Rocks

Rocks  Rocks are solid earth materials formed from a mixture of minerals and sometimes other materials. Rocks are classified into one of three groups based on how they formed. These groups are igneous rocks, sedimentary rocks and metamorphic rocks.  When studying a rock sample, geologist observe the rock’s mineral composition, color, and texture.

Igneous Rocks  Igneous rocks are classified according to their origin, texture, and mineral composition.  Igneous rock form when magma or lava cools and becomes solid. Rocks that form from quickly cooled lava, such as rhyolite, are called extrusive or volcanic and have small mineral grains. Rocks that form from slowly cooled magma, such as granite, are called intrusive or plutonic and have large mineral grains.

Sedimentary Rock  Pieces of rocks, minerals, remains of living things and dissolved minerals that come out of water (such as lime) are all kinds of sediments.  When sediment becomes solid material, it makes sedimentary rock.  Most sedimentary rocks are formed through a series of process: erosion, deposition, compaction, and cementation.

Sedimentary Rock  Erosion-Particles carried away from their source by water or wind.  Deposition-Particles deposited as loosely packed sediment.  Compaction-Particles squeezed together under great pressure.  Cementation-Particles glued together as mineral solutions harden.

Sedimentary Rock  Most sedimentary rocks are made up of broken pieces of other rocks. A clastic rock is a sedimentary rock that forms when rock fragments are squeezed together. Example:Shale,sandstone,limestone.  When minerals that are dissolved in a solution crystallize, Chemical rock forms. Example: rock salt  Not all sedimentary rocks are made from particles of other rocks. Organic rock forms where the remains of plants and animals are deposited in thick layers.

Metamorphic Rocks  Over time, heat and pressure inside Earth squeeze and melt existing rocks. This process changes the grain size and even the minerals that make up those rocks, forming a new type of rock, called metamorphic rock.  Great heat and pressure can change one type of rock into another. Example: Granite  Gneiss, Sandstone  Quartzite, Shale  Slate

Uses of Metamorphic Rock  People have used sedimentary rocks throughout history for many different purposes, including building materials and tools.  Certain metamorphic rocks are important materials for building and sculpture. Marble and slate are the most useful.

The Rock Cycle  The rock cycle is a series of processes on Earth’s surface and in the crust and mantle that slowly change rocks from one kind to another.  Melting- Hot temperatures deep inside Earth melt rocks, forming magma  Cooling and Hardening  Magma that rises from deep inside Earth cools as it reaches the surface, hardening into rock. Magma also cools beneath the surface.  Weathering and Erosion  Weathering breaks apart existing rocks, forming sediment. Erosion moves sediment to new locations. Compacting and Cementing  Pressure compacts sediments together. Water between particles evaporates, leaving minerals that cement sediments. Heat and Pressure  Heat and pressure inside Earth melt and squeeze minerals in rocks. This changes the minerals themselves or their grain size.