Greater Cincinnati by the erosive force of glacial meltwater and the continued action of post-glacial streams; harboring fossils of trilobites, brachiopods and other early marine invertebrates, these are the oldest exposed rocks in the State.
Kamis, 08 Maret 2012
Mono Lake
Mono Lake is a 70 square-mile saline lake at the eastern foot of the Sierra Nevada Range, approximately 13 miles east of Yosemite National Park. Enclosed within a natural basin by the Sierra Batholith to its west and Pliocene-Pleistocene volcanic ranges to its north, east and south, the lake is fed by three primary streams (Lee Vining, Rush and Mill Creeks) and by freshwater springs along its floor. Having formed during the Pleistocene, about 1 million years ago, it is one of the oldest lakes in North America and is the largest lake completely within California.
As a basin lake, its size and depth are a balance between inflow from streams and springs and evaporative loss in the dry climate of the Basin and Range Province. Prior to 1941, its surface elevation hovered above 6400 feet but this was dramatically lowered by diversion of inflow to the Los Angeles Water System; legal challenges by conservation groups eventually restored most of the inflow by 1994.
Despite its high salinity (2.5 times that of ocean water) and alkalinity (which precludes fish survival), Mono Lake supports a spectacular ecosystem. Photosynthetic algae, feeding on minerals from the surrounding mountains, bloom in spring when snowmelt peaks. Feeding on the algae, brine shrimp and brine flies thrive in the lake shallows and provide vital nourishment for migrating shorebirds, eared grebes and phalaropes, among other species; indeed, Mono Lake was added to the Western Hemisphere Shorebird Reserve Network in 1991. California gulls and snowy plovers nest on Paoha Island and smaller islets, escaping predation by fox and coyotes. Beyond the abundant birdlife, Mono Lake is perhaps best known for its tufa towers of calcium carbonate, produced by the interaction of freshwater springs and the alkaline, mineral-rich lake waters. Students of geology will also appreciate the surrounding volcanic landscape, including the relatively recent Panum and Mono-Inyo craters, south of Mono Lake.
As a basin lake, its size and depth are a balance between inflow from streams and springs and evaporative loss in the dry climate of the Basin and Range Province. Prior to 1941, its surface elevation hovered above 6400 feet but this was dramatically lowered by diversion of inflow to the Los Angeles Water System; legal challenges by conservation groups eventually restored most of the inflow by 1994.
Despite its high salinity (2.5 times that of ocean water) and alkalinity (which precludes fish survival), Mono Lake supports a spectacular ecosystem. Photosynthetic algae, feeding on minerals from the surrounding mountains, bloom in spring when snowmelt peaks. Feeding on the algae, brine shrimp and brine flies thrive in the lake shallows and provide vital nourishment for migrating shorebirds, eared grebes and phalaropes, among other species; indeed, Mono Lake was added to the Western Hemisphere Shorebird Reserve Network in 1991. California gulls and snowy plovers nest on Paoha Island and smaller islets, escaping predation by fox and coyotes. Beyond the abundant birdlife, Mono Lake is perhaps best known for its tufa towers of calcium carbonate, produced by the interaction of freshwater springs and the alkaline, mineral-rich lake waters. Students of geology will also appreciate the surrounding volcanic landscape, including the relatively recent Panum and Mono-Inyo craters, south of Mono Lake.
Kamis, 16 Februari 2012
Erosional Remnants
Mention erosional remnants and we tend to think of sea stacks, free-standing buttes and other solitary rock formations that have withstood the forces of wind, water and ice. Some of these landforms, especially mesas and buttes, are protected by a resistant cap of basalt or other non-porous rock while others are igneous plutons or laccoliths, uncovered as their encasing sediments have eroded away.
In reality, all natural landforms are erosional remnants, the current product of underlying geology, tectonic forces and erosion, modified by climate and vegetation. Highlands may represent areas of uplift, volcanism or resistant geologic strata but remain subject to the forces of erosion and, eventually, will wear down to a level plain. Stream valleys and canyons, while seemingly stable during our brief lifespan, are continually molded by floods, deposition, landslides, rock falls and periods of drought; depending upon the interaction of these factors, they may deepen, widen, clog with erosional debris or fill with water behind a natural dam of ice or soil.
Human activity hastens erosion in a number of ways. Deforestation augments erosion on hillsides, the drainage of wetlands increases flooding along river valleys and the construction of highways disrupts natural drainage, increases runoff and, in mountainous regions, hastens the occurrence of rock slides. As we witnessed during the Dust Bowl years, poor farming practices can dramatically augment soil erosion and the over-grazing of natural grasslands continues to threaten those regions. And, of course, our contribution to global warming will significantly affect erosional processes as glaciers retreat, sea levels rise and weather patterns change.
