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