Three Types of Rocks

How are igneous rocks classified? Igneous rocks (ig nee us) are formed from

magma or lava. Igneous means fire-formed. Igneous rocks are classified according to

their origin, texture, and mineral composition.

The origin of a rock is where the rock was formed. Extrusive Rocks: igneous rocks which formed

from lava which erupted on the earth’s surface. Ex. Basalt: forms most of the crust which

includes the crust on the ocean floor.

Intrusive Rocks: igneous rocks that formed when magma hardened beneath the earth’s surface.

Ex. Granite: the most abundant intrusive rock in the part of the crust that makes up the continents. Granite forms the core of many mountain ranges

Texture The texture of an igneous rock depends on

the size and shape of its mineral crystals. The texture may be fine grained, coarse-

grained, glassy, or porphyritic. Rapid cooling lava forms fine-grained

igneous rocks with small crystals. Slow cooling magma forms coarse-grained

rock with large crystals.

A rock with large crystals scattered on a background of much smaller crystals has a porphyritic texture (pawr fuh rit ik). Porphyritic rocks form when intrusive rocks

cool in two stages. As the magma begins to cool, large crystals form slowly. The remaining magma, however, cools more quickly, forming small crystals.

Extrusive rocks have a fine grained or glassy texture.

Mineral Composition Most of Earth's minerals contain silica, a

material formed from oxygen and silicon. The silica content of magma and lava

affects the types of rock they form. Lava that is low in silica usually forms

dark-colored rocks such as basalt. Magma that is high in silica usually forms

light-colored rocks, such as granite.

                                                                                                                                                                                                                                                                                                            

ObservingYou can learn about a rock's mineral composition by looking at a thin slice.

1. The diagram shows a thin slice of an igneous rock. The key identifies different minerals. Which mineral makes up most of this rock? How did you decide?

2. Which mineral is present in the smallest amount?

From Sediment to Rock Sedimentary rocks form from particles

deposited by water and wind. These particles are called sediment.

Sediment are small, solid pieces of material that come from rocks or living things.

Erosion occurs when running water or wind loosen and carry away these fragments of rock.

The moving water or wind slows and deposits the sediment.

After sediment has been deposited, the processes of compaction and cementation change the sediment into sedimentary rock.

Sediment may include shells, bones, leaves, stems, and other remains of living things.

Over time, any remains of living things in the sediment may slowly harden and change into fossils trapped in the rock.

Sedimentary Rock Formation Sedimentary rocks form through the deposition, compaction, and cementation of sediments. A. Water or wind deposits sediment. B. The heavy sediments press down on the layers beneath. C. Dissolved minerals flow between the particles and cement them together.

At first the sediments fit together loosely. But gradually, over millions of years, thick layers of sediment build up. These layers are heavy and press down on the layers beneath them. Then compaction occurs. Compaction is the process that presses sediments together.

While compaction is taking place, the minerals in the rock slowly dissolve in the water. The dissolved minerals seep into the spaces between particles of sediment. Cementation is the process in which dissolved minerals crystallize and glue particles of sediment together.

There are three major groups of sedimentary rocks: clastic rocks, organic rocks, and chemical rocks.

Clastic Rocks Clastic rocks form when rock fragments

are squeezed together. Ex. shale, sandstone, conglomerate and

breccia Usually occurs under water.

Shale forms from tiny particles of clay. Sandstone forms from the sand on beaches, on the

ocean floor, in riverbeds, and in sand dunes. Sandstone is a clastic rock formed from the compaction and cementation of small particles of sand.

Some sedimentary rocks contain a mixture of rock fragments of different sizes. The fragments can range in size from sand and pebbles to boulders. If the fragments have rounded edges, they form a clastic rock called conglomerate. A rock made up of large fragments with sharp edges is called breccia (brech ee uh).

Conglomerate

SandstoneShale

Breccia

Organic Rocks Forms where the remains of plants and

animals are deposited in thick layers. The term “organic” refers to substances

that once were part of living things or were made by living things.

Two important organic sedimentary rocks are coal and limestone.

Coal forms from the remains of swamp plants buried in water. As layer upon layer of plant remains build up, the weight of the layers squeezes the decaying plants. Over millions of years, they slowly change into coal.

Coal

Limestone

How does limestone form? In the ocean, many living things, including coral, clams, oysters, and snails, have shells or skeletons made of calcite. When these animals die, their shells pile up as sediment on the ocean floor.Slowly, the pressure of overlying layers compacts the sediment. Some of the shells dissolve, forming a solution of calcite that seeps into the spaces between the shell fragments. Later, the dissolved material comes out of solution, forming calcite. The calcite cements the shell particles together, forming limestone.

Chemical Rocks Chemical rock forms when minerals that

are dissolved in a solution crystallize. Chemical rocks can also form from mineral

deposits left when seas or lakes evaporate. Rock salt is a chemical rock made of the mineral halite, which forms by evaporation.

Examples of chemical rocks

Gypsum

The word metamorphic comes from the Greek words meta, meaning “change,” and morphosis, meaning “form.”

Heat and pressure deep beneath Earth's surface can change any rock into metamorphic rock.

When rock changes into metamorphic rock, its appearance, texture, crystal structure, and mineral content change.

While metamorphic rocks are forming, high temperatures change the size and shape of the grains, or mineral crystals, in the rock.

In addition, tremendous pressure squeezes rock so tightly that the mineral grains may line up in flat, parallel layers.

Geologists classify metamorphic rocks by the arrangement of the grains that make up the rocks.

Metamorphic Rocks Great heat and pressure can change one type of rock into another. Granite becomes gneiss, shale becomes slate, and sandstone changes to quartzite.

Metamorphic rocks that have their grains arranged in parallel layers or bands are said to be foliated. Ex. Slate, schist, gneiss

Slate Gneiss

Schist

Sometimes metamorphic rocks are nonfoliated. The mineral grains in these rocks are arranged randomly. Metamorphic rocks that are nonfoliated do not split into layers.

Marble Quartzite

Igneous rocks form from magma or lava. Igneous rocks are classified according to their origin,

texture, and composition. Most sedimentary rocks form from sediments that

are compacted and cemented together. The three types of sedimentary rocks are clastic

rocks, organic rocks, and chemical rocks. In a process that takes place deep beneath the

surface, heat and pressure can change any type of rock into metamorphic rock.

Geologists classify metamorphic rock according to whether the rock is foliated or nonfoliated.