21.3 Rocks and the Rock Cycle Rocks are classified into three major groups based on how they formed Igneous Rocks Can be divided into two categories based on where they form

A. Intrusive Rocks – igneous rocks that form underground from magma Cool slowly allowing crystals to grow large giving it a coarse-grain texture

B. Extrusive Rocks – igneous rocks that form at the surface from lava Cool quickly resulting in the crystals to be small giving it a fine grain texture

Igneous Rock Textures

Three Main Groups of Sedimentary Rocks: 1. Clastic 2. Chemical II. Sedimentary Rock – rock that forms over time as sediment is squeezed and cemented together Sediment – small, solid pieces of material that comes from rocks or living organisms Sediment is deposited in layers by running water or wind Sediment is cemented together by pressure from overlying layers Three Main Groups of Sedimentary Rocks: 1. Clastic 2. Chemical 3. Organic

Clastic Rock – sedimentary rocks that form from the broken fragments of other rocks Clastic Rocks are classified based on the size of their fragments Example: Conglomerate – rock fragments that consist of gravel and pebbles

Some clastic rocks are made of smaller particles (sandstone)

2. Chemical Rocks – rocks that form when minerals precipitate out of solution Example: when ocean water evaporates (halite)

Organic Rocks– rocks that form as a result of organic processes Shells of marine animals like corals, clams, mussels sink to the ocean floor when they die. Example: Limestone and calcite (chalk)

III. Metamorphic Rocks – rock that have been changed by temperature, pressure, or chemical reactions. Most metamorphic rocks form under high temperatures and pressures deep underground. (slate and schist– shown below)

Metamorphic Rocks Foliation: a layering of minerals in a rock due to intense pressure squeezing the crystals together.

Metamorphic Rock pictures Metamorphic Rocks like gneiss show banding of light and dark minerals.

Metamorphic Rocks and their Parent Rocks Examples: Slate: parent is shale Gneiss: parent is granite/schist Quartzite: parent is sandstone Marble: parent is limestone

IV. Rock Cycle Rock Cycle – series of processes in which rocks continuously change from one type to another.