Perhaps no other types of landforms so dominate the landscape at regional scales as do those formed from tectonic folding, regional faulting, and intrusion/extrusion of magmas. Space imagery, especially when mosaicked, is an exceptional way to portray the end results of the orogenic processes that produce mountains (always eroded to some degree to carve out the landforms that typify them) and other manifestations of tectonism. One type of mountain is the volcano - usually a stratocone or a bulging upwarp. Tectonic activity also leads to outflows of lavas that can cover vast areas and may result in ususual landscapes.

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:

Here are a sequence of three parallel hogback ridges in Mauritania, imaged from space by SPOT-1:

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.

 

17-3: As a generalization, would you say that the "style" of deformation in the Anti-Atlas and Pakistan scenes is similar or dissimilar? ANSWER

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:

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.

17-4: Broadly speaking, how does the tectonic "style" of this Arabian Shield scene differ from that of the previous two images? ANSWER

All continents have crystalline igneous-metamorphic rock masses that make up the Shields or Cratons 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. We've seen several 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 as the dark area in this (cropped) Large Format Camera photo:

Another remarkable mosaic covers much of northwestern Australia, a region of limited vegetation so that the rocks and valley-fill stand out and reveal much of their underlying structure. The mosaic here has been divided into left (west) and right (east) halves stacked vertically to fit on the page without much reduction. You saw part of this scene in a single Landsat image at the top of page page 6-15. This is the Western Australian shield, containing mostly Precambrian metasedimentary and metavolcanic rocks, interlaced in places by igneous rocks. At the top of the (right half, lower image) mosaic is the Pilbara block, a leading candidate for the classic expression of an ancient greenstone-granite complex anywhere on Earth.

The two halves do not quite match because of copying problems.

This annotated sketch map applies to the mosaic.

For further guidance, look at the above Pilbara features in relation to the geologic map of this part of Australia that covers the right half of the sketch map.

The granite appears as batholiths, up to a 100 km (62 mi) long. These light rocks are diapiric intrusions into the dark greenstones (metamorphosed basalt). To the south is the Hamersley Range (blue area on the map) and the smaller Opthalmia Range (red), bordered on the south by the Ashburton Trough (left) and the Bangemall basin (right). Low-relief hills mark much of the region. The highest area (1,235 m, 4,051 ft) is in the Hamersley Range.

17-5: The upper and lower half of the Australian mosaic are tectonically different. What might this difference be (tectonically)? ANSWER

In Africa, Precambrian mountains often stand out in stark relief as topographic highs midst lowlands covered by sand. Such mountains are found in parts of the Sahara Desert. Here are the Air Mountains in Niger, consisting of peralkaline granite intrusions which appear dark (unusual since most granitic masses are light-toned in the field):

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.

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, which produce a wide variety of surface structures and features, we show a 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:

In this diagram, note the feature labeled "sill". This is caused by lava intruded along a bedding plane (it thus is parallel to beds above and below). In South Africa are curious landforms, shown in this Landsat image, that result from erosion of the layers of sedimentary rock in which the basaltic rock (dark) has intruded, exposing the lava mainly at the rim edges of low mesalike landforms:

The most common lava is the basaltic type, fluid melted rock high in iron and magneium oxides and with a silica (SiO₂) content around 50% by weight. With this composition it is less viscous and flows easily after reaching the surface. But as it cools, it can make strange surface forms, as seen in this close-up photo of pahoehoe (ropy basalt lava).

Basalt flows often build up thick individual units. These may be separated by volcanic ash deposits. This is the case for the John Day region in central Oregon where alternating flow and ash make up the upper part of the distant mountain and a very thick continuous light ash unit from a volcanic explosive eruption to the west lies in thick beds below it:

In general, volcanic units are composed of layers of ash and/or solidified lava wherever these are emplaced. In the interior of the vent at the top of Mount Vesuvius the layers of this stratocone that built it up over time are exposed, evident in this photo:

We look first at these next five images that represent two major types of volcanoes. Then, we look at terrains carved into vast sheets of volcanic flows or flood basalts.

