The Rock Cycle

Learning Objectives

Name the three main types of rocks: igneous, sedimentary, and metamorphic.

Describe how each type of rock is formed.

Explain that any type of rock can be transformed into any other type of rock through the rock cycle.

Earth's Great Recycling Program

A rock is a naturally occurring solid mixture of one or more minerals. The rocks you see around you might seem permanent, but they are constantly changing in a slow process called the rock cycle. The rock cycle describes how rocks are formed, broken down, and reformed over millions of years. It's like Earth's own recycling system.

There are three main classes of rocks: igneous, sedimentary, and metamorphic.

Igneous Rocks: Born from Fire

Igneous rocks (from the Latin ignis, meaning 'fire') are formed from the cooling and hardening of molten rock. Molten rock below the Earth's surface is called magma, and molten rock that has erupted onto the surface is called lava.

Intrusive Igneous Rock: Forms when magma cools slowly beneath the Earth's surface. The slow cooling allows large crystals to form. A common example is granite.

Extrusive Igneous Rock: Forms when lava cools quickly on the Earth's surface. The rapid cooling results in very small crystals or no crystals at all (a glassy texture). Common examples include basalt and obsidian.

Sedimentary Rocks: Made of Pieces

Sedimentary rocks are formed from sediments—small pieces of other rocks, minerals, or organic matter.

The process has several steps:

1.Weathering & Erosion: Existing rocks are broken down by wind, water, and ice into smaller pieces. These pieces are then carried away by erosion.

2.Deposition: The sediments are eventually dropped, or deposited, in layers, often at the bottom of a lake or ocean.

3.Compaction & Cementation: Over a very long time, the weight of the layers above compacts (squeezes) the lower layers. Water seeping through the layers leaves behind minerals that act like a glue, cementing the sediments together. This process is called lithification.

Fossils are almost exclusively found in sedimentary rocks. Examples include sandstone, shale, and limestone.

Metamorphic Rocks: Changed by Heat and Pressure

Metamorphic rocks are rocks that have been changed by intense heat and/or pressure. The original rock, called the protolith, can be an igneous, sedimentary, or even another metamorphic rock. The heat and pressure don't melt the rock, but they do change its texture and mineral composition.

Foliated Metamorphic Rock: The pressure aligns the minerals into layers or bands, giving the rock a striped look. Gneiss (formed from granite) and slate (formed from shale) are examples.

Non-Foliated Metamorphic Rock: The minerals are not arranged in bands. Marble (formed from limestone) and quartzite (formed from sandstone) are examples.

The Cycle Continues

The key idea of the rock cycle is that any rock can become any other rock. An igneous rock can be weathered into sediment to become a sedimentary rock. A sedimentary rock can be buried deep and changed by heat and pressure into a metamorphic rock. A metamorphic rock can melt into magma and cool to become an igneous rock. It's a continuous, dynamic process.

Key Terms

**Igneous Rock: A type of rock that forms from the cooling of molten rock (magma or lava).~|~Sedimentary Rock: A type of rock that forms when particles from other rocks or the remains of plants and animals are pressed and cemented together.~|~Metamorphic Rock: A type of rock that forms from an existing rock that is changed by heat, pressure, or chemical reactions.~|~Rock Cycle: A series of processes on the surface and inside Earth that slowly changes rocks from one kind to another.~|~Magma: Molten rock beneath the Earth's surface.~|~Lava: Molten rock that has erupted onto the Earth's surface.~|~Lithification: The process that converts sediments into solid rock by compaction or cementation.