Tectonic Landforms

01
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Escarpment, Oregon

The biggest of cliffs
Pictures of Tectonic Landforms. Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)

There are different ways to classify landforms, but mine has just three categories: landforms that are built (depositional), landforms that are carved (erosional), and landforms that are made by movements of the Earth's crust (tectonic). Here are the most common tectonic landforms. I take a more literal approach than most textbooks and insist that tectonic motions create, or largely create, the actual landform.

See also:     Depositional Landforms    Erosional Landforms

Escarpments are long, large breaks in the land that separate high and low country. They may result from erosion or from fault activity. (more below)

The escarpment called Abert Rim, in south-central Oregon, is the site of a normal fault where the land in the foreground dropped by several kilometers relative to the plateau behind, one large earthquake at a time. At this point the escarpment is more than 700 meters high. The thick bed of rock at the top is the Steen Basalt, a series of flood basalt flows erupted about 16 million years ago.

Abert Rim is part of the Basin and Range province, where normal faulting due to extension of the crust has created hundreds of ranges, each flanked by basins many of which contain dry lake beds or playas. Abert Rim may be North America's finest example of an escarpment, but the area has several other contenders. The world's premier escarpments, though, are probably in Africa's Great Rift Valley.

This and other pictures of Abert Rim appear in the Oregon Outback Gallery and are also available as free wallpaper images.

02
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Fault Scarp, California

A perishable landform
Pictures of Tectonic Landforms. Photo courtesy Ron Schott of Flickr under Creative Commons license

Motion on a fault may raise one side above the other and create a scarp. This fault scarp formed in the 1872 Owens Valley earthquake. (more below)

Fault scarps are short-lived features in geologic terms, enduring no more than a few millennia at best; they are one of the purest tectonic landforms. But the movements that raise scarps leave a large area of land on one side of the fault higher than the other side, a persistent elevation difference that erosion can obscure but never erase. As fault displacement is repeated thousands of times over millions of years, larger escarpments and whole mountain ranges—like the high Sierra Nevada range beyond—can arise.

03
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Pressure Ridge, California

A mole track from a large earthquake
Pictures of Tectonic Landforms. Photo by Paul "Kip" Otis-Diehl, USMC, courtesy of the U.S. Geological Survey

Pressure ridges form where lateral motions on a curving fault force rocks into a smaller space, pushing them upward. (more below)

Faults like the San Andreas fault are rarely perfectly straight, but rather curve back and forth to some degree. When a bulge on one side of the fault is carried against a bulge on the other side, the excess material is pushed upward. (And where the opposite occurs, the ground is depressed in a sag basin.) The Hector Mine earthquake of October 1999 created this small "mole track" pressure ridge in the Mojave Desert. Pressure ridges occur in all sizes: along the San Andreas fault, its major bends coincide with mountain ranges like the Santa Cruz, San Emigdio and San Bernardino Mountains.

04
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Rift Valley, Uganda-Congo

Where plates split
Pictures of Tectonic Landforms. Photo courtesy Sarah McCans of Flickr under Creative Commons license

Rift valleys appear where the whole lithosphere is pulled apart, creating a long, deep basin between two long highland belts. (more below)

Africa's Great Rift Valley is the world's largest example of a rift valley. This photo looks west from the Butiaba escarpment, in Uganda, across Lake Albert to the escarpment of the Blue Mountains in the Democratic Republic of the Congo.

Other major rift valleys on the continents include the Rio Grande valley in New Mexico and the Lake Baikal rift valley in Siberia. But the greatest rift valleys are under the sea, running along the crest of the midocean ridges where the oceanic plates pull apart.

05
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Sag Basin, California

Downdropped blocks
Pictures of Tectonic Landforms. Photo (c) 2004 Andrew Alden, licensed to About.com (fair use policy)

Sag basins occur along the San Andreas and other transcurrent (strike-slip) faults. They're the counterpart of pressure ridges. (more below)

Strike-slip faults like the San Andreas fault are rarely perfectly straight, but rather curve back and forth to some degree (see the three types of fault). When a concavity on one side of the fault is carried against another on the other side, the ground between sags in a depression or basin. (And where the opposite occurs, the ground rises in a pressure ridge.) Where the ground surface of the sag basin falls below the water table, a sag pond appears. This example is from the San Andreas fault just south of the Carrizo Plain near Taft, California. The two sag ponds lie in a larger rift, a linear valley. Sag basins can be quite large; the San Francisco Bay is an example.

Sag basins can also form along faults with part normal and part strike-slip motion, where the blended stress called transtension operates. They may be called pull-apart basins.

Other sag ponds are shown in the San Andreas fault tour, the Hayward fault gallery and the Oakland geology tour.

06
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Shutter Ridge, California

Blocker in motion
Pictures of Tectonic Landforms. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Shutter ridges are common on the San Andreas and other strike-slip faults. The rock ridge is moving to the right and blocking the stream. (more below)

Shutter ridges occur where the fault carries high ground on one side past low ground on the other. In this case, the Hayward fault in Oakland carries the rocky ridge toward the right, blocking the course of Temescal Creek (here dammed to form Lake Temescal at the site of a former sag pond) and forcing it to flow rightward to get around it. (The result is a stream offset.) On the far side, the stream continues toward San Francisco Bay along the route of the freeway. The motion of the barrier is like the shutter of an old-fashioned box camera, hence the name. Compare this picture to the stream offset picture, which is exactly analogous.

07
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Stream Offset, California

Crooked creeks
Pictures of Tectonic Landforms. Photo courtesy Alisha Vargas of Flickr under Creative Commons license

Stream offsets are the counterpart to shutter ridges, a sign of lateral movement on strike-slip faults like the San Andreas fault. (more below)

This stream offset is on the San Andreas fault in Carrizo Plain National Monument. The stream is named Wallace Creek after geologist Robert Wallace, who documented many of the remarkable fault-related features here. The great 1857 earthquake is estimated to have moved the ground sideways about 10 meters here. So earlier earthquakes clearly helped produce this offset. The left bank of the stream, with the dirt road on it, can be considered a shutter ridge. Compare this picture to the shutter ridge picture, which is exactly analogous. Stream offsets are rarely this dramatic, but a line of them is still easy to detect on aerial photos of the San Andreas fault system.