The Hawaiian islands are entirely volcanic, rising as basaltic volcanoes from the ocean floor, reaching heights that carry them above sealevel. The Big Island of Hawaii is the latest (youngest) in this series of volcanic islands formed from melted lower crustal rocks as the Pacific plate moves northwestward over a fixed hot spot in the Earth's mantle. A newer submarine, volcanic complex, now forming southeast of Hawaii, will eventually surface and replace the Big Island as the center of activity. The Islands to the northwest, including Oahu and Maui, were formed earlier as the Pacific plate passed over them in succession. The next image is a Terra MISR scene that includes all of the larger Hawaiian islands:

(As an aside, note that the green signifying vegetation is much more profuse on the right [eastern] side of the islands. The prevailing winds are easterlies; they come from the east. As winds moving water clouds pass over the islands, the precipitation is confined mainly to the eastern slopes. With much of this water thus lost, the western slopes tend to support considerably less vegetation.)

This early Landsat image of the Big Island shows details of the recent (past few thousand years) volcanic flows (see pages 9-7 and 14-11 for other renditions):

Mauna Loa, near the center of Hawaii, is the central part of a huge shield volcano, which comprises the entire island. Its summit crater, a collapsed caldera named Mokuaweoweo, lies beneath a crest at 4,135 m (13,563 ft). Its base lies about 4,000 m (13,120 ft) below sea level, which makes it the tallest single mountain in the world (Everest, while higher, rises from the valleys of the Himalayas that are thousands of meters above sea level, so its relief is less). Mauna Kea, a crater on the north section of the island, is now extinct. But the most active volcano in the world, Kilauea, lies along the east side of the island and is visible here as a dark patch. This island is quite young, consisting of multiple layers of basaltic flows built up in the last one million years. Numerous lava flows (dark basalt), many extruded over the last few centuries, emanate from Mauna Loa, as seen in this photo taken by astronauts aboard the International Space Station:

A trip to Kilauea at any time stands a good chance of showing some eruption, either along a rift zone connecting it to Mauna Loa or from the caldera that has developed. The next two images were made by SIR-C X and C band radar; check the captions for more information:

The main caldera is evident in this radar interferometric image of part of the Kilauea volcanic edifice.

This caldera is steep-walled with a smaller sink as seen in this ground photo:

Transitional to the stratocones described below are the steeper-sided volcanoes that make up the basaltic Galapagos Islands (see page 6-10), some 500 miles (800 km) west of Ecuador, in the eastern Pacific Ocean. This next illustration shows Volcano Darwin on Isabela Island in a perspective image made from SIR-C radar data and elevation data acquired by TOPSAR (an aircraft-mounted radar system designed to measure topographic elevations):

The other spectacular type of volcano is the stratocone, noted for its steep sides and, often, its symmetrical form. In the U.S. Mainland, the most photogenic stratovolcanoes are in the Cascade Mountains (from Northern California into Northern Washington), and in the Aleutian Islands of Alaska. The photo below shows the north side of Mount Rainier, a massive, still active volcano rising to 4300 m (14411 ft) to the southeast of Seattle, WA. Like most other Cascade stratocones, this volcano is superimposed on older, much eroded volcanic rocks from earlier periods of volcanism. Below the photo is a view from space made from multiband radar imagery acquired by SIR-C.

The classic example of a stratocone is Mount Fujiyama west of Tokyo. It rises to 3776 meters (12460 ft) from sealevel. Here is a vertical photo taken by an ISS astronaut:

Mt Fuji is considered to be the most perfect (symmetrical) stratocone presently on Earth. Judge for yourself from this ground photo:

Below is a Landsat view of a segment of Java, the main island in the Indonesian archipelago, a prime example of an island arc terrain still evolving. Nine stratocones are in the scene; the three most prominent are Muria (top center); Merapi (lower left), and Lawu (lower right).

In the midst of thick sequences of geosynclinal sediments are a series of large composite stratovolcanoes, developed from crustal melt induced by frictional heat, as the Indian-Australian plate dives in subduction below the southernmost extension of the Eurasian plate (see the tectonic map at the top of this page). The stratocone on the north peninsula near the Java Sea is Muria. The highest (2,910 m, 9,545 ft) volcano is the active Merapi, which stands out as the lower of two in the left center. To its right is Lawu. Six other large volcanoes are mainly to the west (left) of Merapi.

