Contact Metamorphism

The lava flow exposed in this roadcut near Alturas, California, has baked the mud beneath it into brick-red shale, a case of contact metamorphism. This form of metamorphism is caused by the heat of magma and is restricted to a narrow zone, unlike regional metamorphism. In this case the metamorphism is mild, but magma intrusions deep underground can cause greater changes as they advance, forming tough, fine-grained hornfels in the country rock at their margins. See a picture of that below, and learn more in About Metamorphic Rocks.

The lava here turned the sediments beneath it red by mobilizing steam (see another example inside a cinder cone). And in the bottom few centimeters of the lava flow, the rocks are full of bubbles that probably also formed from steam. This location may have been the floor of a shallow lake when the lava came. There is no evidence at this roadcut, such as fossil root casts or mudcracks, to show that it was dry land with a soil developed on it. See the rock hammer in the center for scale.

_{Photo by Marli Miller, University of Oregon. Used by permission. (fair use policy)}

This photo shows the Purcell Sill, a thick body of diorite that was injected between layers of limestone of the Helena Formation in northern Montana. The great heat of the intrusion baked the limestone above and below it into white marble.

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