In reality, all natural landforms are erosional remnants, the current product of underlying geology, tectonic forces and erosion, modified by climate and vegetation. Highlands may represent areas of uplift, volcanism or resistant geologic strata but remain subject to the forces of erosion and, eventually, will wear down to a level plain. Stream valleys and canyons, while seemingly stable during our brief lifespan, are continually molded by floods, deposition, landslides, rock falls and periods of drought; depending upon the interaction of these factors, they may deepen, widen, clog with erosional debris or fill with water behind a natural dam of ice or soil.
Human activity hastens erosion in a number of ways. Deforestation augments erosion on hillsides, the drainage of wetlands increases flooding along river valleys and the construction of highways disrupts natural drainage, increases runoff and, in mountainous regions, hastens the occurrence of rock slides. As we witnessed during the Dust Bowl years, poor farming practices can dramatically augment soil erosion and the over-grazing of natural grasslands continues to threaten those regions. And, of course, our contribution to global warming will significantly affect erosional processes as glaciers retreat, sea levels rise and weather patterns change.
Rabu, 25 Januari 2012
Harney Basin
Harney Basin is a geologic and topographic basin in southeast Oregon; while it sits adjacent to the northwest corner of the Great Basin, it is separated from that province by the massive fault-block of the Steens Mountain ridge. On its north side, the Harney Basin is bordered by the southern edge of the Blue Mountains while a high lava plain separates the basin from the watersheds of the John Day and Klamath Rivers to the northwest and southwest, respectively.
During warm interglacial periods of the Pleistocene, glacial meltwater from the adjacent highlands filled the Harney Basin, spilling northeastward into the Malheur River, a tributary of the Snake River. Today, as the climate has warmed through the Holocene, the floor of Harney Basin has become a high desert, receiving only 6 inches of precipitation each year; a low divide along the basin's northeast edge, formerly a spillway, now completes the basin topography and all streams flow inward toward Harney and Malheur Lakes, on the basin's floor. Burns, Oregon, is the only sizable town in this remote, high desert basin.
While most of the basin floor is high and dry, with elevations between 4000 and 5200 feet, Malheur Lake, fed by the Silvies River from the Blue Mountains and the Blitzen River from Steens Mountain, provides a rich and welcome oasis for migrant waterfowl, shorebirds, white pelicans and sandhill cranes. Home to many other species as well, the lake and its wetlands are protected as the Malheur National Wildlife Refuge; during seasons with heavy precipitation or snowmelt, Malheur Lake spills west toward Harney Lake, an ephemeral, salt pan lake that is the topographic sink of Harney Basin.
During warm interglacial periods of the Pleistocene, glacial meltwater from the adjacent highlands filled the Harney Basin, spilling northeastward into the Malheur River, a tributary of the Snake River. Today, as the climate has warmed through the Holocene, the floor of Harney Basin has become a high desert, receiving only 6 inches of precipitation each year; a low divide along the basin's northeast edge, formerly a spillway, now completes the basin topography and all streams flow inward toward Harney and Malheur Lakes, on the basin's floor. Burns, Oregon, is the only sizable town in this remote, high desert basin.
While most of the basin floor is high and dry, with elevations between 4000 and 5200 feet, Malheur Lake, fed by the Silvies River from the Blue Mountains and the Blitzen River from Steens Mountain, provides a rich and welcome oasis for migrant waterfowl, shorebirds, white pelicans and sandhill cranes. Home to many other species as well, the lake and its wetlands are protected as the Malheur National Wildlife Refuge; during seasons with heavy precipitation or snowmelt, Malheur Lake spills west toward Harney Lake, an ephemeral, salt pan lake that is the topographic sink of Harney Basin.
Jumat, 06 Januari 2012
The Raton Basin
Driving along I-25 between Walsenburg, Colorado, and Raton, New Mexico, one enjoys spectacular scenery, including lofty peaks, high mesas and rugged canyons. One would certainly not imagine that this area was once a basin!
