Unconformity
Unconformity
An unconformity is a widespread surface separating rocks above and below, which represents a gap in the rock record. The gap, or interval of geologic time that goes unrecorded, is called a hiatus. Unconformities occur when either erosion wears away rocks, or rock deposits never form. Therefore, a time gap exists between when the rocks below the unconformity formed and when those above it formed.
Unconformities are classified as three types. The most easily recognized are angular unconformities, which show horizontal layers of sedimentary rock lying on tilted layers of sedimentary rock. The upper layers may not be perfectly horizontal, but they do not lie parallel to the lower layers. The second type of unconformities are disconformities, which lie between parallel layers of sedimentary rock. The third type are nonconformities, which divide sedimentary layers from metamorphic and intrusive (cooled inside the Earth) igneous rocks. Common to all three, erosion causes them to form, and younger rocks sit on older rocks.
Angular unconformities
Four basic steps create angular unconformities. In step one, sediment weathered from land and carried to the sea accumulates on the sea floor and, over millions of years, turns to rock layers. Then, the collision of plates, giant sections of Earth’s crust that constantly shift, lift and tilt the layers until the layers rise above sea level and then weather and erode. They erode for millions of years until the edges of the tilted layers become a flattened plane (a peneplain is a broad land surface flattened by erosion). Finally, in step four, sea level rises or land sinks. Sediments wash down, forming new horizontal layers that cover the submerged, tilted layers. These four steps could take hundreds of millions of years to complete.
The Colorado River at the Grand Canyon, in the United States, exposed one of the best angular unconformities in the world. From even miles away on the rim of the canyon, observers can see tilted layers of rock truncated roughly 550 million years ago by a horizontal sedimentary layer called the Tapeats Sandstone. The inner gorge of the Grand Canyon provides a great example of angular unconformity formation. Except, instead of four steps, the rocks tell of seven: (1) Over two billion years ago, layers of sediment accumulated and turned to rock. (2) Around two billion years ago, plate collisions lifted mountains and turned the sedimentary rocks into the Vishnu Schist, a metamorphic rock. (3) One-half billion years later, the mountains eroded into a peneplain. (4) The land subsided or sea level rose to deposit new layers (known as the Grand Canyon Series) on the old Vishnu Schist. (5) New plate collisions tilted and uplifted the Grand Canyon Series. (6) Erosion truncated the tilted series and created another peneplain. The erosional episode lasted almost one billion years. (7) The land subsided eventually and the Tapeats Sandstone accumulated on the tilted Grand Canyon Series. In some places in the canyon, the Tapeats lies not on the angled series but directly on the metamorphic Vishnu Schist—making this a nonconformity.
Another famous angular nonconformity is Scotland’s Siccar Point, a site which played a part in the development of modern geology. In the eighteenth century, most people believed Earth to be only 6,000 years old, a figure arrived at earlier by Bishop James Ussher (1581–1656), a prominent theologian who added the ages of Biblical characters and, thus, concluded the world was created in 4004 BC. Scottish geologist James Hutton (1726–1797), however, realized that thousand-year-old Roman ruins in Great Britain were barely touched by weathering and erosion. He, therefore, wondered how long it takes for whole mountains, like those in Scotland, to wear down.
The angular unconformity Hutton discovered at Siccar Point in Scotland provided dramatic evidence for his time expansion. He saw nearly horizontal sandstone resting on nearly vertical graywacke (a sedimentary rock similar to sandstone) and marveled at how long it took to deposit the graywacke, tilt it, erode it, and then lay sandstone across it. As his friend, John Playfair, wrote “The mind seemed to grow giddy by looking so far into the abyss of time.”
Disconformity
from about one mile away, or perhaps from a few hundred feet away, disconformities can hide. The layers appear regular and parallel. However, between the layers, a disconformity can lie. Not until a geologist closely examines the fossils in the layers for the presence or absence of certain organisms can he or she recognize the gap in time—an erosional period when sediment accumulation or deposition halted or perhaps when anything laid down washed away.
As with an angular unconformity, disconformities form in steps. In step one, sediments collect on an ocean floor (or perhaps on the bed of a large lake). They compact and become rock layers. In the second phase, sea level falls or the sea floor rises to expose the layers to weathering and erosion. The main difference in the formation of disconformities and angular unconformities lies in this second step. As the layers of the future disconformity rise above sea level, they remain horizontal—no tilting occurs. If they tilt in this step, they later form an angular unconformity. Then, in step three, the land subsides or sea level rises, and new sediments collect on the older, still horizontal, layers.
Back in the Grand Canyon, where the story paused roughly 550 million years ago, the Tapeats Sandstone draped across the Grand Canyon Series and the Vishnu Schist to form the Great Unconformity. At least two more layers of sediments—the Bright Angel Shale and the Muav Limestone—collected on the Tapeats over the next 50 million years. Other layers may have been formed, too, but they disappeared when the land rose and eroded for the next 80 million years. When the area again sank beneath the sea, the Temple Butte Formation, 80 million years younger than the Mauv, accumulated on the disconformity. The cycle of deposition, uplift, erosion, subsidence, and more deposition repeated at least four times from 550 to 250 million years ago.
