- cinder cone
- composite volcano
- shield volcano
A volcano is a vent through which molten rock and gas escape from a magma chamber. Volcanoes differ in many features such as height, shape, and slope steepness. Some volcanoes are tall cones and others are just cracks in the ground (Figure below). As you might expect, the shape of a volcano is related to the composition of its magma.
Mount St. Helens was a beautiful, classic, cone-shaped volcano. The volcano
Composite volcanoes are made of felsic to intermediate rock. The viscosity of the lava means that eruptions at these volcanoes are often explosive (Figure below).
Mt. Fuji, the highest mountain in Japan, is a dormant composite volcano.
The viscous lava cannot travel far down the sides of the volcano before it solidifies, which creates the steep slopes of a composite volcano. Viscosity also causes some eruptions to explode as ash and small rocks. The volcano is constructed layer by layer, as ash and lava solidify, one upon the other (Figure below). The result is the classic cone shape of composite volcanoes.
A cross section of a composite volcano reveals alternating layers of rock and ash: (1) magma chamber, (2) bedrock, (3) pipe, (4) ash layers, (5) lava layers, (6) lava flow, (7) vent, (8) lava, (9) ash cloud. Frequently there is a large crater at the top from the last eruption.
Shield volcanoes get their name from their shape. Although shield volcanoes are not steep, they may be very large. Shield volcanoes are common at spreading centers or intraplate hot spots (Figure below).
Mauna Loa Volcano in Hawaii is the largest shield volcano on Earth with a diameter of more than 112 kilometers (70 miles). The volcano forms a significant part of the island of Hawaii.
The lava that creates shield volcanoes is fluid and flows easily. The spreading lava creates the shield shape. Shield volcanoes are built by many layers over time and the layers are usually of very similar composition. The low viscosity also means that shield eruptions are non-explosive.
This Volcanoes 101 video from National Geographic discusses where volcanoes are found and what their properties come from (3e): http://www.youtube.com/watch?feature=player_profilepage&v=uZp1dNybgfc(3:05).
Cinder cones are the most common type of volcano. A cinder cone has a cone shape, but is much smaller than a composite volcano. Cinder cones rarely reach 300 meters in height but they have steep sides. Cinder cones grow rapidly, usually from a single eruption cycle (Figure below). Cinder cones are composed of small fragments of rock, such as pumice, piled on top of one another. The rock shoots up in the air and doesn’t fall far from the vent. The exact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder cones usually have a crater at the summit.
In 1943, a Mexican farmer first witnessed a cinder cone erupting in his field. In a year, Paricut
Cinder cones are often found near larger volcanoes (Figure below).
This Landsat image shows the topography of San Francisco Mountain, an extinct volcano, with many cinder cones near it in northern Arizona. Sunset crater is a cinder cone that erupted about 1,000 years ago.
Supervolcano eruptions are extremely rare in Earth history. It’s a good thing because they are unimaginably large. A supervolcano must erupt more than 1,000 cubic km (240 cubic miles) of material, compared with 1.2 km3 for Mount St. Helens or 25 km3 for Mount Pinatubo, a large eruption in the Philippines in 1991. Not surprisingly, supervolcanoes are the most dangerous type of volcano.
Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated. However, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion. This creates a huge hole or caldera into which the surface collapses (Figure below).
The caldera at Santorini in Greece is so large that it can only be seen by satellite.
The largest supervolcano in North America is beneath Yellowstone National Park in Wyoming. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2.1 million, 1.3 million, and 640,000 years ago. Yellowstone has produced many smaller (but still enormous) eruptions more recently (Figure below). Fortunately, current activity at Yellowstone is limited to the region’s famous geysers.
The Yellowstone hotspot has produced enormous felsic eruptions. The Yellowstone caldera collapsed in the most recent super eruption.
Long Valley Caldera, south of Mono Lake in California, is the second largest supervolcano in North America (Figure below). Long Valley had an extremely hot and explosive rhyolite about 700,000 years ago. An earthquake swarm in 1980 alerted geologists to the possibility of a future eruption, but the quakes have since calmed down.
The hot water that gives Hot Creek, California, its name is heated by hot rock below Long Valley Caldera.
- An interactive image of the geological features of Long Valley Caldera is available here:
A supervolcano could change life on Earth as we know it. Ash could block sunlight so much that photosynthesis would be reduced and global temperatures would plummet. Volcanic eruptions could have contributed to some of the mass extinctions in our planet’s history. No one knows when the next super eruption will be.
Interesting volcano videos are seen on National Geographic Videos, Environment Video, Natural Disasters, Earthquakes: http://video.nationalgeographic.com/video/player/environment/. One interesting one is “Mammoth Mountain,” which explores Hot Creek and the volcanic area it is a part of in California.
- Composite, shield, cinder cones, and supervolcanoes are the main types of volcanoes.
- Composite volcanoes are tall, steep cones that produce explosive eruptions.
- Shield volcanoes form very large, gently sloped mounds from effusive eruptions.
- Cinder cones are the smallest volcanoes and result from accumulation of many small fragments of ejected material.
- An explosive eruption may create a caldera, a large hole into which the mountain collapses.
