Volcanic Landforms: Extrusive Igneous

Mt.Drum Volcanic Crater Wrangell-St.Elias National Park
Mt.Drum Volcanic Crater in Wrangell-St.Elias National Park and Preserve

NPS Photo/ Betts and Hannan

Volcanic Landforms: Extrusive Igneous

Extrusive igneous landforms are the result of magma coming from deep within the earth to the surface, where it cools as lava. This can happen explosively or slowly, depending on the chemical composition of the lava and whether there is an easy path for it to take to the surface. If there is not a pathway, pressure builds up over time (like a shaken soda) until the magma forcibly explodes outward.
Formation of a Caldera
Diagram of the formation of a caldera

Trista L. Thornberry-Ehrlich, Colorado State University, after Stearns H.T. 1946 and Keating, B.H. 1992.

Volcanic processes are constantly changing the Earth. Eruptions can create new islands, build and destroy mountains, and alter landscapes. Active, dormant, and ancient remnants of eruptions are all contained within our National Parks.

Volcanic Features

Volcanic processes create many features we see when visiting the National Parks such as:

Types of Volcanoes

  • Shield volcanoes- Volcanoes ranging in size from small to truly massive, created by steady, non-violent outpouring of lava. Mauna Loa and Mauna Kea of Hawaii are shield volcanoes rising nearly 9 km (5.6 mi) fromt the seafloor.

  • Stratovolcanoes- Also known as composite volcanoes, stratovolcanoes erupt both as flows and violently. Mt. St. Helens and Mt. Rainier are stratovolcanoes.

  • Cinder cones- Short, steep volcanoes associated with limited eruptive events.

  • Lava Domes
  • Maar-Diatreme Volcanoes

Where Volcanoes Form

There are three places that volcanism commonly occurs. These are at hot spots, spreading centers and fault zones, and subduction zones.
Spreading centers are places where tectonic plates are diverging (moving away from one another). As the plates separate, a pathway is created for magma to move toward the surface. Spreading centers can extend into continental plates, such as The Great Rift Valley in East Africa. Magma produced at spreading centers is mafic. Therefore, much of the oceanic crust is made of basalt, a mafic rock.

Some of the magma produced at spreading centers erupts as lava flows and pyroclastic material, but most of it cools internally below Earth's crust. Fault zones are areas where the crust is cracking, usually due to the movement of the tectonic plates. Earthquakes and volcanism are common around fault zones.

There are no areas of active sea-floor spreading found within a national park of the United States. The country of Iceland, however, continues to grow due to the phenomenon. There are a few national parks that have evidence of past volcanism in rift valleys.

Isle Royale National Park, Michigan [Geodiversity Atlas] [Park Home]
Keweenaw National Historical Park, Michigan [Geodiversity Atlas] [Park Home]
Petroglyph National Monument, New Mexico [Geodiversity Atlas] [Park Home]
Subduction occurs when two tectonic plates converge, and the denser of the two plates is pushed beneath the other plate. Volcanism will be present at the leading edge of the top plate. When a continental plate and oceanic plate converge, the denser oceanic plate is subducted.

The descending plate is heated by pressure and Earth's geothermal gradient. This leads to the formation of magma. The magma rises to the surface, and a belt of composite volcanoes forms. Subduction commonly generates felsic and intermediate magmas. There is not much volcanic activity at the convergence of two continental plates because continental crust is typically not dense enough to be subducted.

The volcanic activity on the Aleutian Islands and the Alaskan Peninsula is caused by subduction. The denser Pacific Plate is being subducted below the North American Plate. Visit volcanic parks in Alaska that are associated with subduction zones.

Aniakchak National Monument and Preserve
Lake Clark National Park and Preserve
Katmai National Park and Preserve
Wrangell-St. Elias National Park and Preserve
Volcanic activity can occur in areas that are in the interior of a plate, far away from spreading centers or subduction zones. Rising magma somewhere inside the borders of a plate can create a local "hot spot." There are between 50 and 100 hot spots identified around the world, and they occur in both continental and oceanic plates.

Hot spots originate deep inside Earth, so they remain stationary while the plates above them move. That is how island chains like the Hawaiian Islands are formed. The magma associated with hot spots is mafic, so shield volcanoes are commonly formed. Hot spots are also associated with many geothermal features.

The Hawaiian Islands were formed when the Pacific Plate passed over a hot spot. Yellowstone National Park is also a result of hot spot volcanism.

Hawaii Volcanoes National Park, Hawaii
Haleakala National Park, Hawaii
Yellowstone National Park, Wyoming, Idaho, and Montana

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