Igneous Rocks

Columnar jointing on South Table Mountain in Golden

Igneous rock is formed from magma that has cooled and become solid. Molten rock is extraordinarily hot, sometimes exceeding 2,000 degrees Fahrenheit. If this molten and partially crystallized material (magma) crystallizes underground before it reaches the surface, the rock is “intrusive” or “plutonic”. If magma travels up through Earth’s crust and reaches the surface, the resulting rocks are “extrusive” or “volcanic”.

(Left):Columnar jointing on South Table Mountain in Golden, formed when basalt cools into a hexagonal structures.

Illustration of various forms of igneous intrusions. Original artwork by R. W. Krantz,

Magma that moves upward and cuts across pre-existing layers of rock forms what are generally known as plutons. The largest plutons are batholiths, such as the granitic rocks of Pike’s Peak, which are part of a 1,300-square-mile batholith. Smaller plutons take a variety of shapes, each with its own name, such as stocks, plugs, dikes and sills.

Idealized diagram of an ash flow eruption from a caldera.

Magma that reaches the surface forms a variety of volcanic landforms and deposits. Today, resistant volcanic flows cap mesas such as Grand Mesa, White River Plateau, Raton Mesa, and those near Basalt, Colorado. In the southwestern part of the state, ash-flow tuffs cover thousands of square miles. The tuff is created from the ash that is blown from the volcano to blanket the landscape.

The ash flow eruption creates a roughly circular depression called a caldera. There are at least nineteen calderas in Colorado, making the state one of the world’s best outdoor laboratories in which to study their formation.

The map below shows the distribution of Proterozoic granitic intrusions in Colorado. Those in green are a billion years old. Those in yellow were originally interpreted as being 1.4 billion years old. Those in fuchsia were originally interpreted as being 1.7 million years old. Recent dating of some of these rocks has revealed more precise and different ages. However, this generalized map still gives a good feel for the distribution and ages of the intrusions.

granitic rock of colorado