Sections 3.2, 3.3 and 3.4 Types of Rocks

Igneous 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 crystal size and silica content! 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.

Basalt

Igneous rocks 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

Granite

Igneous rocks Crystal Size 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.

Igneous rocks A rock with large crystals scattered on a background of much smaller crystals has a porphyritic texture 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.

Igneous rocks 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.

Observing Igneous rocks                                                                                                                                                                                                                                                                                                              Igneous rocks Observing You 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?

Sedimentary rocks From Sediment to Rock Sediment means “something that settles” 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.

Sedimentary Rocks 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

Sedimentary rocks 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. 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.

Sedimentary rocks 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.

Sedimentary Rocks 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.

Types of sedimentary rocks 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.

Sandstone Shale Conglomerate Breccia

Sedimentary rocks 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.

Sedimentary rocks Coal 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

Sedimentary rocks 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. Limestone

Sedimentary rocks 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. Gypsum also forms this way!

Rock Salt Examples of chemical rocks Gypsum

Metamorphic rocks 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.

Metamorphic Rocks With the added heat and pressure, the bonds between atoms can break When new bonds form and the atoms join together differently we call this recrystallization This is one way that metamorphic rocks form

Metamorphic rocks 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

Gneiss Slate Schist

Sometimes metamorphic rocks are nonfoliated 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

Nonfoliated One reason that metamorphic rocks do not show foliation is that it is made up of only one type of mineral, so the different minerals cannot separate and line up in layers Another reason that a metamorphic rock may lack foliation is that it has not been subjected to high pressure!

Summary 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.