Yet, back in the Cretaceous, some 100 million years ago, when Tyrannosaurus rex and his cohorts roamed the planet and a broad seaway stretched from Texas to western Canada, a geologic basin extended across this region, tilted upward to the west; marine shale, known as Pierre Shale, covered the floor of the basin, deposited by the Cretaceous Sea. As the Rockies began to rise and the sea retreated to the south and east, coastal sands settled on the shale and have since hardened into the sandstone of the Trinidad Formation. Then, toward the end of the Cretaceous, a delta ecosystem spread across the region, destined to yield the shales, siltstones, sandstones and coal seams of the Vermejo Formation; coal has been mined from this formation since the 1800s and methane has been retrieved since the 1980s. As the Cretaceous gave way to the Tertiary Period, alluvial debris from the rising mountains covered the area, the source of conglomerates that now comprise the Raton Formation; harboring pockets of coal, this Formation is famous for its exposure of the K-T boundary, within which a high level of iridium supports the theory that a catastrophic meteor impact ended the Cretaceous and wiped out the dinosaurs. Finally, during the Paleocene, alluvial sands and muds of what is now the Poison Canyon Formation covered any remaining evidence of the underlying geologic basin.
Since then, in concert with continued uplift along the Rocky Mountain corridor, granite intrusions pushed up into the sediments, later to be uncovered by erosion to reveal the spectacular Spanish Peaks of southern Colorado, complete with Eocene-Oligocene lava dikes that formed as these massive plutons rose. In the late Miocene, about 9 million years ago, the Raton-Clayton Volcanic Field developed across northeast New Mexico, spreading thick pools of basalt across what is now the Colorado-New Mexico line. This hard cap of basalt protected the underlying Tertiary and Cretaceous sediments, yielding a chain of high mesas that stretch from the base of the Rockies to the Oklahoma Panhandle, separating the watersheds of the Arkansas and Cimarron Rivers.
Yet, back in the Cretaceous, some 100 million years ago, when Tyrannosaurus rex and his cohorts roamed the planet and a broad seaway stretched from Texas to western Canada, a geologic basin extended across this region, tilted upward to the west; marine shale, known as Pierre Shale, covered the floor of the basin, deposited by the Cretaceous Sea. As the Rockies began to rise and the sea retreated to the south and east, coastal sands settled on the shale and have since hardened into the sandstone of the Trinidad Formation. Then, toward the end of the Cretaceous, a delta ecosystem spread across the region, destined to yield the shales, siltstones, sandstones and coal seams of the Vermejo Formation; coal has been mined from this formation since the 1800s and methane has been retrieved since the 1980s. As the Cretaceous gave way to the Tertiary Period, alluvial debris from the rising mountains covered the area, the source of conglomerates that now comprise the Raton Formation; harboring pockets of coal, this Formation is famous for its exposure of the K-T boundary, within which a high level of iridium supports the theory that a catastrophic meteor impact ended the Cretaceous and wiped out the dinosaurs. Finally, during the Paleocene, alluvial sands and muds of what is now the Poison Canyon Formation covered any remaining evidence of the underlying geologic basin.
Since then, in concert with continued uplift along the Rocky Mountain corridor, granite intrusions pushed up into the sediments, later to be uncovered by erosion to reveal the spectacular Spanish Peaks of southern Colorado, complete with Eocene-Oligocene lava dikes that formed as these massive plutons rose. In the late Miocene, about 9 million years ago, the Raton-Clayton Volcanic Field developed across northeast New Mexico, spreading thick pools of basalt across what is now the Colorado-New Mexico line. This hard cap of basalt protected the underlying Tertiary and Cretaceous sediments, yielding a chain of high mesas that stretch from the base of the Rockies to the Oklahoma Panhandle, separating the watersheds of the Arkansas and Cimarron Rivers.
Sabtu, 31 Desember 2011
The Great Basin
Defined by its hydrology, the Great Basin is a large area of the American West within which the rivers and streams drain from an irregular ring of topographic divides toward the interior of the basin, never to reach the sea. The west edge of the Great Basin runs along the crest of the southern Cascades, the Sierra Nevada Range, the Tehachapi Range and the San Bernardino Mountains while the east edge follows the crest of the Wasatch Plateau and the westernmost Uintas. Along its northern boundary, an irregular, low divide separates the watersheds of the basin rivers (dominated by the Bear and Humboldt Rivers) from that of the Snake River. The southern edge of the Great Basin is even more complex, separating basin river watersheds from that of the Colorado River, extending southward into northwest Mexico.
Since the Great Basin is surrounded by topographic divides, winds are downsloping from all directions, warming and drying the air; as a result, the Basin floor, with elevations ranging from 6500 feet to below sea level (in Death Valley), is a mosaic of deserts, covered by sage grasslands, salt flats and, in parts of the Mojave, joshua trees. The largest lakes within the Great Basin are the Great Salt Lake and Utah Lake, both in Utah, Pyramid Lake, in northwest Nevada, and Lake Tahoe and the Salton Sea in California.
