3.2 Rocks

What is a rock? Rocks are any solid mass of mineral or mineral-like matter occurring naturally as part of our planet. The three major types of rocks are igneous, sedimentary and metamorphic rocks Interactions among Earth’s water, air, and land can cause rocks to change from one type to another The continuous processes that cause rocks to change make up The Rock Cycle

The Rock Cycle Most changes in the rock cycle take place over long periods of time Types of Rocks Igneous rock is formed when magma cools and crystallizes beneath the surface or as a result of a volcanic eruption

The Rock Cycle 2. Sedimentary rock is formed from the weathered products of preexisting rocks that have been transported, deposited, compacted, and cemented

The Rock Cycle 3. Metamorphic rock is formed by the alteration of pre-existing rock deep within Earth by HEAT, PRESURE, and/or CHEMICALLY ACTIVE FLUIDS

The Rock Cycle The Rock Cycle shows how the three rock types are related to each other Terms to know: Magma is molten material that forms deep beneath Earth’s surface Lava is magma that reaches the surface Weathering is a process in which rocks are broken down by water, air, and living things Sediment is weather pieces of Earth materials

The Rock Cycle Energy that drives the Rock Cycle Processes driven by heat from the Earth’s interior are responsible for forming both igneous and metamorphic rock Weathering and movement of weathered material are EXTERNAL processes powered by energy from the Sun External processes produce sedimentary rock

Bowen’s Reaction Series As magma cools, elements in it bond together to form crystals of minerals Not all minerals form at same temperature during cooling Some grow when it is hot Others grow when magma is cooler Bowen’s Reaction series shows when different minerals form as magma cools

If olivine allowed to cool further, olivine would react with the residual magma and change to pyroxene. With dropping temp. we have decreasing thermal vibration of molecules, allows silica to form more complex structures

Igneous Rocks Recall that igneous rocks form when magma or lava cools or hardens In the picture to the right, a dark-colored igneous rock called Basalt will form Different rock would have been made if it would have stayed inside Earth

Formation of Igneous Rocks Rock formed when magma hardens beneath Earth’s surface are known as Intrusive igneous rocks Cooling below the surface is very SLOW, forming large crystals in the rock

Formation of Igneous Rocks When lava hardens, the rocks that form are called Extrusive Igneous Rocks Cooling at the surface is much FASTER, thus making SMALLER crystals in the rock

Classification of Igneous Rocks Igneous rocks can be classified based on their composition and texture Composition is what they’re made of or the amounts of light and dark minerals in the rock Texture describes the appearance based on its size, shape and arrangement of crystals

Classification of Igneous Rocks Texture Coarse-grained is caused by SLOW cooling resulting in LARGER crystals (known as Phaneritic texture) Fine-grained texture is caused by RAPID cooling in SMALLER, interconnected mineral grains (known as Aphanitic texture)

Phaneritic Igneous Texture Example: Granite

Aphanitic Igneous Texture Example: Rhyolite

Classification of Igneous Rocks Texture (continued) Glassy texture is caused by VERY RAPID cooling (ex. Obsidian) Porphyritic texture is a mixture of both large and small crystals (ex. Andesite) Phenocrysts – larger crystals surrounded by a background of smaller crystals

Classification of Igneous Rocks Composition Felsic composition rocks are made mostly of light- colored quartz and feldspar silicate minerals Felsic rocks are the MAJOR ROCKS of the continental crust Examples are Granite (intrusive) and Rhyolite (extrusive)

Classification of Igneous Rocks Composition (continued) Intermediate composition rocks are between granitic light-color minerals and mafic composition Examples are Andesite (extrusive) and Diorite (intrusive)

Classification of Igneous Rocks Composition Mafic composition rocks are made MOSTLY of dark- colored silicate minerals and plagioclase feldspar Typically DARKER and DENSER than Felsic rocks Examples are Basalt (extrusive) and Gabbro (intrusive)

Classification of Igneous Rocks Composition Ultramafic composition rocks are made mostly of iron and magnesium- rich minerals Rare at Earth’s surface Examples are Komatiite (extrusive) and Peridotite (Intrusive)

ONE LAST IMPORTANT POINT For Igneous rocks The amount of silica content changes (SiO2) Mafic/Ultramafic rocks have the least amount of silica content Felsic rocks have the most amount of silica content This is all due to the minerals that they are made of

Sedimentary Rocks All sedimentary rock begin to form when existing rocks break down into sediments Sediments, which is weathered rock debris, are often transported around

Formation of Sedimentary Rocks 1st Step - Weathering Any process hat breaks up rocks into sediments Physical weathering is when physical forces break up rocks Chemical weathering is when minerals in the rocks change into new substances

Formation of Sedimentary Rocks 2nd Step – Erosion Involves the WEATHERING and the REMOVAL of rock Agents of erosion: Air Water Wind Gravity 3rd Step - Deposition occurs when an agent of erosion loses energy and drops sediments Deposited according to size (large to small)

Formation of Sedimentary Rocks 4th Step – Compaction and Cementation Compaction is a process that squeezes, or compacts sediments Cementation takes place when dissolved minerals are deposited in the tiny spaces among the sediments HINT: Glue together

Classification of Sedimentary Rocks Classified into groups according to the WAY THEY FORM Three main groups Clastic Chemical Organic

Classification of Sedimentary Rocks Three Main Groups Clastic sedimentary rocks are composed of weathered bits of rocks and minerals Classified by particle size Common rocks include: Shale (most abundant) Sandstone Conglomerate

Classification of Sedimentary Rocks Three main groups 2. Chemical Sedimentary rocks form when dissolved precipitate (separate from water) Common rocks include: Limestone – most abundant chemical rock Evaporites such as rock salt or gypsum Coal

Classification of Sedimentary Rocks Three main groups 3. Organic Sedimentary Rocks Some are the shells and skeletal remains of organisms that settle to the ocean floor Examples include coquina, limestone, and chalk

Features of Some Sedimentary Rocks The many unique features of sedimentary rocks are clues to how, when, and where they formed Some features include layering, and fossils

Features of Some Sedimentary Rocks Sedimentary rock layering Graded bedding – when different sizes and shapes of sediment settle to different levels in the rock Cross bedding – the slanting of layers in sedimentary rocks

Sedimentary Rock Chart

Metamorphic Rocks Metamorphism means “to change form” Most metamorphic changes occur at elevated temperatures and pressures Conditions for formation are found a few kilometers below the Earth’s surface

Formation of Metamorphic Rocks Most metamorphism occurs in one of two settings: Contact metamorphism Regional metamorphism

Formation of Metamorphic Rocks Contact metamorphism occurs when magma move into rock Happens near a body of magma Changes are driven by a rise in temperature Example: Marble forms when magma intrudes a limestone body

Formation of Metamorphic Rocks Regional metamorphism is metamorphism of rocks over a large area During mountain building, large areas of rocks are subjected to extreme pressures and temperatures Produces a large amount of metamorphic rock

Agents of Metamorphism The agents of metamorphism are: Heat Pressure Hydrothermal solutions

Agents of Metamorphism Heat (most important) Provides the energy needed to drive chemical reactions Heat comes from magma and the change in temperature with depth Pressure Causes a more compact rock with greater density Pressure also increases with depth

Origin of Pressure in Metamorphism

Agents of Metamorphism Hydrothermal solutions Hot water-based solutions escaping from the mass of magma Promote recrystallization by dissolving original minerals then depositing new ones

Classification of Metamorphic Rocks Two main categories Foliated Metamorphic Rock Has a banded or layered appearance Ex. Gneiss Nonfoliated Metamorphic Rock Does not have a banded texture Ex. Marble

Metamorphic Rock Chart