Rock Forming Processes Big Idea Understanding natural processes and Earth materials allows Earth history to be discerned. The Rock Cycle The rock cycle allows us to view different relationships between the parts of the Earth system. No beginning or an end. We begin with magma. Magma cools and forms crystals in a process called crystallization. When cooling occurs, depending on the conditions and time, atoms are arranged forming geometric shapes. The rocks formed from cooling magma are called igneous rocks. Once igneous rocks are formed, they can undergo various changes. Igneous rocks can be weathered and eroded through sedimentation. When sediment undergoes a natural process of cementation called lithification, they become sedimentary rock. Igneous rocks can also be buried, undergoing metamorphism, or experiencing extreme pressure and temperatures. Rocks that experience metamorphism become distorted from their original forms. Any of the three types of rocks under the right circumstances can become another type of rock. They can even cycle between the same types. For example, sedimentary rocks can become sediment again and become part of a different type of sedimentary rock. The rock cycle takes place at locations where there is activity. 1. Convergent and 2. Divergent plate boundaries 3. Rivers/Streams 4. Deserts, etc. Convergent boundaries can create volcanoes and many different types of igneous rocks as well as cause erosion, and metamorphism as one plate moves beneath another. Divergent boundaries allow magma to rise to the surface of the earth creating new igneous rocks. Igneous Rocks Igneous rocks form from magma or lava, depending on the location. There are two types of igneous rocks based on where they form. Intrusive (plutonic) igneous rocks form inside the earth and intrude upon existing rock structures. These rocks cool slowly because the Earth around them acts as insulation. Intrusive rock features: Coarse or large grained texture Visible crystal arrangement Minerals are identifiable Examples: Granite, Diorite, Gabbro, Peridotite Extrusive (volcanic) rocks are igneous rocks that form when lava solidifies or extrudes onto the surface of the earth. These rocks often cool quickly when they are exposed to the air. Extrusive rock features: Fine or small grained texture No crystals apparent Voids (vesicles) left by escaping gasses (Scoria) Examples: Basalt, Rhyolite, Andesite Some igneous rocks form when lava or magma flows into large bodies of water or is ejected into the air. The rapid cooling caused by the cooler water/air temperatures forms volcanic glass. Obsidian is an example of volcanic glass, and so is pumice. Sometimes, a rock will cool underground and then suddenly be erupted. This results in an igneous rock with a porphyritic texture, or large and small crystals mixed together. One magma can produce many different types of igneous rocks. Bowens Reaction Series is a chart that shows the sequence of mineral crystallization in magma. All igneous rocks are classified by their texture and composition. The four textures of igneous rocks are: Coarse-grained Fine-grained Porphyritic Glassy The four different compositions are: Granitic Andesitic Basaltic Ultramafic (*rare on Earths surface) Sedimentary Rocks Sedimentary rocks are made from the sediments, or tiny pieces, of other rocks. Weathering of other rocks with the help of gravity and erosional agents carry sediment to new locations. The sediment becomes lithified, or turned to rock by compaction and cementation. The word sedimentary comes from the Latin word sedimentum, meaning, settling. Over time, weathering and deposition of sediment leads to a build up of materials. The layers on the bottom become compacted and together with mineral matter form solid rock. Sedimentary rocks make up about 5% (volume) of the Earths crust, but make up 75% of the rocks that are exposed at the surface. Sedimentary rocks are used to look back at the history of the Earth and give answers to ancient climates and life. Sedimentary rocks are very valuable to world economies. For example, the following are all found in sedimentary deposits: Coal Petroleum Natural gas Iron Aluminum Sand Gravel Classifying sedimentary rocks There two ways that sedimentary rock can form: They can originate from solid particles weathered from other rocks. These are called detrital sedimentary rocks. They can form from chemical sediment, or precipitate out of liquids to reform solids. These are called chemical sedimentary rocks. Detrital sedimentary rocks Particle size is used to distinguish between detrital rocks. There are four size categories that are used to organize them. Conglomerate- large rounded pieces of rock dominate Breccia- large angular pieces of rock dominate Sandstone- sand size grains are present Shale- fine-grained sediment is present, also the most abundant These size classifications also provide clues about the environment in which the sediment was deposited. The size reflects the amount of energy present in the environment and the shape can reflect the time of sediment transport. For example, glacier movement vs. stream vs. wind vs. lakes Names can be mixed when more than one size of sediment is present, i.e. sandy siltstone, depending on which size predominates. Chemical sedimentary rocks Formed from material carried in solution, or dissolved in liquids. Precipitation (not rainfall) occurs by physical processes or by organisms. Limestone is the most abundant chemical sedimentary rock. It is composed of mostly biochemical sediment, or stuff that was once alive. (Coquina or chalk) Other chemical sedimentary rocks form from solids precipitating out of solution. This can occur because of water chemistry changes or temperature changes. Travertine is an example of a chemical sedimentary rock. It is often found in caves, forming as groundwater drips or trickles. Agate is a popular Michigan rock that is a chemical sedimentary rock. Evaporation can also form chemical sedimentary rocks. As the water leaves the minerals are left behind. Halite and gypsum are both common evaporates. Halite - cooking and seasoning Gypsum - drywall or sheet rock. Coal is a special type of sedimentary rock. It forms when organic matter such as forest or woods are buried for extended periods of time. Swampy areas have the right conditions for coal to form. Lithification of sediment Compaction and cementation are the two processes that are needed to turn sediment into rock. Compaction is the smooshing or crushing of sediment beneath a buildup of more sediment. Cementation is the use of a natural cementing agent such as calcite, silica or iron oxide to glue or stick pieces of sediment together. The cementing agent can clearly be distinguished because: Calcite- fizzes in diluted acid Silica- the hardest cementing agent Iron oxide- orange or red color present Features of sedimentary rocks Strata, or beds- flat surfaces on which sediments are deposited and form Fossils- traces or remains of prehistoric life, environments, etc. Ripple marks- form in areas that had gently lapping waves Mud cracks- mud or clay dries and shrinks, leaving gaps Geodes- rocks that form from dissolved minerals getting inside sedimentary rocks Graded bedding-sorted layers of rocks with larger at bottom Metamorphic Rocks Metamorphism means, to change form. Metamorphic rocks are existing rocks that have been changed in size, shape or texture due to three agents: Heat Pressure Chemically active fluids There are different levels of metamorphism ranging from low-grade (parent rock is identifiable) to high- grade (parent rock is obliterated). Metamorphic rocks do not melt!! If they did, they would be igneous. Metamorphism usually occurs in one of two settings: *Regional metamorphism like mountain building, or Contact metamorphism when rocks are near magma masses *Most metamorphic rocks form from regional metamorphism. The three agents of metamorphism can work together, but most often work in ones or twos. Heat is the most important metamorphic agent. Temperature and Pressure increase with depth, so it makes sense that rocks at great depths under go intense metamorphism. In these situations rocks can experience the great bending and rippling that are often seen in metamorphic formations. Water is the mostly common chemically active fluid. The texture changes that occur with metamorphism create the two types of metamorphic rocks: Foliated metamorphic rocks Nonfoliated metamorphic rocks Foliated metamorphic rocks Rocks that undergo recrystallization often do so in a preferred direction creating bands or stripes of certain minerals or rocks. This usually occurs during regional metamorphism. Foliated rocks form when minerals are brought into alignment. Example: Shale becomes slate; any schists; gneiss Nonfoliated metamorphic rocks Do not exhibit banding or stripes. These rocks are often formed from only one mineral. This usually occurs during contact metamorphism. Example: limestone becomes marble; sandstone becomes quartzite
