Tectonic landforms usually dominate the scenery in any region that has experienced significant crustal disturbances, and this activity often shows as truly spectacular expressions in remote sensing images. For this reason, the theme chapter by this title in "Geomorphology from Space" is by far the longest. These landforms frequently reveal surface manifestations of the type of underlying deformation caused by plate tectonic interactions. Some of these interactions characterize orogenic (mountain) belts at subduction zones (convergence of two or more plates) or pull-apart regions where plates diverge. For anyone unfamiliar with the first-order framework of the global tectonic system, examine this map produced by Paul D. Lowman, Jr. (author of Section 12) of the lithospheric plates, spreading ridges, transform faults, and other tectonic features. Consult any introductory Geology textbook for more information on the Plate Tectonic paradigm. Or, better yet, work through this Website: USGS Tutorial.
We described some exceptional examples (drawing upon mostly Landsat images) of tectonic landforms in Sections 2, 6, and 7, which you can review (look particularly at the Zagros folds, the Pindus thrust belts, the Atlas Mountains, and the Altyn Tagh fault in Section 2 and the Appalachian folds and Basin and Range block fault mountains in Section 6, and the European Alps in Section 7.
These images focused on folds and faults, the most common types of tectonic deformation. The resulting landforms commonly have elevation differences (relief) that may be sufficient to change ecosystems developed at these heights. Thus, mountains in a semi-arid climate may be heavily vegetated (dark toned in visible band images) and adjacent basins less so (light), thus, showing strong contrasts in black and white images (the Nimbus 3 image of the Wyoming mountains in Section 14 is a good example). Mountainous terrains appear clearly in Landsat, HCMM, and radar images by virtue of shadowing, which causes tonal variations related to slope/sun positions.
One of the simplest tectonic landforms is the "hogback" - a ridge (straight to curved) made up of harder, more slowly eroding inclined rock units, that lags behind as the general surface level is worn down by erosion (as is found along the front of the Colorado Rocky Mountains; see Section 6). This aerial photo shows a typical hogback:
As you saw on page 2-6, space images in arid lands are particularly suited to showing folds - mainly anticlines - as they are expressed by the contortions revealed as erosion cuts into the inclined strata. We show five examples of this; the first four are SPOT images, the last is a Landsat image covering a larger area:
HCMM is especially suited to showing large segments of a mountain belt, providing a small-scale overview. Perhaps the most famous in the world, in terms of how its origin has been interpreted to lead to some earlier hypotheses on formation of orogenic belts, is the Appalachians. Examine the HCMM mid-Appalachians image found on page 6-3 (while there, scroll down to see the frontal hogbacks referred to above).
The Rocky Mountains in the U.S. were examined on page 6-6. They continue into Canada and in Alberta and British Columbia almost merge with the Coast Range and other Cordilleran mountain chains. Here is a Landsat mosaic that shows some of the Canadian Rockies. Below it is a strip across those Rockies made from Radarsat imagery.
And here is a aerial oblique view of typical mountain terrain in part of the Canadian Rockies; the broad valley has been widened by glaciation and backfilled with post-glacial deposits:
Recall the Landsat mosaic that showed much of the vast chains of interrelated mountain belts in southern Asia where the "crash" of the Indian subcontinent over the last 40 million years created the Himalayas on the north and folds in Pakistan and Iran in the west and others in Burma (Myomar) (see page 5-5) and Malaysia in the east. A spectacular oblique view of the main Himalayas, taken by an astronaut using a film camera, was shown on page 12-4. Here is another astronaut photo, made with the Large Format Camera, covering much of the same scene, including the Siwalik Hills (dark, near bottom right), the snow-covered main high Himalayas, and the southern Tibetan Plateau (on average, the highest generally flat landmass in the world).
To see more detail in the flanking mountains in western Pakistan, here is part of the fold belt that was shoved up by the huge collision between the Indian subcontinent and southern Asia (the context of this is evident in the mosaic examined earlier in Section 7). The scene shows the Sulaiman fold belt, consisting of echelon (offset) anticlines (some closed), making up the ridges (flat valleys occupy intervening synclines). The Kingri fault passes through the image center (look for an abrupt discontinuity). The crustal block to its west (left) has moved northward relative to the block on the east.
The tectonics of southern Asia is dominated by the Himalayan docking event. Subsidiary tectonic disturbances occur beyond the Himalayas. In central China is this scene (which includes the through-flowing Yangtze River) of what are known as decollement folds formed within thrust sheets (like wrinkles on a sliding rug).
Major strike-slip faults occur in the mountains of southern Asia, as land units moved laterally as a result of the Himalayan collision. This mosaic shows part of the 1500 km (1000 mile) Altyn Tagh (Altai) fault, which serves as a zone of crustal rock erosion that forms a valley:
Similar slip zones - the Kunlun and Karakorum faults - in the Kunlun mountains also form valleys, as seen in this astronaut photo.