Nonconformities
nonconformities separate sedimentary rock layers from metamorphic rock layers and from intrusive
KEY TERMS
Angular unconformity —An unconformity, or gap, in the rock record, where horizontal rock layers overlie tilted layers.
Disconformity —An unconformity, or gap, in the rock record, situated between parallel rock layers.
Nonconformity —Anunconformity, orgap, intherock record, where sedimentary rocks overlie metamorphic or igneous rocks.
igneous rock (like granite). In a step-by-step process similar to the other two unconformities, sediment accumulates and becomes rock. Then, plate collisions deform these layers and change them into metamorphic rocks. Associated with this mountain-building, molten rock often squeezes upward into the metamorphic rock fractured by the deformities and solidifies, forming igneous rock (usually granite). In phase three, the mountains erode to a peneplain. Then, finally, new layers collect over the flattened metamorphic and igneous rocks. As described earlier, the Tapeats Sandstone layer in the Grand Canyon forms an angular unconformity where it overlies the tilted Grand Canyon Series and a nonconformity where it rests on the Vishnu Schist.
An ongoing process
As rocks continue to wear away, more unconformities appear. As road crews cut through mountains, they expose unconformities for the speeding motorist as well as the geologist to enjoy. However, these new exposures, and the mountains that contain them, will erode flat. The Appalachians, the Himalayas, the Alps, the Rockies, even the Grand Canyon, will die their slow erosional deaths as nature levels the continents, which may then subside beneath the seas. However, more sediment will soon accumulate, which will uplift and erode, and so on into eternity—an unbroken cycle of geologic processes.
See also Geologic time.
Resources
BOOKS
Baars, Donald L. The Colorado Plateau: A Geologic History. Albuquerque, NM: University of New Mexico Press, 2000.
Biornerud, Marcia. Reading the Rocks: The Autobiography of the Earth. Cambridge, MA: Westview Press, 2005.
Chronic, Halka. Pages of Stone: Geology of the Grand Canyon & Plateau Country National Parks & Monuments. Seattle, WA: Mountaineers Books, 2004.
Duebendorfer, Ernest M.,ed. Geologic Excursions in Northern and Central Arizona: Field Trip Guidebook for Geological Society Of America. Flagstaff, AR: Northern Arizona University, 1998.
Harris, Ann G., Esther Tuttle, and S. D.Tuttle. Geology of National Parks. Dubuque, IA: Kendall/Hunt Publishing Co., 2006.
McPhee, John. Basin and Range. New York: Farrar, Strauss & Giroux, 1982.
Monroe, James Stewart. The Changing Earth: Exploring Geology and Evolution. Pacific Grove, CA: Thomson-Brooks/Cole, 2006.
Unconformity
Unconformity
An unconformity is a widespread surface separating rocks above and below, which represents a gap in the rock record. Unconformities occur when either erosion wears away rocks, or rock deposits never form. Therefore, a time gap exists between when the rocks below the unconformity formed and when those above it formed.
Unconformities are classified as three types. The most easily recognized are angular unconformities, which show horizontal layers of sedimentary rock lying on tilted layers of sedimentary rock. The upper layers may not be perfectly horizontal, but they do not lie parallel to the lower layers. The second type of unconformities are disconformities, which lie between parallel layers of sedimentary rock. The third type are nonconformities, which divide sedimentary layers from metamorphic and intrusive (cooled inside the earth ) igneous rocks . Common to all three, erosion causes them to form, and younger rocks sit on older rocks.
Angular unconformities
Four basic steps create angular unconformities. In step one, sediment weathered from land and carried to the sea accumulates on the sea floor and over millions of years turns to rock layers. Then the collision of plates, giant sections of the earth's crust that constantly shift, lift and tilt the layers until the layers rise above sea level and then weather and erode. They erode for millions of years until the edges of the tilted layers become a flattened plane (a "peneplain" is a broad land surface flattened by erosion). Finally, in step four, sea level rises or land sinks. Sediments wash down, forming new horizontal layers that cover the submerged, tilted layers. These four steps could take hundreds of millions of years to complete.
The Colorado River at the Grand Canyon exposed one of the best angular unconformities in the world. From even miles away on the rim of the canyon, observers can see tilted layers of rock truncated roughly 550 million years ago by a horizontal sedimentary layer called the Tapeats Sandstone. The inner gorge of the Grand Canyon provides a great example of angular unconformity formation. Except, instead of four steps, the rocks tell of seven: (1) Over two billion years ago, layers of sediment accumulated and turned to rock. (2) Around two billion years ago, plate collisions lifted mountains and turned the sedimentary rocks into the Vishnu Schist, a metamorphic rock . (3) A half a billion years later, the mountains eroded into a peneplain. (4) The land subsided or sea level rose to deposit new layers (known as the Grand Canyon Series) on the old Vishnu Schist. (5) New plate collisions tilted and uplifted the Grand Canyon Series. (6) Erosion truncated the tilted Series and created another peneplain. The erosional episode lasted almost a billion years. (7) The land subsided eventually and the Tapeats Sandstone accumulated on the tilted Grand Canyon Series. In some places in the canyon, the Tapeats lies not on the angled Series but directly on the metamorphic Vishnu Schist—making this a nonconformity.