17-6: What is missing volcanically in the Java image that is present in the Hawaiian scene? ANSWER

Stratocones come in various sizes and can occur in dispersed swarms. This is particularly a hallmark of volcanoes in the South American Andes Mountains as seen below. Use the sketch map as an aid to picking out the individual cones, many of which are snow-capped in this southern Fall image, owing to the high elevations of the flanks of the High Andes.

This color version of similar volcano coned-dotted terrain in Chile is interesting:

On a much smaller size-scale are cinder cones - in some respects miniature stratocones. Here is a swarm (right side) of cinder cones on the flank of a caldera (left) on the Isla de la Palma, one of the Canary Islands off the Mid-Atlantic Ridge:

Lavas (magmas that reach the surface) extrude not only from discrete individual volcanoes but from deep-reaching fractures in the crust that can tap into the upper mantle. The result is widespread flows covering large areas. We saw one example in Section 3 of basaltic flows in the East African Rift. This huge fracture zone runs across much of the eastern side of that continent as one of the "arms" of splitting tectonic plates. Two other arms or dividing zones, where Africa is breaking off from the Arabian plate and from the Australo-Indian plate, meet the newly developing East African arm at a "triple junction" located in the Afar of Ethiopia. This junction was captured photographically by astronauts on the Earth-orbiting Apollo 7 (pre-lunar) mission:

Associated with the rifts, beyond the apex of these spreading centers, great quantities of basaltic lavas are pouring over the surface in the Afar Triangle. This locale was photographed with the Large Format Camera, from the Shuttle, as seen here:

Well south along the East African Rift, the fault zone in the basalts narrows. Here is that segment, part in Kenya and the southern part in Tanzania. In the upper right is the great stratocone of Mt Kenya; its larger companion, Mt. Kilimanjaro is in the lower right:

Flood basalts, extruding from many fissures, and moving out to cover 100s of thousands of square kilometers, are found on several continents. They occur mainly in active tectonic zones. Here is a map showing the global distribution of flood basalts and similar oceanic basalt fields, of various ages, but occurring at the crustal surface:

The Columbia River and Snake River basalts of Oregon, Washington, and Idaho form one such plateau of lavas piled on lavas, the many successive outflows producing distinct layering. Here is a field photo showing a series of flood basalt flow layers in the Columbia Plateau.

In western India are the Deccan plateau basalts, whose extrusions for more than 70 million years are related to the collision of India against the southern margin of the Asian plate. A Landsat view shows the landscape, often barren, mountainous, and with a lower population density. The photo below it reveals the expression of the flow layers.

Large areas covered by basaltic lava will develop distinctive terrain forms. This next Landsat image shows flat to somewhat rounded hills in thick flood basalt flows ("trap rock") overlying Permo-Triassic sedimentary rocks in the northwestern Siberia Plateau.

Let us look at a small volcanic field in the Mexican state of Sonora just south of the border with Arizona. Little was known of it among American volcanologists until this Gemini photo was taken in the 1960s. The Pinacate field, covering an area of 1500 km², consists of basaltic flows coming from more than 200 vents:

The same field is shown in color made from individual radar bands in this SIR-C image:

Cinder cones and eroded maars (volcanic structures partly blown away when lava encounters water which turns to steam; another example from Pinacate is seen on page 18-1) are characteristic of this Field.

Taller volcanic cones and lava flows are evident in the eastern part of the Pinacate Field, as seen in this photo:

Finally, we examine the Hopi Butte volcanic landforms that show up as small dark flat-topped hills in a swarm of more than 200 individuals in the Painted Desert of the Colorado Plateau in northern Arizona. This is a subscene extracted from an early Landsat image:

Seen from the ground, the distinctive flat-topped nature of individual structures makes this, along with Monument Valley to the north, a popular spot for western movies (especially favored John Wayne):

Photo credit: Louis Maher

Most of these structures are diatremes (steam-driven volcanic material that punches its way through sedimentary rocks); some are the maars (crater-shapes developed when the surface rocks are expelled) that form above the intruding diatremes which eventually are exposed to make the volcanic necklike surface shapes. Some are volcanic caps on sedimentary rocks.

It is worth commenting to close this page that much/most of the exteriors of the other inner or terrestrial planets, and our Moon, are surfaced by countless basalt flows. (And, of course, this applies to the bedrock below marine sediments on the Earth's ocean floors.)

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Primary Author: Nicholas M. Short, Sr.