Igneous Rock FeaturesChapter 12, Section 3

Classification Igneous rocks are classified into two areas

depending on where thy formed. Intrusions

Underground igneous rock masses.

Extrusions Above ground igneous rock masses.

Extrusive Features Include volcanoes, flood basalts, lava flows,

etc.

Intrusive Features Form when magma cools and hardens

underground. These structures may later be exposed at the

surface through erosion. Include:

Batholiths, dikes, sills, and volcanic necks.

Intrusive Features Batholiths

Largest intrusive structures. Cover over 100 square kilometers.

May be many hundreds of kilometers in width and length and several kilometers thick.

Tend to form the cores of major mountain ranges.

Example: Boulder Batholith

Intrusive Features Stock

Similar to a batholith, but… Covers less then 100 square kilometers.

Intrusive Features Laccolith

Forms when magma flows between rock layers and spreads upward, pushing the overlying rock layers into an arc.

The floor of a laccolith is parallel to the rock layer beneath it.

Intrusive Features Dike

Forms when magma that has been forced into a crack that cuts across rock layers hardens.

Intrusive Features Sill

Forms when magma that has been forced into a crack running parallel to rock layers hardens.

Volcanic Neck Forms when the magma inside a volcano

cools and hardens and the overlaying softer layers are eroded.

Calderas Form when the top of a volcano collapses into

an emptied magma chamber. Example – Crater Lake – Formed after the

eruption of Mount Mazama around 7,000 years ago.