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Background Information for Plate Tectonics & Rock Formation Background concepts needed to achieve objectives. Basic understanding of the Earth’s Interior 1. Crust: made up of oceanic crust (basalt – dense) & continental crust (granite – less dense than basalt), solid 2. Mantle – made up of three parts. Lithosphere (solid, floats on asthenosphere). Asthenosphere (soft material that can slowly flow). Outer mantle (solid). 3. Core – Outer Core (liquid metal). Inner Core (Solid metal). 4. Temperature & Pressure increase as you go towards the center of the Earth Heat Transfer & Convection 1. Heat is always transferred from a warmer substance to a cooler substance 2. Convection is heat transfer by the movement of a heated fluid. It is caused by differences in temperature and density within the fluid. Warmer materials are less dense because the molecules move faster and spread apart. Cooler materials are more dense because the molecules move slower and are closer together. Gravity pull the more dense materials down and the less dense materials rise. 3. A convection current is the flow that transfers heat within a fluid. In the Earth’s Mantle the Asthenosphere flows. The heat from the Earth’s core heats up the lower asthenosphere. This materials becomes less dense than the material above it and it starts to rise. Gravity pulls the cooler material down towards the core. Because the lower mantle and lithosphere are solid the asthenosphere continuously flows up and down. 4. These convection currents inside the asthenosphere cause the lithosphere to move. This movement allows the continents to drift. Continental Drift is the hypothesis that all continents had once been joined together in a single landmass know as Pangaea. Sea-Floor Spreading 1. Ocean thermal vents are places in the Ocean floor where ocean water sinks through cracks, or vents, into the crust. This water is heated by contact with hot material from the mantle and then spurts back into the ocean. In these areas where there is no light and where temperatures would normally be freezing there is tons of life living around theses vents. 2. Mid-Ocean Ridge is the longest chain of mountains in the world. This ridge extends into all of the Earth’s oceans. At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. 3. Sea-Floor Spreading is the process that continually adds new material to the ocean floor at the mid-ocean ridge. As new material is added the old rock moves aside in both directions. 4. Deep-ocean Trenches are underwater canyons where the oceanic crust bends downwards and is pushed back into the mantle. Without the existence of these trenches the ocean floor would continuously increase because of sea-floor spreading. 5. Subduction is the process by which the ocean floor sinks through a deepocean trench and back into the mantle. The ocean floor near a deep ocean trench is cooler and more dense than the ocean floor at the mid-ocean ridge. This more dense material is heavier and over tens of millions of years will sink back into the mantle at the trenches. Plate Tectonics 1. Plate Tectonics is the geological theory that states that pieces of the Earth’s crust are in constant, slow motion, driven by convection currents in the mantle. As the plates move, they collide, pull apart, or grind past each other, producing volcanoes, earthquakes, mountain ranges and deep-sea trenches. 2. There are three types of plate boundaries. a. Transform Boundary. Here plates slide past one another moving in opposite directions. No crust is created or destroyed. Earthquakes are common here. b. Divergent Boundary. A place where two plates move apart. Most occur at the mid-ocean ridge. On land a deep valley called a rift valley is formed. c. Convergent Boundary. A place where two plates come together. The density of the plates decides what happens. When oceanic crust collides with oceanic crust or continental crust the more dense material will sink below the other into a deep-ocean trench. Volcanoes are common here. When continental crust collides with continental crust the less dense granite squeeze into mountain ranges. Minerals 1. A mineral is a naturally occurring, inorganic solid that has a crystal structure and a definite chemical composition. There are over 3,000 minerals. About 20 of these minerals make up most of the rocks of the Earth’s crust. a. Inorganic: means that the mineral cannot arise from materials that were once part of a living thing. b. Crystal structure: particles in a mineral line up in a unique pattern that repeats over and over again. It has flat sides that meet at sharp edges and corners. c. Chemical composition: a mineral always contains the same elements in the same proportions. 2. Identifying minerals. a. Hardness: A mineral can scratch any mineral softer than itself, but will be scratched by any mineral that is harder. Compare to Mohs Hardness Scale. b. Color. c. Streak: the color of its powder. This can be observed by rubbing a mineral against a piece of unglazed tile. d. Luster: how a mineral reflects light from its surface. Terms to describe luster include: bright, metallic, shiny, glassy, earthy, waxy and pearly. e. Density: the mass in a given space. Dividing the samples mass by its volume gives the density of the mineral. f. Crystal Shape: Look at the number and angle of the crystal faces. These can be classified into six groups. g. Cleavage and Fracture: the way a mineral breaks. Cleavage is a mineral splits easily along flat surfaces. Fracture describes how a mineral looks when it breaks. Some terms to identify fracture include: shell-shapes, hackly (jagged), earthy (crumble easily), and uneven (rough, irregular surfaces). h. Special properties: Some glow (fluorescence), magnetic, radioactive, react to acid, electrical, etc. 3. Minerals can be formed in two ways: a. Crystallization of melted materials: As hot magma cools inside the crust or as lava hardens on the surface. The size of the crystals depends on the rate at which the magma cools, the amount of gas it contained, and the chemical composition of the magma. Magma deep below the surface cools slowly and forms large crystals, magma towards the surface or lava on the surface cools quickly and forms smaller crystals. b. Crystallization of materials dissolved in water: Magma can heat water causing minerals to dissolve inside it producing a solution. A solution is a mixture in which one substance dissolves inside another. When the hot water solution begins to cool minerals are left behind. Many minerals form from solutions at places where tectonic plates spread apart along the mid-ocean ridge and at ocean thermal vents. c. Minerals formed by evaporation: Minerals can form when solutions evaporate. Ex. Several minerals form from the evaporation of seawater (halite, gypsum, calcite, and minerals containing potassium) 4. Minerals are the source of metals, gemstones, and other materials used to make many products. a. Gemstones: rubies & saphires b. Metals: aluminum, iron, copper, silver c. Other uses: foods, medicines, fertilizers, and building materials. Ex. Talc is made into talcum powder, