Chapter 20: Earth Materials

Section 1: Minerals

Common Elements * The crust is the outermost layer of Earth. It includes all continental material and the material that forms the ocean bottom. * Mineral: a naturally occurring element or compound that is inorganic, solid, and has a crystalline structure. * Inorganic means that minerals are materials that are not produced by living organisms. * The composition of minerals are indicated by their chemical formulas.
Physical Properties * A mineral has a particular chemical composition. * Different amounts of the chemical impurity chromium in the crystalline structure of corundum cause the difference in color. * Some physical properties are controlled by the orderly arrangement of atoms in a mineral’s structure. * This orderly pattern is what makes a mineral crystalline. * When minerals break along planes that cut across relatively weak chemical bonds, a smooth, flat surface is created. * Cleavage: The ability of a mineral to do this is the physical property * All parallel cleavage planes define a single direction of cleavage. * Because mica has one direction of cleavage, it can be separated in layers. * Feldspar is an example of a mineral with two planes of cleavage. * Fracture: When a mineral breaks unevenly * Bonds connecting atoms in materials often have different strengths. * Hardness: The physical property that measures resistance to scratching * When a hardness test is performed by rubbing two objects together, the softer of the two will wear away. * The way a mineral reflects light is the physical property known as luster. * Two main types of luster, metallic and nonmetallic, help subdivide minerals and often give a clue about their compositions. * Streak: The color of a mineral in powdered form * The streak of a mineral may be the same color as the mineral specimen. When a mineral shows different colors, the streak powder color generally stays the same, which helps identify the mineral. * A streak test is performed by rubbing a mineral on an unglazed, white porcelain tile. * The orderly internal arrangement of atoms in a mineral often is related to its external crystal shape. * Minerals can be classified using six basic crystal forms. * The types of symmetry shown by the crystal are key elements in determining the crystal system to which a mineral belongs.
Mineral Formation * A mineral crystal grows as atoms are added to its surfaces, edges, or corners. * The types of atoms that are added depend on the atoms in the growing crystal’s surroundings. * Growth also is controlled by how fast atoms can migrate to the crystal and by the temperature and pressure of the surroundings. * Mineral crystals can form in different ways. * One way is by precipitation from hot, water-rich fluids. * Another way is by solidification from molten rock. * A third way is by the evaporation of water rich in dissolved salts at low temperatures near Earth’s surface. * Some minerals are produced from hot-water solutions containing dissolved mineral matter. * Hydrothermal minerals form on the rims of hot springs. * When hot water passes through cracks in cooler rock, minerals may form within the cracks * Magma: Molten rock material found inside Earth * When the temperature of magma drops well below the solidification temperature of a mineral, crystals of that particular mineral may form and grow. * When water slowly evaporates, dissolved mineral material may be left behind to form crystals.
Mineral Groups * About 3,800 minerals have been identified in nature. * The atomic arrangement and composition of minerals allow them to be sorted into groups. * Most minerals contain silica. * Silica is a common term for a compound that contains silicon plus oxygen or silicon dioxide (SiO2). * In silicate minerals, the elements silicon and oxygen bond together to form a geometric structure called a tetrahedron. * The simplest silicate structures have silicon-oxygen tetrahedrons that are not linked together. * Several important silicate groups form most of Earth’s crust. * Earth’s oceanic crust is denser and contains a larger percentage of silicates whose tetrahedrons are linked together as single chains or are not linked. * Some important mineral groups are not silicates. These include the carbonates, oxides, halides, sulfides, sulfates, and native metals. The non-silicate groups are a source of many valuable ore minerals and building materials.
Mineral Uses * People use minerals either directly as objects of wealth, or as raw materials to make things. * Not all minerals need to provide metals to be valuable. Nonmetallic minerals are valuable as well.

Section 3: Igneous Rocks

What’s A Rock? * Rock: a naturally formed mixture containing minerals, rock fragments, or volcanic glass bound together. * Texture: describes the size, shape, and arrangement of the rock’s \n components. * The rock-making process is a continuous cycle.
Intrusive Igneous Rocks: form within, or push into, regions of Earth’s crust * Igneous rocks are those that form from molten magma. * Minerals have different melting temperatures. * This interaction between magma and the rock it pushes into can cause changes in the rock, changes in the magma, or both. * As magma cools, different minerals crystallize at different temperatures. * When the temperature of the magma is high, olivine, pyroxene group minerals, and plagioclase feldspars crystallize first. * The Bowen’s reaction series, shown below, illustrates the sequence in which minerals crystallize from magma at different temperatures. * Igneous rocks form from three types of magma—granitic magma, basaltic magma, or andesitic magma. * Granitic rocks include rocks, such as granite, that contain the minerals quartz, potassium feldspar, mica, and hornblende. * Basaltic rocks contain the minerals plagioclase feldspar, pyroxene, and olivine. * Finally, andesitic rocks have compositions intermediate between granitic and basaltic rocks. * The size of the mineral crystals in a rock is called the grain size. * Grain size depends on how quickly the magma cooled that formed the rocks.
Extrusive Igneous Rocks: rocks that form from lava erupted at Earth’s surface. * If a volcanic eruption is on land, lava pours out into the air. * If a volcanic eruption is on the ocean floor, lava flows into water. * When magma cools inside Earth’s crust and forms intrusive igneous rocks, the crustal rock surrounding the magma can be hot. * Extrusive igneous rocks have different textures than intrusive igneous rocks. * Rocks with small grain sizes are called fine-grained. * Fine-grained, extrusive igneous rocks often have grain sizes that are too small to be seen without magnification. * The difference between extrusive igneous rocks and intrusive igneous rocks is due mainly to the difference in their textures. * Extrusive igneous rocks are fine-grained with small crystals. * Intrusive igneous rocks are coarse-grained with large crystals. * Different rocks form from granitic or basaltic magmas, depending on how quickly the magma cools. Coarse-grained granite and fine-grained rhyolite both form from granitic magmas. Coarse-grained gabbro and fine- grained basalt both form from basaltic magmas. * When lava erupts at Earth’s surface, other types of extrusive igneous rocks can be formed.

Section 3: Sedimentary Rocks

Rocks from Surface Materials * Clasts: small bits and pieces * Rocks that are tumbled more than 3,000 km along the bottom of the Colorado River, shown below, can be broken into fine particles before they reach the Gulf of California. * Mechanical weathering occurs when physical forces break rocks into smaller clasts. * As clasts are transported they grind against each other and other hard objects in their environment. * Sandstone can for when sand grains are deposited, compacted, and cemented together. * Pore Space: The empty space between clasts * Water, oil, and natural gas found beneath Earth’s surface are stored in the pore spaces of sedimentary rocks. * The process by which clasts stick together by being pushed together is called compaction. * Cementation: When minerals slowly precipitate out of water and fill spaces between clasts
Detrital Sedimentary Rocks * Clasts have different sizes and geologists classify clasts according to their size. In order of decreasing size, clasts ar e classified as gravel, sand, silt, or clay. * Sand is defined by size, not by composition. Sand doesn’t even have to be made from rock material; it can be made from shells. * The size of a clast determines how the clast can be transported. * The separation of clasts according to size by wind or water is called sorting. * Sorting can also occur as clasts are deposited. Deposition of clasts occurs when the clasts are no longer being transported. * Detrital sedimentary rock composition depends on sources of rock material that were eroded, transported, and eventually deposited. * The number of possible combinations of different kinds of clasts is large. * Some minerals tend to be more common in detrital sediments because they are harder or more resistant to being dissolved. * Geologists examine sedimentary rock compositions and try to reconstruct what happened to form them. * Just as igneous rocks are classified according to composition and texture, similar observations are used to classify detrital sedimentary rocks. * Mineral composition is extremely variable, so adjectives are used to modify the general name of the rock. * Clast size also provides clues to help determine the depositional environment of the sediment that formed the detrital rock.
Biochemical Sedimentary Rocks * If sedimentary rocks contain the remains of living organisms they are called biochemical sedimentary rocks. * Most of Earth’s limestone is composed, at least partially, of the remains of marine organisms that had hard parts made of calcium carbonate. * Another common rock that originates from the remains of organisms is coal. * Coal usually develops from peat, a brown, lightweight deposit of moss and other plant matter. Peat forms shallow swamps or bogs in a temperate or tropical climate. * As sediment accumulates above a layer of peat the peat becomes more compressed. Continual compression drives out water and other compounds, leaving behind a form of carbon called coal.

Section 4: Metamorphic Rocks and the Rock Cycle

Metamorphic Rocks * Sharp folds sometimes display intense transformations in metamorphic rocks. * Any igneous, sedimentary, or metamorphic rock can be changed through metamorphism. * Metamorphic rocks form under conditions that are between the conditions that form igneous and sedimentary rocks.
Metamorphic Rock Composition * Metamorphic changes in rocks are caused by thermal energy, pressure, and chemical reactions. * Clay minerals, micas, and amphiboles are examples of minerals that contain water in their crystal structures. * Regional movements of Earth’s tectonic plates can cause rocks to be buried deeply, producing large increases in the temperature of the rocks. * Metamorphic changes in rocks that occur over large areas are called regional metamorphism.
Metamorphic Rock Textures * Metamorphic processes produce rocks with different textures. * Folio means “leaf.” Foliated texture has the appearance of layered leaves or pages of a book. * Foliated: crystals are arranged in layers and bands. * Foliated textures are formed under high pressure. * Metamorphic textures can also be nonfoliated where crystals are in more random orientations. * The most common sedimentary rocks in Earth’s crust are rocks, such as shale and siltstone, that are formed from mud. * Mineral grains are randomly oriented when no directed force is involved. * Orientation of mineral grains is perpendicular to the direction of pressure caused by compression. * Mineral grains are parallel to the direction of shearing force. * The smallest-grain sizes in foliated textures occur in slate, which forms thin layers and exhibits rock cleavage. * Gneiss rock textures often are banded, and gneisses generally represent the limit between metamorphic and igneous conditions. * Similar in texture to intrusive igneous rocks, nonfoliated metamorphic rocks tend to have random crystal orientation and uniform grain size. * Regional and contact metamorphism cause changes that can occur over millions of years.
Classifying Metamorphic Rocks * Metamorphic rocks can be classified by their texture. * Metamorphic rocks can be foliated or nonfoliated. * A rock with a schist-like texture made of garnet and mica is a garnet-mica schist.
The Rock Cycle * Rocks above and below Earth’s surface are continually being changed into other types of rocks. * Sedimentary and metamorphic rocks can be melted to form igneous rocks. * Weathering, compaction and cementation can change igneous and metamorphic rocks into sedimentary rocks. A rock can even be changed into a different rock of the same type. * Rock Cycle: The continual changing of rocks into different types. * As rocks move through the various stages of the rock cycle, matter is always conserved