Classifying Rocks • When studying a rock sample, geologists observe three things: • Mineral Composition • Color • Texture
Mineral Composition • Rocks are made of minerals and other materials. • Some rocks contain only one mineral. Others can contain many different types of minerals.
Mineral Composition • There are about 20 minerals that make up most rocks. • These minerals are known as rock-forming minerals.
Mineral Composition • Geologists look at the shape and color of the mineral crystals in the rock to help identify what the minerals are.
Mineral Composition/Color • A rock’s color can help geologists know what minerals are in it. • For example, when we were studying volcanoes, we know that magma with more silica was lighter in color. The same is true of rocks with more silica.
Texture • A rock’s texture is a very useful way to identify the rock. • A rock’s texture is the look and feel of the rock’s surface. • Some are smooth and glassy. • Some are rough or chalky.
Texture • Most rock’s are made up of particles of minerals or other rocks called grains. • Grains give the rock its texture.
Texture • To describe a rock’s texture, geologists use: • Size of grains • Shape of grains • Pattern of grains
Grain Size • If grains are large and easy to see, they are said to be coarse-grained. • If the grains are so small they need a microscope to be seen, they are said to be fine-grained. • Some rocks do not have any visible grains even under a microscope.
Grain Shape • Some rocks have grains that have very smooth, rounded shapes. • Some rocks have grains with very jagged, sharp shapes. • Some rocks have grains that are rounded AND grains that are jagged.
Grain Pattern • The grains in rocks can form different patterns. • Sometimes the grains form flat layers. • Sometimes they occur randomly. • Sometimes they form swirling patterns.
Igneous Rock • Igneous rock forms from magma and lava hardening back into a solid. • The name igneous comes from the Latin word ignis, meaning “fire”. • This is also where we get the word ignite from.
Igneous Rock • Igneous rocks are classified according to three things: • Origin • Texture • Mineral Composition
Igneous Rock - Origin • Igneous rock can form either on Earth’s crust, or under Earth’s surface. • Extrusive rock is formed from lava that has erupted and cools when it is outside of the volcano and on Earth’s surface. • Intrusive rock forms when magma hardens beneath Earth’s surface.
Igneous Rock - Texture • Lava that cools quickly forms very fine-grained igneous rocks with small mineral crystals. • There will be small to no visible grains in the rock.
Igneous Rock - Texture • Slow-cooling magma will form coarse-grained rocks with large mineral crystals. • The rocks will have large, easy to see grains. • Intrusive rocks have larger crystals than extrusive rocks.
Igneous Rock – Mineral Composition • Remember that different types of lava have different amounts of silica. • Minerals in rock that have a lot of silica will be very lightly colored and minerals that have less silica will be darker. • Rocks with more silica will be lighter in color than rocks with less silica.
Sedimentary Rocks • Sedimentary rocks form over millions of years as particles of mud, sand, and volcanic ash are deposited and compacted and become solid rock.
Sedimentary Rocks • Most sedimentary rocks are formed through a series of processes. • Erosion • Deposition • Compaction • Cementation
Sedimentary Rocks (Erosion) • Destructive forces are constantly breaking up and wearing away the rocks on Earth’s surface. • These forces include heat and cold, rain, waves, and grinding ice.
Sedimentary Rocks (Erosion) • In erosion, running water, wind, or ice loosen and carry away fragments of rock. • These fragments of rock are called sediments. • Sediments are also particles of shells, bones, leaves, stems, and remains of other living things.
Sedimentary Rocks (Deposition) • The sediments that were moved by the running water, wind, or ice will eventually sink to the bottom of the water or be dropped by the wind or ice. • Deposition is the process where sediments settle out of water or wind. • The particles of rock are now laying on the earth.
Sedimentary Rocks (Compaction) • The process that presses sediments together is called compaction. • Thick layers of sediments builds up over millions of years. • The new layers push down on the older, lower layers, squishing them together tightly.
Sedimentary Rocks (Cementation) • During compaction, the minerals in the sediments dissolve into water. • Cementation is when the dissolved minerals recrystallize and act as a glue holding all of the sediment together.
Types of Sedimentary Rocks • Geologists classify sedimentary rocks based on the sediments that are in the rock. • There are three major types of sedimentary rock: • Clastic • Organic • Chemical
Types of Sedimentary Rock • Clastic • A clastic rock is a sedimentary rock that forms from rock fragments that have been squeezed together and cemented. • Most sedimentary rocks are clastic. • Clastic rocks are further classified based on the size of the rock fragments (grains).
Types of Sedimentary Rock • Organic • Not all sedimentary rocks form from particles of other rocks. • Organic rocks form from the remains of plants and animals. • The term “organic” means that the substances were once part of living things.
Types of Sedimentary Rock • Chemical • When minerals that are dissolved in a solution crystallize, chemical rock forms.
Metamorphic Rocks • Metamorphic rocks are rocks that have changed form. • Any type of rock can change into metamorphic rock. (Igneous, sedimentary, or metamorphic) • Meta means “change”. • Morphosis means “form”.
Metamorphic Rocks • Rock can change form when one or both of the following things are applied to it: • Heat • Pressure
Metamorphic Rock • Collisions between plates pushes rock downwards. • This adds pressure from the crust of the Earth and heat from the mantle, causing the minerals to change into other minerals, forming metamorphic rock.
Metamorphic Rock • When rock changes into metamorphic rock, its appearance, texture, crystal structure, and mineral content change.
Types of Metamorphic Rock • Geologists classify metamorphic rocks according to the arrangement of the grains that make up the rocks. • There are two main types of metamorphic rock: • Foliated • Nonfoliated
Types of Metamorphic Rock • Foliated • Metamorphic rocks that have their grains arranged in parallel layers or bands are said to be foliated.
Types of Metamorphic Rock • Nonfoliated • Some metamorphic rocks are nonfoliated. • The mineral grains in these rocks are arranged randomly.