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IGNEOUS ROCKS
Igneous comes from the Latin word ignis, meaning “fire”. Igneous rock is any rock that
forms from cooled magma or lava.
Extrusive igneous rocks are formed from lava that erupted at the Earth’s
surface. Basalt, pumice and obsidian are common extrusive rocks. Intrusive igneous
rocks are formed when magma cools and hardens below the Earth’s surface. Granite
and gabbro are the most common intrusive rocks.
The texture of igneous rocks depends on the size of their mineral crystals. Rapidly cooling lava
forms fine-grained extrusive rock with small crystals, such as basalt. If lava cools fast enough to
trap air bubbles inside, it can produce a lightweight extrusive rock called pumice. If lava encounters
water, it cools instantly. Since there is no time for crystals to form, a type of volcanic glass called
obsidian is produced. Below the Earth’s surface, slow-cooling magma forms coarse-grained intrusive
rock with large crystals, like granite.
1. Formation of Igneous Rock
Fill in the blanks by looking at the diagram below.
Word Bank
Granite
Basalt
Pumice
Obsidian
Instantly
Very Quickly
Quickly
Slowly
Frothy lava is thrown into the air
Lava flows into a water and cools
and cools ____________, trapping
_____________. There is no time for
pockets of air inside. Rock Name =
crystals to form. Rock Name =
_______________
_______________
Magma below the surface cools
Lava outside the volcano cools
____________. The igneous rock
____________. The igneous rock
contains large crystals.
contains small crystals.
Rock Name = _______________
Rock Name = _______________