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Unit 10 Earth Science Chapter 29 Formation of Rocks Name________________________________ 29.1 Volcanoes The eruption of Mount St. Helens in 1980 reduced the height of this mountain from 2,932 meters (9,677 feet) to 2,535 meters (8,364 feet). Early in the morning of May 18, 1980, an earthquake triggered a landslide that caused the bulge to eject magma, water, and gases. Solid rock melts and becomes magma under certain conditions that lower the melting point of the material. Volcanologists (scientists who study volcanoes) use several indicators to predict when a volcano will erupt. They look for and/or monitor: Upward movement of magma Bulging of a volcano’s side Temperature of the ground water and the release of gases Liquid rock material below the Earth’s surface is called magma; liquid rock material at or above the Earth’s surface is called lava. At subduction zones, water is the key for solid rock to melt and become magma. The cup-like depression at the top of a volcano where lava usually exits is called the crater. Most volcanic activity is found at the edges of tectonic plates, namely at divergent and convergent plate boundaries, but does not occur at transform plate boundaries. Unit 10—Earth Science Chapter 29 Formation of Rocks Unit 10—Earth Science Chapter 29 Formation of Rocks A large belt of mountain ranges and volcanoes surround the Pacific Ocean. Earthquakes are common in this area known as the Ring of Fire. There are three main types of volcanoes: shield volcanoes, stratovolcanoes (or composite volcanoes), and cinder cones. Low viscosity, fast-flowing lava, low in silica content and made principally from melted basalt, is associated with shield volcanoes. Because this lava easily flows down hill, shield volcanoes are gently sloped and flattened. The gently-sloped, dome-shaped Hawaiian Islands are examples of hot-spot shield volcanoes, as are the volcanoes of Iceland. Unit 10—Earth Science Chapter 29 Formation of Rocks High viscosity lava is associated with stratovolcanoes (also called composite volcanoes). High silica content can be found in stratovolcano lava These volcanoes range in height from 500 to 10,000 meters high. Mt. St. Helens is a stratovolcano. Cinder cone volcanoes are steep stacks of loose pyroclasts (clumps and particles of lava). Cinder cones are rarely higher than 300 meters. Crater Lake National Park (Oregon) contains a cinder cone, Wizard Island, which actually formed in the caldera of a stratovolcano. Lava viscosity also determines how explosive an eruption will be. Explosive eruptions occur when the lava has a lot of water and dissolved gases. Gentle eruptions are associated with fast-flowing lava from oceanic crust. Unit 10—Earth Science Chapter 29 Formation of Rocks Volcanoes also form when an oceanic plate slides under another oceanic plate. Hydrothermal vents are deep sea, chimney-like structures that occur along midocean ridges. Unit 10—Earth Science Chapter 29 Formation of Rocks Some gemstones are also associated with volcanic activity. For example, diamonds form at high temperatures deep underground when carbon crystallizes inside rocks called kimberlites. Geothermal energy is the useful product of volcanic activity. Geothermal energy is energy derived from the heat and steam associated with magma close enough to the earth’s surface to harness. When steam from magma collects below ground, it can be tapped just like water in a well. The pressurized steam can be used to generate electricity. Volcanic activity results in the formation of two kinds of igneous rocks: extrusive intrusive A batholith is a large underground rock that formed when a mass of magma cooled underground. Unit 10—Earth Science Chapter 29 Formation of Rocks 29.2 The Surface of Earth Earth’s surface is constantly changing. Recall that earthquakes, volcanoes, mountains, and the construction of new lithosphere are events that occur at plate boundaries. These events are changing the appearance of Earth’s surface all the time. The features we see on Earth’s surface represent the dynamic balance between constructive processes versus destructive processes. Mountain-building is a major constructive process. Mountains form in three main ways: by folding at convergent plate boundaries (fold mountains) by movement of chunks of land at faults (faultblock mountains) by volcanic activity (volcanic mountains) Unit 10—Earth Science Chapter 29 Formation of Rocks Topography and Erosion Topography refers to features and formations, like bodies of water and mountains, that characterize Earth’s surface. Observing hill slopes, stream patterns, and bedrock structures would help you describe landscape topography. Local climate and geology have the greatest influence on the development of a natural landscape. Erosion (also known as weathering) is a major destructive process. Weathering is defined as the collective processes in which soil and rock are eroded as a result of climate and seasonal changes. Erosion describes the continuous physical and chemical events that cause land and rock to wear down. The substance that has the greatest effect on the rate at which rocks weather is water. Unit 10—Earth Science