What is a Rock? A rock is a naturally occurring solid mixture of one or more minerals and/or organic material.
3 Different Types of Rocks
The Rock Cycle Geologic processes cause rock to change from one type to another. The rock cycle is the process in which rocks are changed by geologic activity into different types of rocks. Rocks may follow various pathways within the rock cycle. Several variables such as time, heat, pressure, weathering, and erosion may alter a rock’s identity. Weathering is a process in which rocks are physically and chemically broken down by water, air, and living things to produce sediment.
Bowen’s Reaction Series In the 1900’s, a Canadian geologist named Norman L. Bowen began studying how minerals crystallize from magma. He learned that as magma cools, certain minerals tend to crystallize first (changes composition of the magma). In 1928, Bowen proposed a simplified pattern explaining the order in which minerals form as magma solidifies, or crystallizes, called Bowen’s Reaction Series. According to this, minerals form in one of two ways: 1. Continuous formation of plagioclase feldspar: Calcium-Sodium (Ca-Na) rich minerals. 2. Discontinuous formation of Iron-Magnesium (Fe-Mg) rich minerals.
Bowen’s Reaction Series
Chemical Stability of Minerals The rate at which a mineral chemically breaks down is dependent on the chemical stability of the mineral. Chemical stability is a measure of the tendency of a chemical compound to maintain its original chemical composition rather than break down to form a different chemical. Most stable minerals formed at lowest temperatures (similar to Earth’s surface conditions). Least stable minerals formed at highest temperatures. A fine ceramic is more chemically stable than a metal when subjected to HCl.
Physical Stability of Minerals Rocks also have natural zones of weakness, determined by how and where the rocks form. Example: Sedimentary rocks may form as a series of layers of sediment. These rocks tend to break between layers. Massive igneous rock structures commonly have unevenly spaced zones of weakness, called joints, that form as the rock cools and contracts. Below is “Devils Postpile National Monument” in CA.
Igneous Rocks – Formation Magma forms when rock melts, when pressure is released, or when rock changes composition. Magma is called lava when it is exposed at Earth’s surface. An igneous rock is a type of rock forming when magma or lava cools and solidifies.
Igneous Rocks – Formation Minerals with low melting points are the first to melt (composition of the magma changes). The process by which different minerals in rock melt at different temperatures is called partial melting.
Igneous Rocks – Formation When magma cools, the cooling process is the reverse of partial melting. Minerals that melted last will crystallize out of the magma first. The crystallization and removal of different minerals from the cooling magma, as occurs in Bowen’s Reaction Series, is called fractional crystallization.
Igneous Rocks – Texture Magma cooling deep inside of Earth’s crust results in the formation of intrusive, or plutonic, igneous rocks. The magma intrudes, or Granit enters, into other rock e masses beneath Earth’s surface. The magma then slowly cools and hardens. Lava cooling at Earth’s surface forms extrusive, or volcanic, igneous rocks. Basa lt
Igneous Rocks – Texture Intrusive igneous rocks commonly have LARGER crystals because they have a slow loss of heat and longer to cool. Igneous rocks composed of large mineral grains are described as having a coarse-grained, or phaneritic, texture.
Igneous Rocks – Texture Extrusive igneous rocks commonly have smaller crystals because they cool very rapidly. Igneous rocks composed of small mineral grains are described as having a fine-grained, or aphanitic, texture.
Igneous Rocks – Texture Some igneous rocks can have a mixture of both larger and smaller crystals and their texture is known as porphyritic. The larger crystals (phenocrysts) are surrounded by a background mass of smaller crystals (groundmass).
Igneous Rocks – Composition Light-colored igneous rocks are called felsic rocks. Large proportions of silica are present along with common minerals including quartz, feldspar, and mica. Rhyolit e Dark-colored igneous rocks are called mafic rocks. Large proportions of iron and magnesium are present along with common minerals including pyroxene and olivine. Gabbr o
Igneous Rocks – Examples GabbroBasa Rhyolitlt e Diorite Andesit e Obsidia Granite n Pumic e
Sedimentary Rocks – Formation Loose fragments of rock, minerals, and organic material resulting from natural geologic processes are called sediment. Combinations of different types of sediment make up what is known as sedimentary rocks. The most noticeable features of sedimentary rock is the layering, or stratification.
