Volcanic Landforms
Different geological processes produce different kinds of landforms. Eruptions through central vents produce common landforms known as volcanoes. Volcanoes build up the surface and erosion wears them down forming volcanic rocks and exposing batholiths such as those in Yosemite National Park. Volcanic activities take place underground as magma cools forming igneous rocks. Some magma squeezes into horizontal cracks forming sills or into vertical cracks forming dikes. When sills push rock layers upward, rock domes called laccoliths are formed. (Synder, 405).
Note: Most magma never reaches the surface of the Earth, so most of the igneous activity takes place underground.
Some of the more familiar volcanoes are as follows: Shield Volcanoes, Composite Volcanoes, Cinder Cones, and Lava Domes.Craters and calderas are also recognizable volcanoes.
Shield volcanoes are usually huge. Some are as large as 190 km wide. They are formed mainly of basaltic lavas. They have long duration of activity, ten thousands of years. They have gentle sloping sides of 2 to 10 degrees and usually erupt non-violently. Shield volcanoes are typical of volcanoes found in Hawaii, e.g., Mauna Kea and also Olympus Mars.
Cinder cones are relatively small, about a km in width and have steep sides up to 30 degrees. They are short lived, usually a single event.
Composite volcanoes are the most recognized types of volcanoes. They are made mainly of layers of alternating pyrocalstic deposits and andesitic lava flows. They slope intermediate in steepness and are quite large 10 to 15 km wide. Composite Volcanoes are highly explosive and have intermittent eruptions over a long time span, thousands of years. Example, Mount St. Helens, Mount Rainer, and Mount Shasta all part of the Cascade Range.
Lava domes are associated with violent eruptions. They are small, steep sided and are hundreds of meters wide. They are formed from viscous felsic lavas, i.e., Mono Craters, California.
Craters and calderas are also volcanic landforms. Calderas are produced when volcanoes collapse into partially drained magma chambers, i.e., Crater Lake, Oregon and Valles Caldera, New Mexico. They are steep-walled depressions and may have younger domes within them. They can be several kilometers wide.
Shapes of Volcanoes
Shapes of volcanoes depend on the type of eruptions (quiet or explosive) and whether the lava flows are basaltic or granitic. Shield volcanoes erupt quietly and spread out basaltic lava in flat layers that form broad volcanoes with gently sloping sides. Examples of shield volcanoes are the Hawaiian Islands and Iceland, located in rift zones. For example, in the Iceland volcanic eruptions, the eruptive force was moderate to low, the silica content was low, and the water content was high. Cinder cone volcanoes are explosive eruptions, throwing lava, in the form of volcanic ash, cinders, gas, and large rocks, high into the air. The magma content is high in silica and water. The lava hardens into layers of tephra. Composite volcanoes are formed between periods of violent eruptions (gas, ash, formation of tephra) and quieter eruptions (spewing lava over tephra). The composite volcano is made up of alternating layers of tephra and lava. They are formed mostly at convergent plate boundaries such as in the formation of Mount Rainer.
Occurrences of Volcanoes
Volcanoes occur at three places directly related to plate tectonics: divergent plate boundaries, convergent plate boundaries, and at hot spots.
Divergent Plates
Iceland, a large island in the North Atlantic Ocean, has volcanic activity because it sits on top of the Mid-Atlantic Ridge which is a divergent plate boundary. Two divergent plates move apart forming deep cracks called rifts. Magma flows through these rifts as lava and is cooled by the seawater. This process continues over long periods of time causing the lava to build up from the sea floor often rising to form volcanic islands such as Iceland and the historical Thera.
Convergent Plate Boundaries
All volcanoes are not islands. Mount Saint Helens is one of several volcanoes that make up the Cascade Mountain Range in the Northwestern United States. These volcanic mountains in the Cascade Mountain Range formed because of convergent plate boundary between the Juan de Fuca Plate and the North American Plate. Magma rises from the subduction zones of convergent plate boundaries, as one plate pushes underneath the other, forming the volcanoes of the Cascades. All volcanoes in the Pacific Ring of Fire are formed at the convergent boundary of where the Pacific Plate collides with other plates. There are numerous earthquakes and volcanoes located around the Pacific Plate of which Mount Saint Helens, Mount Rainer, and Mount Shasta are among them.
Hot Spots
The Hawaiian Islands, like those of Iceland, are volcanic islands. The Hawaiian Islands are formed over hot spots. Hot spots are areas in the mantle that are hotter than other areas. Hot spots melt rocks that rise toward the Earth crust as magma. As the Pacific Plate moved over hot spots, the Hawaiian Islands were formed. Most of this magma rose above sea level forming islands such as Maui, Molokai, and Kauai.
Volcanic Necks
A volcanic neck is formed when a volcano stops erupting. The magma hardens inside the vent forming solid igneous rocks. These rocks are harder than the volcano crater. Over time, erosion wears away the softer crater first, leaving behind the solid core that is called a volcanic neck. Volcanic necks are common in the Southeast United States. Ship Rock is a volcanic neck located in New Mexico.
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