Tectonic/Volcanic Landforms¶
Perhaps no other types of landforms so dominate the landscape at regional scales than those formed from tectonic folding, regional faulting, and intrusion 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 set up 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/Volcanic Landforms¶
tectonism, including the plates, the spreading ridges, and location of volcanic belts of the last million years.|
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.
HCMM is especially suited to showing large segments of a mountain belt. 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
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), the snow-covered main high Himalayas, and the southern Tibetan Plateau (on average, the highest generally flat landmass in the world).
` <>`__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 <Sect17_answers.html#17-3>`__
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 (with the through-flowing Yangtze River) of what are known as decollement folds formed within thrust sheets (like wrinkles on a sliding rug).
|Landsat image showing detachment folds in the Sichuan Province, China. |
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 edged 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 <Sect17_answers.html#17-4>`__
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 (peridoties; some gabbros), first extruded as lavas with shallow intrusives below, that moved as ocean floor 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.
(top), a Precambrian greenstone belt intruded by granitic batholiths.|
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 <Sect17_answers.html#17-5>`__
Turning now to volcanic landforms, we show three 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:
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.
Below is a Landsat view of a segment of Java, the main island in the Indonesian archipelago, a prime example of an island arc terrane still evolving.
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 <Sect17_answers.html#17-6>`__
Stratocones come in various sizes and can occur in 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.
seen by Landsat.|
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. 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. In western India is 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 nature of the flow layers.
It is worth commenting 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.)