The rock that we see, except for meteorites that come from space, is formed from magma. This means that the rock is “Igneous”. The magma can be contained within the Earth’s crust and protected from the atmosphere. This is called “Igneous Intrusive”. The magma may have escaped or vented to the surface of the crust and exposed to the atmosphere. This is called “Igneous Extrusive”.
After the magma cools, intrusive magma cools far more slowly. There is less or none of the chemical interaction, venting of gases, and weathering that goes on in the atmosphere. Crystals are allowed to grow, often until they are visible to the naked eye.
Extrusive magma cools far more quickly, gasses vent off quickly, and weathering begins immediately. Crystals are not allowed to grow until they are visible to the naked eye, creating such rocks as volcanic glass and obsidian.
On a huge scale, there are landforms that are either igneous intrusive or igneous extrusive. The land form may have been intrusive, then later exposed as the surface rock and smaller bits were removed by atmospheric weathering or lifted up in a process that is called isostatic uplift.
Or, the land form may have been extrusive and subject to weathering from the time that the magma was extruded onto the surface. This may have happened before the atmosphere was even created through volcanic eruptions! The atmosphere, once created, has many ways of changing magma and rock from one thing into another.
Igneous Intrusive
The igneous intrusive landforms include “Batholithic” formations. These are huge granitic formations of 100 square kilometers or more, where the magma cooled slowly, forming large crystals, and were exposed through weathering or uplife. The major batholitic formations are the Sierra Nevada Batholith of California, Enchanted Rock Batholith, Texas, and Mt. Desert Island, Maine.
Stone Mountain, Georgiais called a “stock”. It is like a batholith, but is less than 100 square kilometers.
Volcanic necks or throats are crystallized magma that formed in the interior of extinct volcanoes, followed by weathering that removed the rest of the volcano. Devil’s Tower, Wyoming, is an example of a volcanic neck.
There are also Dikes (or fissures), concordant plutons, (all of the above examples are discordant plutons), sills and lacoliths to round out the inventory of igneous intrusive landforms.
Igneous extrusive
Volcanoes are the most obvious igneous extrusive land form. The three main types are shield, cinder cone and composite cone.
Volcanic Depressionsare another type of igneous extrusive land form. These can be in the form of craters that form over a volcanic vent or calderas that are larger craters and basins caused by extreme pyroclastic action from violent eruptions.
Basalt flowsor plateaus are extruded from dikes (or fissures) in floodlike ways. These formations may have cinder cones across the landscape.
Ignimbrite sheets and plateaus happen when two or more violent pyroclastic flows resulted from caldera eruptions. Katmai is an example of an ignibrite sheet at Yellowstone Plateau.
The incredible variety of exposed intrusive and extrusive igneous landforms that are volcanic magma in origin, then weathered at various times in the atmosphere serve as both permanent and evolving features of the Earth.
“Volcanism And Volcanic Landforms”, Salem State University
Geomorphology 33 J Bret Bennington
