Rocks and Minerals

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.