Chapter 29 Formation of Rocks The rate of erosion of mountains (previous page) is related to the height and steepness of the mountain—the steeper the mountain is, the faster it erodes because it is easier to push material down a steep slope than a gradual slope. A soil profile is a cross-section that shows the different layers of soil in the ground. It shows the weathered products of rock. It takes a long time and a lot of weathering for soil to have all the layers. Young soil does not have each of these layers. Topsoil, in Horizon A, is important for growing crops. It contains the most nutrient-rich soi. The topsoil can increase in thickness when biological activity increases since more roots will hold soil in place and plants will serve as a barrier to erosion at the surface. Horizon D is rock or bedrock. This is the horizon where the least amount of weathering has taken place. A glacier is a huge mass of ice on land that can be many kilometers thick and thousands of kilometers wide. Glaciers erode the land as they move. Glaciers at the poles are a frozen form of about 2 percent of all the water on Earth. Glaciers are formed from the accumulation of snow over hundreds or thousands of years. Unit 10—Earth Science Chapter 29 Formation of Rocks Geologic hazard maps indicate the location of faults where earthquakes occur, areas where volcanoes are active, and where landslides, avalanches, floods, or other natural hazards are possible. The term urban sprawl refers to how living areas around a city “sprawl” as they grow instead of concentrate near facilities that serve the people of the community. Building roads changes the land. Roads and parking lots prevent water from slowly seeping into the ground to replenish the water supply in aquifers. Increased pollution and loss of open space are typical results of urban sprawl. The most practical way individuals can help control the adverse effects associated with urban sprawl is by becoming aware of the problems associated with urban sprawl and carefully plan the use of our environment. Unit 10—Earth Science Chapter 29 Formation of Rocks 29.3 Rocks and Minerals A mineral is a solid, naturally-occurring, in organic object with a defined chemical composition. Minerals are inorganic and have a crystalline structure. Minerals are the building blocks of a rock. Granite, at the right, is a type of rock. The minerals horneblende, quartz, mica, and feldspar are found in this rock. Faulted rock systems (areas where rocks are exposed) would be the best place to start prospecting for precious minerals like gold and silver . Graphite and diamonds are two different minerals that are made of pure carbon. Diamond is the hardest naturally occurring substance, while graphite is quite soft (pencil “lead”). Their very different properties arise from the fact that the carbon atoms in the diamond are bonded or connected differently than the carbon atoms in the graphite. A mineral is a material that is naturally occurring, inorganic, and crystalline. There are more than 3,000 minerals on Earth. About 20 minerals make up Earth’s crust. Some Common Minerals Name and Uses chemical formula Silver, Ag Jewelry, electrical wire, coins Corundum, Al2O3 Sandpaper,gems (ruby, sapphire) Quartz, SiO2 Glass making, gems (onyx, amethyst) Gypsum, Used to make Plaster of CaSO4∙2H2O Paris Mohs hardness scale was developed in 1812 by Friedrick Mohs (an Austrian mineral expert) as a method to identify minerals. The hardness of a mineral can be determined by using it to scratch other solid material of known hardness. Unit 10—Earth Science Chapter 29 Formation of Rocks Most minerals (except metals) have one or more cleavage planes that also help in determining their identity. Cleavage planes are flat surfaces that minerals break along when struck. A rock is a naturally formed solid usually made of one or more minerals. The terms igneous, sedimentary, and metamorphic refer to the origin of a rock, how a rock was formed. Igneous rocks form by the cooling and solidification of magma or lava. Igneous rocks usually contain crystals since the cooling often occurs underground where the process takes a long time so that crystals can form. A crystal is a solid with a definite internal structure of atoms arranged in a regular, repeating pattern. Sedimentary rocks are found in many areas on most continents. Sedimentary rocks are formed from the compression and cementing of weathered particles (called sediment). Sediment must be compressed and cemented before it can change to sedimentary rock. Preserved animal remains are most commonly found in sedimentary rock. Metamorphic rocks are formed by intense pressure or heat applied to an existing rock. The rock cycle is the set of processes leading to the formation and recycling of various kinds of rock. The rock cycle is depicted on the next page and illustrates the formation and recycling of rocks by geological processes. Unit 10—Earth Science Chapter 29 Formation of Rocks Identifying Rocks--Observe and ask questions: What does the rock look like? Examine the grain of a section. What is the rock's composition? Determine what minerals are in it. Where was the rock found? Mountain Stream or river Volcano Ocean floor