Sedimentary Rocks – Formation Two main processes convert loose sediment into sedimentary rock: Compaction: the process in which sediment is squeezed and the pore space between sediment grains is reduced. Cementation: the process in which sediments are glued together by minerals that are deposited by water.
Sedimentary Rocks – Classes1. Chemical: forms when the ions from dissolved minerals crystallize, or precipitate, out of a solution. • One reason minerals precipitate is due to evaporation (minerals are left behind once H20 evaporates, thus being called evaporites). • Halite is an example of this type of sedimentary rock, made of sodium chloride, NaCl, or table salt (recall … this is also a mineral). • The Bonneville Salt Flats near the Great Salt Lake in UT are a good example of evaporite deposits. Halit Bonneville Salt e Flats
Sedimentary Rocks – Classes2. Organic: forms from the remains of once-living plants and animals. • Most limestone forms from the remains, or fossils, of animals that once lived in the oceans. Some are made of the skeletons of tiny organisms called coral (coral reefs). • Coal forms from plant remains that are buried before they decay and are then compacted into matter composed mainly of carbon.Fossiliferous Coral BituminousLimestone Reef Coal
Sedimentary Rocks – Classes 3. Clastic: forms when rock or mineral fragments, or clasts, of pre-existing rocks are compacted or cemented together. • Classified by the size of the sediment they contain: • Clastic sedimentary rock made of round fragments ranging from 2 mm. to boulders is called a conglomerate. If the fragments are angular with sharp corners, the rock is called a breccia. • Clastic sedimentary rock made up of sand-sized grains, mainly consisting of quartz, is called sandstone. • Clastic sedimentary rock consisting of flaky clay-sized particles is called shale.Conglomera Breccia Sandston Shalete e
Sedimentary Rocks – Features The tendency for currents of air or water to separate sediments according to size is called sorting.
Sedimentary Rocks – Features The place or setting in which sediment is deposited is called a depositional environment. Common ones include: Rivers Beache s Delta Ocea s ns
Sedimentary Rocks – Features Some sedimentary rocks are characterized by slanting layers within the rock called cross-beds. Generally form in sand dunes or river beds.
Sedimentary Rocks – Features A type of stratification (layering) in sedimentary rock when different sizes and shapes of sediment settle to different levels of rock is called graded bedding. Commonly transition from largest grains on the bottom to smallest grains on the top.
Sedimentary Rocks – Features Some sedimentary rocks clearly display ripple marks. Caused by the action of wind or water on sand; often preserved in sandstone and shows the sediment was once part of a beach or river bed. Another feature of sedimentary rocks are mud cracks, which form when muddy deposits dry and shrink.
Sedimentary Rocks – Examples Brecci a FossiliferouBituminous sCoal Conglomera Limestone te Arkos e Sandsto neLimesto Redne Shal Sandstone e
Metamorphic Rocks – Formation The process by which heat, pressure, or chemical processes change one type of rock into another is called metamorphism. Most metamorphic rock, or rock having undergone metamorphism, forms deep within Earth’s crust.
Types of Metamorphism1. Contact Metamorphism: a change in the texture, structure, or chemical composition of a rock due to contact with magma.
Types of Metamorphism2. Regional Metamorphism: a change in the texture, structure, or chemical composition of a rock due to changes in temperature and pressure over a large area.
Metamorphic Rocks – Classification Metamorphic rocks are classified according to texture. The metamorphic rock texture in which mineral grains are arranged in planes or bands is Gneiss known as foliation. The metamorphic rock texture in which mineral grains are not arranged in planes or bands is called non-foliation. Quartzit e
Metamorphic Rocks – Examples Gnei ssAmphibolite Marble Quartzit e Anthracit e Coal Phyllit Slat e e Schi st