Within the Great Basin, Earth's crust is being stretched in an east-west direction, caught between the uplifts of the Sierra Nevada, Rocky Mountains and Colorado Plateau. This has created a maze of linear faults, aligned north to south, along which fault-block mountains have risen to produce waves of ranges, separated by broad, flat valleys; while this aptly named Basin and Range Province includes all of the Great Basin, it also extends southeastward into the Sonoran Desert.
Since the Great Basin is surrounded by topographic divides, winds are downsloping from all directions, warming and drying the air; as a result, the Basin floor, with elevations ranging from 6500 feet to below sea level (in Death Valley), is a mosaic of deserts, covered by sage grasslands, salt flats and, in parts of the Mojave, joshua trees. The largest lakes within the Great Basin are the Great Salt Lake and Utah Lake, both in Utah, Pyramid Lake, in northwest Nevada, and Lake Tahoe and the Salton Sea in California.
Within the Great Basin, Earth's crust is being stretched in an east-west direction, caught between the uplifts of the Sierra Nevada, Rocky Mountains and Colorado Plateau. This has created a maze of linear faults, aligned north to south, along which fault-block mountains have risen to produce waves of ranges, separated by broad, flat valleys; while this aptly named Basin and Range Province includes all of the Great Basin, it also extends southeastward into the Sonoran Desert.
Sabtu, 17 Desember 2011
The Owens Valley
The Owens River of southeast California rises along the eastern flank of the Sierra Nevada Mountains, at the southeast edge of Yosemite National Park. After flowing eastward through the Long Valley Caldera, the river angles to the south-southeast and begins its journey through the magnificent landscape of the Owens Valley.
The Owens Valley is a geologic graben, a block of crust that dropped between the parallel faults of the Sierra batholith to its west and the White-Inyo fault block mountains to its east; as the mountains rose on either side, this block slipped downward. The floor of the valley has an elevation of 4000 to 3500 feet (north to south) while its steep walls rise toward some of the highest summits in the Lower 48; the latter include Mt. Whitney in the Sierra Nevada (14,498 feet) and White Mountain Peak (14,252 feet) in the White Mountains. The latter range also includes Boundary Peak (13,167 feet), the highest point in Nevada, while the Inyo Mountains, known for their exposure of Cambrian sediments, top out below 12,000 feet.
Representing the southwest edge of the Basin and Range Province, the Owens Valley lies in the rain shadow of the Sierra Nevada and receives very little moisture from the east due to the high wall of the White and Inyo Mountains. The waters of the Owens River have thus long been diverted for crop production across the Valley and, since 1913, much of its flow has been directed into the Los Angeles Aqueduct, leaving Owens Lake, the natural terminus of the river, mostly dry. Back in the Pleistocene, when mountain glaciers fed the Owens River, its lake basin spilled to the east (around the southern end of the Inyo Range), merging with lakes from other Great Basin rivers to produce an inland sea across Death Valley and adjacent lowlands.
The Owens Valley is a geologic graben, a block of crust that dropped between the parallel faults of the Sierra batholith to its west and the White-Inyo fault block mountains to its east; as the mountains rose on either side, this block slipped downward. The floor of the valley has an elevation of 4000 to 3500 feet (north to south) while its steep walls rise toward some of the highest summits in the Lower 48; the latter include Mt. Whitney in the Sierra Nevada (14,498 feet) and White Mountain Peak (14,252 feet) in the White Mountains. The latter range also includes Boundary Peak (13,167 feet), the highest point in Nevada, while the Inyo Mountains, known for their exposure of Cambrian sediments, top out below 12,000 feet.
Representing the southwest edge of the Basin and Range Province, the Owens Valley lies in the rain shadow of the Sierra Nevada and receives very little moisture from the east due to the high wall of the White and Inyo Mountains. The waters of the Owens River have thus long been diverted for crop production across the Valley and, since 1913, much of its flow has been directed into the Los Angeles Aqueduct, leaving Owens Lake, the natural terminus of the river, mostly dry. Back in the Pleistocene, when mountain glaciers fed the Owens River, its lake basin spilled to the east (around the southern end of the Inyo Range), merging with lakes from other Great Basin rivers to produce an inland sea across Death Valley and adjacent lowlands.
Selasa, 13 Desember 2011
The Wichita Mountains
Rising from the flat terrain of southern Oklahoma, northwest of Lawton, ridges of igneous and metamorphic rock trend northwest to southeast. Thirty miles long, these rock formations originated in a rift zone that developed from the late Precambrian Era into the Cambrian Period, some 550-600 million years ago. Though the rifting process was aborted, plutons of granite, intrusions of gabbro and deposits of rhyolite were encased within and atop the surrounding crust and, over the next 20 million years, were covered by layers of younger Paleozoic sediments.
