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Notes on Classifying Rocks


Rock: a mixture of minerals


Rocks are classified in two ways:



By composition:  what minerals are in the rock


·       The minerals can determine the rock’s color

·       Granite (usually light-colored – has high silica content)

·       Basalt (dark colored – has low silica content)



     By texture: how a rock’s surface feels


·       Texture is described three ways

·       By grain size:

Fine-grained: small grains or crystals – smooth

Coarse-grained: large grains or crystals - rough

·       By grain pattern:

Flat layers



Bands – like gneiss

Rocks form in three ways:

Igneous Rocks:

·       Form from lava or magma cooling

·       Form from two processes:

-melting in the mantle

-cooling on or near Earth’s surface


Sedimentary Rocks: rocks form when particles of other rocks or the remains of plants and animals are pressed and cemented together. – they form in layers

·       Form on Earth’s surface

·       Form from three-four processes

-erosion or weathering


-compacting and cementing


Metamorphic Rocks: These rocks are formed deep in the mantle by

·       Great heat

·       Great pressure