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.
- 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