Satellite imagery is particularly useful in change detection studies because of the short times involved in most eruptions. These two ASTER images show the Caribbean island of Montserrat, with its Soufriere volcano, one of the world's most active. The top scene shows the dark gray lava flows produced during the 20th century; the bottom scene shows the lighter-colored ash from an eruption in January of 2010.
In the diagram depicting volcanic landforms, shown above, 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.)
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:
Calderas occur wherever large volcanoes either have a collapse of their peaks or eruptions have "blown off the top", as at Mt. St. Helens. Here is a large caldera in the Sudan of eastern Africa:
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:
Another photogenic volcano is Teide in the Canary Islands along the Mid-Atlantic ridge. In this astronaut photo, almost as striking are the numerous lava channels, many bounded by lava levees:
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.
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:
This photo taken from the International Space Station shows cones in North Africa.
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; the same region was shown earlier on this page:
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:
As examples of flood basalts consider first the Deccan plateau basalts in western India, 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 flows spread over a wide area The photo below it reveals the expression of the flow layers.
In the United States the largest outpourings of basic lava are the Columbia River and Snake River basalts of Oregon, Washington, and Idaho. They form widespred plateaus built from lavas piled on lavas, the many successive outflows producing distinct layering. This map shows the three major volcanic provinces of the Pacific Northwest:
Although extensive is areal distribution and thick (multiple flows), these basalts are not readily apparent in satellite imagery. They tend to have formed soils that support agriculture. The Snake River Plains of Idaho are typical but the role of volcanism is revealed by some recent lava flows, such as those making up the Craters of the Moon National Monument:
Localized flows such as those at Craters of the Moon often occur in apparent isolation from their source volcanoes. Here are two small flows in the arid mountainous terrain of New Mexico (see captions for their names); both stand out because they are dark basalt:
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):
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.)