There are three main types of rocks: sedimentary, igneous, and metamorphic. The document describes how to identify samples of different rocks and determines their classification. Sedimentary rocks form from compressed sediments and often contain layers and fossils. Igneous rocks usually form from volcanic activity and are made of cooled lava with interlocking crystals that lack layers or fossils. Metamorphic rocks were previously sedimentary or igneous rocks that were altered by heat and pressure and may show signs of recrystallization.

1. Rocks
There are three different types of rocks. You should know all about
sedimentary, igneous and metamorphic rocks, so have a look at
these pictures and see if you can identify each specimen.
Classify these rocks. Try and work out whether they are
sedimentary, igneous or metamorphic.
Think about how they were formed.
Basalt
Limestone
Sheared gneiss
Granite
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2. Sandstone
Marble
Pitchstone
Chalk
Slate
Lava
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3. Conglomerate
Garnet amphibolite
Gabbro
Marble
Granite
Gritstone
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4. Faulted gneiss
Clay (top), mudstone (bottom)
Identifying rocks
If you are stuck and cannot decide whether a rock is sedimentary,
igneous or metamorphic here are some clues.
Sedimentary rocks are made up of grains of sediment that have
been weathered from a pre-existing rock or mineral. The grains can
be big or small, they are often rounded, and are cemented together
in a matrix. Sedimentary rocks may be layered and may also contain
fossils.
Igneous rocks often form from volcanic activity. They are made up
of interlocking crystals that form when molten rock cools. The
crystals can be big (if the rock formed underground), or small (if it
formed from lava that was extruded from a volcano). Igneous rocks
rarely contain fossils and they are not layered, but may contain gas
bubbles.
Metamorphic rocks form when a pre-existing rock is subjected to
heat and/or pressure. They may show evidence of re-crystallisation.
Copyright © 2005, Oxford University Museum of Natural History