Igneous Rock Features Chapter 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.