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Essential Science for Teachers: Earth and Space Science

The Engine That Drives the Earth The Engine That Drives the Earth | A Closer Look

A Closer Look

Look for the following topics in the video, indicated by the onscreen icon, and click below to learn more.


Plate Boundaries

Hot Spots

A Closer Look: Volcanoes

What is a volcano?

A volcano is a landform that is formed through the eruption and accumulation of lava and other solid material. It starts as a vent, hole, or crack in the Earth’s surface, through which hot molten rock (lava), gases, and tephra erupt. Tephra is a generic term for any fragments of volcanic rock that are blasted into the air, such as ash and chunks of rock, which, depending on their size, have a variety of names.

Why do volcanoes erupt?

Volcanoes erupt because of changes in density, buoyancy, temperature, and pressure. A volcanic eruption requires magma, or melted rock. Rock melts by either an increase in temperature, a decrease in pressure, or by the addition of water to the system (since water lowers the temperature at which a rock can melt). Melted rock is less dense than the solid rock surrounding it. Buoyancy causes less dense material to rise through more dense material. As the magma rises, pressure decreases, which causes additional melting and a continued decrease in density. Magma rises until it either erupts, or enters material with the same density, at which point it will form a magma chamber.

Throughout this ascent, bubbles can form from gas in the magma. This gas increases the pressure in the magma. If the pressure becomes great enough, the overlying rock can fracture, at which point an eruption occurs. Generally, volcanoes stop erupting because all the trapped volatile gasses have degassed and there is no longer sufficient pressure to drive the magma out of the Earth. Alternately, volcanoes stop erupting because enough heat is lost so that the magma cools and is no longer buoyant.

What are the different types of volcanoes?

The United States Geological Survey has identified four principal types of volcanoes:

Cinder cone.

Cinder cones are the simplest types of volcano. They are built from pieces of lava and tephra that have been ejected from a single volcanic vent. As the high gas-content lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a cone. Most cinder cones have a bowl-shaped crater at the summit. Cinder cones are commonly found on the flanks of shield volcanoes and stratovolcanoes. One of the most famous cinder cones, Paricutin, grew in the middle of a cornfield in Mexico in 1943.

Shield volcano in Hawaii.

Shield volcanoes are large and gently sloping. The Hawaiian volcanoes typify them. Shield volcanoes generally erupt fluid basaltic lava. Shield volcanoes are built up slowly by the accumulation of many of these highly fluid lava flows that spread widely over great distances. They are characterized by low gas contents and therefore low-exclusivity, and they often produce fountain-like eruptions. Made almost entirely of lava, shield volcanoes are a common product of hotspot volcanism.


Stratovolcanoes are characterized by eruptions of lava that is more viscous (resistant to flow) and higher in gas content. They are often found at subduction-related arcs. Viscous lavas allow gas pressures to build up to high levels by chemically accommodating water well, as well as effectively “plugging” the volcano. As a result, stratovolcanoes often erupt explosively. Typically steep-sided, they are built of alternating layers of lava flows and tephra (volcanic ash and rocks that are blasted into the air upon eruption). The layering of these volcanic materials gives stratovolcanoes their other common name, Composite volcanoes. Well-known examples of stratovolcanoes are Mount St. Helens in the United States and Mount Fuji in Japan.

Volcanic domes, also referred to as lava domes, commonly occur within the craters or on the sides of large stratovolcanoes. Volcanic domes are rounded, steep-sided mounds built by lava too viscous to flow any great distance. This viscous lava piles over and around its volcanic vent. Domes may consist of one or more individual lava flows. A dome grows largely by expansion from within. As a dome swells with hot magma inside, its outer surface cools and hardens, and then shatters, spilling loose fragments down its sides. Monte Pelee in Martinique is an example of a volcanic dome.

What determines the shape and size of a volcano?

The shape of a volcano depends primarily on how viscous the erupting lava is, which is determined by the lava’s chemical composition. Although magma is made of several different chemical compounds, the relationship between a volcano’s shape and the chemical composition of the magma is largely determined by a single component: silica (SiO2). The more silica in magma, the more viscous or resistant to flow it is. Higher silica content also allows magma to trap more gas, which produces violent eruptions.

Gas Content Silica Content Volcano Type
Low Low Shield
Low High Dome
High Low Cinder cone
High High Stratovolcano


A Closer Look: Plate Boundaries

What are plate boundaries?

A central concept of plate tectonics is that large portions of the Earth’s lithosphere — the crust and the rigid part of the upper mantle — move relative to one another. This relative movement of the plates creates varying conditions at the plate edges, or boundaries. Geologists describe three basic types of plate boundaries: divergent boundaries, convergent boundaries, and transform boundaries.

