Volcanic Landforms: Intrusive Igneous

Wrangell St Elias National Park and Preserve
Wrangell St Elias National Park and Preserve

NPS Photo

Intrusive or plutonic igneous rocks are formed from magma that has slowly cooled deep beneath the Earth’s surface. The mass of cooling magma is called a pluton, and the rock around is known as country rock. Slow cooling over thousands to millions of years allows large visible crystals to form. Common igneous rock types include granite, gabbro, and diorite. Large plutons can form along convergent tectonic plate boundaries.

Plutonic rocks are hard and erode slowly, so in many places they have become exposed at the surface after the rocks above erode away over millions of years. In the U.S. massive granite landforms can be found in over 30 U.S. states, including many that are iconic National Park features. El Capitan in Yosemite National Park is the largest granite monolith in the world.
Devils Tower is an intrusive igneous formation
Devils Tower National Monument (Wyoming).

NPSphoto

  • Batholiths- Plutons that have been exhumed (exposed on the surface through uplift and erosion).
  • Sills and Dikes are tabular bodies of magma that intrude into a fracture. Sills follow bedding planes, whereas dikes cross-cut beds.
  • Monadnocks, also called Inselbergs, are isolated rock hills standing in a level plain. These are often the result of softer sedimentary rocks eroding around a hard intrusive igneous body. Devils Tower National Monument (Wyoming) is an example of a monadnock.
  • Lava tubes- Former passages of flowing lava. As the outside of the lava cools, the inside continues to flow and eventually leaves the tube hollow.

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