Another famous angular nonconformity is Scotland's Siccar Point, a site which played a part in the development of modern geology . In the eighteenth century, most people believed the earth to be only 6,000 years old, a figure arrived at earlier by Bishop Ussher, a prominent theologian who added the ages of Biblical characters and thus concluded the world was created in 4004 b.c. Scottish scientist James Hutton, however, realized that thousand-year-old Roman ruins in Great Britain were barely touched by weathering and erosion. He therefore wondered how long it takes for whole mountains, like those in Scotland, to wear down.
The angular unconformity he discovered at Siccar Point in Scotland provided dramatic evidence for his time "expansion." He saw nearly horizontal sandstone resting on nearly vertical graywacke (a sedimentary rock similar to sandstone) and marveled at how long it took to deposit the graywacke, tilt it, erode it, and then lay sandstone across it. As his friend, John Playfair, wrote "The mind seemed to grow giddy by looking so far into the abyss of time."
Disconformity
From a mile away, or perhaps from a few hundred feet away, disconformities can hide. The layers appear regular, all parallel. However, between the layers, a disconformity can lie. Not until a geologist closely examines the fossils in the layers for the presence or absence of certain organisms can he or she recognize the gap in time—an erosional period when sediment accumulation or deposition halted or perhaps when anything laid down washed away.
As with an angular unconformity, disconformities form in steps. In step one, sediments collect on an ocean floor (or perhaps on the bed of a large lake ). They compact and become rock layers. In the second phase, sea level falls or the sea floor rises to expose the layers to weathering and erosion. The main difference in the formation of disconformities and angular unconformities lies in this second step. As the layers of the future disconformity rise above sea level, they remain horizontal—no tilting occurs. If they tilt in this step, they later form an angular unconformity. Then, in step three, the land subsides or sea level rises, and new sediments collect on the older, still horizontal, layers.
Back in the Grand Canyon, where our story paused roughly 550 million years ago, the Tapeats Sandstone draped across the Grand Canyon Series and the Vishnu Schist to form the Great Unconformity. At least two more layers of sediments—the Bright Angel Shale and the Muav Limestone—collected on the Tapeats over the next 50 million years. Other layers may have been formed, too, but disappeared when the land rose and eroded for the next 80 million years. When the area again sank beneath the sea, the Temple Butte Formation, 80 million years younger than the Mauv, accumulated on the disconformity. The cycle of deposition, uplift , erosion, subsidence , and more deposition repeated at least four times from 550 to 250 million years ago.
Nonconformities
Nonconformities separate sedimentary rock layers from metamorphic rock layers and from intrusive igneous rock (like granite). In a step-by-step process similar to the other two unconformities, sediment accumulates and becomes rock. Then plate collisions deform these layers and change them into metamorphic rocks. Associated with this mountain-building, molten rock often squeezes upward into the metamorphic rock fractured by the deformities and solidifies, forming igneous rock (usually granite). In phase three, the mountains erode to a peneplain. Then, finally, new layers collect over the flattened metamorphic and igneous rocks. As described earlier, the Tapeats Sandstone layer in the Grand Canyon forms an angular unconformity where it overlies the tilted Grand Canyon Series and a nonconformity where it rests on the Vishnu Schist.
An ongoing process
As rocks continue to wear away, more unconformities appear. As road crews cut through mountains, they expose unconformities for the speeding motorist as well as the geologist to enjoy. However, these new exposures and the mountains that contain them will erode flat. The Appalachians, the Himalayas, the Alps, the Rockies, even the Grand Canyon, will die their slow erosional deaths as nature levels the continents, which may then subside beneath the seas. However, more sediment will soon accumulate, which will uplift and erode, and so on into eternity—an unbroken cycle of geologic processes.
See also Geologic time.
Resources
books
Baars, Donald L. The Colorado Plateau: A Geologic History. Albuquerque: University of New Mexico Press, 1983.
Chronic, Halka. Pages of Stone: Geology of Western NationalParks and Monuments. Vol. 4. Grand Canyon and the Plateau Country. Seattle: The Mountaineers, 1988.
Chronic, Halka. Roadside Geology of Arizona. Missoula, MT: Mountain Press, 1984.
Dixon, Dougal, and Raymond L. Bernor, ed. The Practical Geologist. New York: Simon and Schuster, 1992.
Harris, Ann G., Esther Tuttle, and S. D. Tuttle. Geology of National Parks. 4th ed. Dubuque: Kendall/Hunt Publishing Co., 1990.
McPhee, John. Basin and Range. New York: Farrar, Strauss & Giroux, 1982.
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Angular unconformity
—An unconformity, or gap, in the rock record, where horizontal rock layers overlie tilted layers.
- Disconformity
—An unconformity, or gap, in the rock record, situated between parallel rock layers.
- Nonconformity
—An unconformity, or gap, in the rock record, where sedimentary rocks overlie metamorphic or igneous rocks.