During the Pennsylvanian Period, about 325 million years ago, the continents began to merge into Pangea. The collision of North America and Africa crumpled up the Southern Appalachians and the docking of South America extended that collision zone toward the west, lifting the Ouachita and Wichita Mountains. Erosion coincided with this uplift and continues today; having carried away the overlying and encasing sediments, wind and water have left behind outcrops of the ancient rock that formed within the Cambrian rift. Of course, the ridges of the Wichita Mountains, like all ranges on this planet, will eventually wear down to a level plain, their aprons of debris carried off to the sea or dispersed by the wind.
Wichita Mountains NWR protects a portion of this scenic landscape, providing habitat for bison, elk and the more common wildlife of southern Oklahoma. Several of the peaks rise above 2400 feet, offering a cool retreat during the summer months and attracting a wide variety of birds to their oak woodlands. From I-44, about 8 miles north of Lawton, head west on Route 49; the refuge will be approximately 12 miles ahead.
During the Pennsylvanian Period, about 325 million years ago, the continents began to merge into Pangea. The collision of North America and Africa crumpled up the Southern Appalachians and the docking of South America extended that collision zone toward the west, lifting the Ouachita and Wichita Mountains. Erosion coincided with this uplift and continues today; having carried away the overlying and encasing sediments, wind and water have left behind outcrops of the ancient rock that formed within the Cambrian rift. Of course, the ridges of the Wichita Mountains, like all ranges on this planet, will eventually wear down to a level plain, their aprons of debris carried off to the sea or dispersed by the wind.
Wichita Mountains NWR protects a portion of this scenic landscape, providing habitat for bison, elk and the more common wildlife of southern Oklahoma. Several of the peaks rise above 2400 feet, offering a cool retreat during the summer months and attracting a wide variety of birds to their oak woodlands. From I-44, about 8 miles north of Lawton, head west on Route 49; the refuge will be approximately 12 miles ahead.
Minggu, 20 November 2011
Detour to the Divide
Leaving our Littleton, Colorado, farm early yesterday morning, low clouds obscured the finale of the annual Leonid meteor shower. The clouds and chilly air were courtesy of a northeast, upslope wind behind a cold front that had pushed across the Front Range overnight.
The prospect of driving across the Plains beneath this low ceiling prompted a detour to the south and I followed US 85 and then I-25 to Colorado Springs. Crossing the Palmer Divide at Monument Hill, just north of the Air Force Academy, I left the clouds behind and descended through the Fountain Creek Valley under sunny skies. Once in the city, I turned east on US 24 and drove across the south flank of the Palmer Divide all the way to Limon, watching the cloud bank lap against the crest of the ridge like an angry sea.
An erosional remnant, the Palmer Divide consists of Tertiary deposits overlying a Cretaceous base. Stretching nearly 70 miles from west to east, it connects the foothills with the High Plains escarpment and separates the watersheds of the South Platte and Arkansas Rivers. Elevations along the crest of the Divide range from 7500 feet at its junction with the foothills to 6500 feet where it merges from the High Plains, northeast of Limon. This geophysical barrier plays a significant role in the regional weather, catching upslope precipitation from the north or south, igniting thunderstorms and granting either Denver or Colorado Springs downsloping winds that warm the air and clear the skies.
The prospect of driving across the Plains beneath this low ceiling prompted a detour to the south and I followed US 85 and then I-25 to Colorado Springs. Crossing the Palmer Divide at Monument Hill, just north of the Air Force Academy, I left the clouds behind and descended through the Fountain Creek Valley under sunny skies. Once in the city, I turned east on US 24 and drove across the south flank of the Palmer Divide all the way to Limon, watching the cloud bank lap against the crest of the ridge like an angry sea.
An erosional remnant, the Palmer Divide consists of Tertiary deposits overlying a Cretaceous base. Stretching nearly 70 miles from west to east, it connects the foothills with the High Plains escarpment and separates the watersheds of the South Platte and Arkansas Rivers. Elevations along the crest of the Divide range from 7500 feet at its junction with the foothills to 6500 feet where it merges from the High Plains, northeast of Limon. This geophysical barrier plays a significant role in the regional weather, catching upslope precipitation from the north or south, igniting thunderstorms and granting either Denver or Colorado Springs downsloping winds that warm the air and clear the skies.
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