Minerals and the Rock Cycle

The Rock Cycle

Every rock contains clues about its origin Every rock contains clues about its origin. Rocks are divided into types based on the way they formed.

Igneous Rocks Remember that new crust material forms from molten material beneath the surface (magma). When magma reaches the surface it is called lava.

When lava or magma cools and hardens, it forms igneous rock When lava or magma cools and hardens, it forms igneous rock. The term igneous means “from fire”, so the term describes how igneous rock forms.

Magma that cools and hardens inside the earth forms large crystals within the rock because it cools slowly. These are called intrusive igneous rocks.

Lava that cools and hardens on the surface of the earth forms extrusive igneous rock. Lava at the surface forms smaller crystals since it cools faster. These are called extrusive igneous rocks.

Metamorphic Rock Metamorphic means “change form”.

Existing rocks that undergo great heat and pressure change size, shape, texture and even mineral content due to chemical reactions.

Force from pressure can cause the rock to warm, bend and fold Force from pressure can cause the rock to warm, bend and fold. The resulting metamorphic rock will have a banded appearance with flattened crystals. This is called foliation. Not all metamorphic rocks are foliated. Nonfoliated metamorphic rock does not have the banded appearance.

Sedimentary Rocks Over time, rocks are continually broken down into smaller pieces. The breaking down of rock at the earth’s surface is called weathering. The pieces or particles of rock are called sediment. Rocks undergo chemical weathering (ex: acid rain) or mechanical weathering (ex: wind).

Sediment is transported from one place to another by gravity, wind, water, and ice. Moving sediment from one place to another is called either erosion or mass wasting. Eventually, sediment is deposited into lakes, river valleys and oceans.

Sediment may eventually be converted into rock Sediment may eventually be converted into rock. Rock formed from sediment is called sedimentary rock.

There are two main ways that sediment can become sedimentary rock There are two main ways that sediment can become sedimentary rock. One way is by compaction. Compacted sedimentary rock forms when the weight of overlying sediment presses the bottom sediments together over time.

The second method is called cementation The second method is called cementation. Sediment in water can be cemented together by materials carried in the water. The materials move the sediment and over time, glues the particles together.

Sedimentary rocks are classified into 3 types: clastic, chemical and organic.

Clastic sedimentary rocks are formed by the cementing and compacting of sediments.

Chemical sedimentary rocks are formed when a body of water evaporates and leaves behind minerals that were dissolved in the water.

Organic sedimentary rocks form from material left behind by decaying plants or organisms.

Rocks are continuously changed from one type of rock to another.