- Supervolcano eruptions are devastating but extremely rare in Earth history.
- Rank, in order, the four types of volcanoes from smallest to largest in diameter.
- What factor best determines what type of volcano will form in a given area?
- Which type of volcano is most common?
- Why do pahoehoe and a’a lava erupt from shield volcanoes? Why don't they erupt from composite volcanoes?
- Why are cinder cones short-lived?
- If supervolcanoes are so big, why did it take so long for scientists to discover them?
Points to Consider
- Composite volcanoes and volcanic cones usually have craters on the top. Why are the craters sometimes U- or horseshoe-shaped?
- Think about plate boundaries again. What type of volcanoes do you think are found at convergent, divergent, and transform boundaries? How about at intraplate sites?
- Some people have theorized that if a huge asteroid hits the Earth, the results would be catastrophic. How might an asteroid impact and a supervolcano eruption be similar?
Volcanic Landforms and Geothermal Activity
- hot spring
- lava dome
- lava plateau
Volcanoes are associated with many types of landforms. The landforms vary with the composition of the magma that created them. Hot springs and geysers are also examples of surface features related to volcanic activity.
Landforms from Lava
Volcanoes and Vents
The most obvious landforms created by lava are volcanoes, most commonly as cinder cones, composite volcanoes, and shield volcanoes. Eruptions also take place through fissures (Figure below). The eruptions that created the entire ocean floor are essentially fissure eruptions.
A fissure eruption on Mauna Loa in Hawaii travels toward Mauna Kea on the Big Island.
When lava is viscous, it is flows slowly. If there is not enough magma or enough pressure to create an explosive eruption, the magma may form a lava dome. Because it is so thick, the lava does not flow far from the vent. (Figure below).
Lava domes are large, round landforms created by thick lava that does not travel far from the vent.
Lava flows often make mounds right in the middle of craters at the top of volcanoes, as seen in the Figurebelow.
Lava domes may form in the crater of composite volcanoes as at Mount St. Helens
A lava plateau forms when large amounts of fluid lava flows over an extensive area (Figure below). When the lava solidifies, it creates a large, flat surface of igneous rock.
Layer upon layer of basalt have created the Columbia Plateau, which covers more than 161,000 square kilometers (63,000 square miles) in Washington, Oregon, and Idaho.
Lava creates new land as it solidifies on the coast or emerges from beneath the water (Figure below).
Lava flowing into the sea creates new land in Hawaii.
Over time the eruptions can create whole islands. The Hawaiian Islands are formed from shield volcano eruptions that have grown over the last 5 million years (Figure below).
A 3-D computer generated view of the Big Island of Hawaii with its five volcanoes.
Landforms from Magma
Magma intrusions can create landforms. Shiprock in New Mexico is the neck of an old volcano that has eroded away (Figure below).
The aptly named Shiprock in New Mexico.
Hot Springs and Geysers
Water sometimes comes into contact with hot rock. The water may emerge at the surface as either a hot spring or a geyser.
Water heated below ground that rises through a crack to the surface creates a hot spring (Figure below). The water in hot springs may reach temperatures in the hundreds of degrees Celsius beneath the surface, although most hot springs are much cooler.
Even some animals enjoy relaxing in nature's hot tubs.
Geysers are also created by water that is heated beneath the Earth’s surface, but geysers do not bubble to the surface -- they erupt. When water is both superheated by magma and flows through a narrow passageway underground, the environment is ideal for a geyser. The passageway traps the heated water underground, so that heat and pressure can build. Eventually, the pressure grows so great that the superheated water bursts out onto the surface to create a geyser. Figure below.
Conditions are right for the formation of geysers in only a few places on Earth. Of the roughly 1,000 geysers worldwide and about half are found in the United States.
Castle Geyser is one of the many geysers at Yellowstone National Park. Castle erupts regularly, but not as frequently or predictably as Old Faithful.
- Viscous lava can produce lava domes along a fissure or within a volcano.
- Lava plateaus form from large lava flows that spread out over large areas.
- Many islands are built by or are volcanoes.
- Igneous intrusions associated with volcanoes may create volcanic landforms.
- When magma heats groundwater, it can reach the surface as hot springs or geysers.
- What are four different landforms created by lava?
- What is the major difference between hot springs and geysers?
- The geyser called Old Faithful has been erupting for perhaps hundreds of years. One day, it could stop. Why might geysers completely stop erupting?
- After earthquakes, hot springs sometimes stop bubbling, and new hot springs form. Why might this be?
Points to Consider
- What might the Earth look like if there were no tectonic plates? Are there any planets or satellites (moons) that may not have tectonic plates? How is their surface different from that of the Earth?
- The largest volcano in the solar system is Olympus Mons on Mars. How could this volcano have formed?
- What kind of land formations are the result of volcanic activity? Are all of these created by extrusive igneous rocks?
- How are hydrothermal vents at mid-ocean ridges like the geysers of Yellowstone?
Opening image courtesy of Robert Simmon and NASA's Earth Observatory,http://earthobservatory.nasa.gov/IOTD/view.php?id=43999, and is in the public domain.