Western China, in and around Sinkiang Province, is mostly arid lands marked by deserts and mountains. Some of the mountain terrains are bounded by faults as in this Landsat image that includes part of southern Mongolia:
Crystalline (igneous and metamorphic) rocks make up distinctive terrains and landforms. Intrusions of granite, in particular, often accompany orogenies that produce extensive mountain chains. Granites are the most common rock type in batholiths that can form the core of a mountain system. In the United States the best known batholiths are in the Idaho Rocky Mountains and the California Sierra Nevada. Here is a Landsat image that shows the terrains of notable relief (emphasized by glaciation) in the High Sierra; beneath it is a ground photo that typifies the mountainous character of this terrain:
All continents have crystalline igneous-metamorphic rock masses that make up the Shields or Cratons (multiple intrusions and deep burial metamorphism over extended spans of geologic time) around which the continents have grown. In North America, the Canadian (subset Laurentian) Shield is the core around which the continent has grown by accretion and marine overlap of sedimentary rocks on this crystalline basin. In northern Canada, near Churchill, is a classic exposure of barren granitic/metamorphic shield rocks:
To the west of the Indian subcontinental plate is the Arabian tectonic plate, caught between the African, Eurasian, and Indian-Australian plates as they move in different directions. The western part of this plate is a crystalline shield (a continental nucleus containing ancient igneous and metamorphic rocks). Below is a mosaic (from 12 individual Landsat scenes) of the shield as exposed in southern Saudi Arabia and the Yemen Arab Republic.
Dominant features in this scene are the numerous granitic intrusions, whose boundaries show as distorted oval shapes. The shield is a region of low mountains separated by valleys, many of which are sand-covered. A prominent escarpment (near the upper, left edge) bounds the western edge of the shield. The coastal plain is bounded by a fault-controlled scarp. Another scarp (lower right) also relates to the fault.
The Arabian Shield is part of a Precambrian craton that extends beyond Saudia Arabia. In Egypt it forms part of the Nubian Shield that is exposed on both sides of the Red Sea. The Southern Sinai Highlands is an offshoot that is found on both sides of the Gulf of Aden, seen here in an astronaut photo. A ground photo indicates how rugged the Sinai terrain can be; the rocks shown are granitic:
We've seen several other examples of shield terrane on this and other pages. Here is another, the Nigerian Shield (astride the Nigeria/Cameroon border). Like most shields, large criss-crossing fractures cut into the surface and the hills tend to be of low relief. The red indicates vegetation; the blue is sedimentary rocks with barren surfaces.
On the east side of the Arabian Peninsula, in Oman, are the Oman mountains, large parts of which are composed of ophiolites. These are ultramafic igneous rocks, first extruded as lavas with shallow intrusives below (peridotites; some gabbros), that made up new ocean floor that moved away from a spreading ridge. On contacting a continental mass at a subduction zone, the ophiolites may subduct but otherwise can also be thrust on (obducted) to the continental edge. In this Landsat image the ophiolites are the dark bluish-black masses.
The island of Cyprus in the eastern Mediterranean contains a distinctly different wedge of subducted basaltic rock making up an ophiolite inclusion within rocks of a different lithologic nature. It is prominent in this (cropped) Large Format Camera photo:
Another swarm of mountains made up of crystalline rocks occurs in the Namib Desert of Namibia. Here is a Landsat-1 image of isolated mountains, some rising to 1800 m (5300 ft) above the desert sands:
Most striking of these is the Branberg Massif (located beyond the above subscene), a Precambrian intrusion, now weathered out, of near-circular shape (occupying 650 km2) that reaches an elevation of 2573 m (8480 ft). It is seen here in a Landsat-7 image and then a perspective view (January 3, 2001) made from ASTER imagery and its own DEM measurements.
Plutons of the West African Shield in Mali show up differently depending on the type of sensor used. In the left image they are evident in this radar view but do not contrast with the terrain outside the intrusions. In the right Landsat image, the plutons are dark in sharp contrast with the desert sands that occupy the lower terrain (the plutons are low hills not easily covered by sand).
A landform that has aspects both of a structural and a igneous nature is produced by intrusion of magma or lava into a major fault or fracture in the crust to form what is called a dike. By far the biggest such feature known on Earth is the Great Dyke of Zimbabwe in Africa. The dike is more than 450 km (280 miles) in length and up to 15 km (10 miles) wide. It is filled with coarse-grained gabbro rather than the customary basalt, which serves to identify it as the feeder vent that produced several lopoliths (now eroded away) that formed in the Zimbabwe craton. Here is part of the Great Dyke as imaged by Terra's ASTER:
A landform that is both tectonic and volcanic is the ring dike. It forms by intrusion of lava into a conical fracture that is circular in plan cross-section. As the surface is eroded, the dike, being more resistant than surrounding sedimentary rocks, stands out as a ring rising above its surroundings. A classic example is the Kondyor massif, in which the intrusive igneous rock is an alkaline dunite which hosts platinum minerals and gold, in eastern Siberia, seen in this ASTER image:
Turning now to volcanic landforms, these are especially evident in images made from space. Volcanism is widespread worldwide and eruptions have been monitored in near real time using satellites. Compared with most other landform types, those resulting from eruptions tend to form over short timeframes. Volcanic activity produces a wide variety of surface structures and features. Examine this well-known block diagram prepared by the U.S. Geological Survey that sketches surface volcanic landforms and ties some of these to subsurface structures that serve as the magma chambers and feeders that bring the molten rock (lava) to the surface:
This image introduces how volcanic landforms often stand out in space imagery. Try to identify specific landforms in the scene and name them from the above sketch. The scene includes the Harrat Khaybar in Saudi Arabia:
Volcanic landforms are continuously being produced on Earth, since active volcanism is widespread today. Here are two images made by Terra sensors that caught the eruption of the Chaiten volcano near the coast of Chile south of Santiago. This volcano had been dormant for nearly 9000 years.