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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.