Divergent boundaries exist where two plates are moving apart. This occurs most commonly at mid-ocean spreading ridges. At such ridges, plates move apart and the mantle exposed is melted by the decrease in pressure, becoming magma. That magma then rises up to “fill in” as new oceanic crust. As the plates separate, the geologic feature known as a “rift valley” is created. Iceland, which sits on the Mid-Atlantic ridge, is splitting apart on the divergent boundary between the North American and Eurasian Plates.

Convergent boundaries exist where two plates are moving toward each other.

Convergent boundaries can be of three types:

Subduction demonstration

Ocean-Ocean Convergence: If the plates moving toward each other are both made of oceanic crust, one of the plates will move downward, or subduct, under the other plate. This is called a subduction zone. A deep trench forms on the ocean floor at the location where one oceanic plate subducts under another. Also, a volcanic arc (a chain of volcanoes parallel to the trench) typically develops above the subduction zone. These volcanoes are generated as water brought down on the subducting plate melts the overlying mantle, causing magma to rise through the mantle and crust, erupting at the surface. The Mariana trench and volcanic arc mark where the Pacific and Philippine plates converge. The Aleutians, Japan, and the Philippines are other examples of volcanic arcs that exist as a chain of islands.

Ocean-Continent Convergence: If one plate topped by oceanic crust moves toward another topped by continental crust, the more dense, oceanic plate will subduct under the less dense, continental plate and a trench will develop off the shore of the continent. Off the coast of South America, along the Peru-Chile trench, the oceanic Nazca Plate is being subducted beneath the continental South American Plate. As a result, the Andes, a mountainous volcanic arc, have developed in South America. The Cascade Range, where Mt. St. Helens is found, is another example of a volcanic arc formed by the convergence of oceanic and continental plates.

Continent-Continent Convergence: If both converging plates carry continental crust, neither of the plates fully subduct. Continental rocks have a relatively low density and, like two colliding icebergs, usually resist downward motion. During a collision, the crust is compressed and subjected to very high temperatures and pressures. The lithosphere thickens and crustal rocks are folded and faulted. Large amounts of uplift push rock high into the sky, forming mountain ranges such as the Alps or the Himalayas.

Transform Boundaries: Transform boundaries exist where two plates slide past each other. Here, no lithosphere is created or destroyed. Often these large faults, or fracture zones, connect divergent or convergent plate boundaries. Most transform faults are found on the ocean floor. Examples include the Alpine fault in New Zealand, which forms the boundary between the Australian and Pacific plates, the Dead Sea fault, which forms the boundary between the African and Arabian plates, and the San Andreas fault, which lies between the Pacific and North American plates in California.

Is it that simple?

Not all plate boundaries are as simple as the main types discussed above. For example, there are several places on the Earth where three plate boundaries intersect. These are referred to as “triple junctions.” An example of a triple junction exists where the African, Australian, and Antarctic plates intersect. There, three spreading ridges intersect. What does that tell us about this area of the African, Australian, and Antarctic plates? — the plates are moving apart in this region.

In some parts of the world, plate boundaries are not well defined. These regions, called “plate-boundary zones,” are large areas where the effects of plate interactions are unclear. Plate boundary zones involve at least two large plates and one or more microplates (small plate fragments) caught up between the larger plates. The geology of these areas can be very complex. The Mediterranean-Alpine region between the Eurasian and African plates is an example of a plate boundary zone.

A Closer Look: Hot Spots

The majority of volcanoes occur near plate boundaries, but there are some exceptions to this. For example, the Hawaiian Islands, which are volcanic in origin, formed in the middle of the Pacific Plate more than 3,200 km from the nearest plate boundary. How can this be?

Kilauea, Hawaii, a hot spot volcano.

In 1963, J. Tuzo Wilson, the Canadian geophysicist for whom the Wilson Cycle of supercontinent assembly, breakup, and re-assembly was named (Session 2), noted that in several locations around the world, such as Hawaii, volcanism was occurring in the interior of tectonic plates. Scientists think that deep in the mantle, possibly even near the core-mantle boundary, large pockets of heat generate plumes of melt which rise through the Earth, regardless of plate boundaries, and generate ‘intraplate’ volcanism. While plates on the surface of the Earth move, “hotspots” are thought to be fairly stationary, generating melts until their source of heat dies away.

The hotspot beneath Hawaii has remained fairly fixed in the Earth’s interior. However, as the Pacific plate moves northwest above the plume, volcanic islands are formed in a chain in the middle of the plate.

Most hotspots occur in the interior of plates but some can be found near mid-ocean spreading ridges, such as beneath the Azores Islands of Portugal and Iceland. Many geological phenomena related to hotspots, far from plate boundaries, have been identified around the world. For example, the geysers of Yellowstone National Park exist because magma from the Yellowstone hotspot is close enough to the surface to generate the heat that drives geyser “eruptions.”

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Essential Science for Teachers: Earth and Space Science


Produced by Harvard-Smithsonian Center for Astrophysics. 2004.
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  • ISBN: 1-57680-742-8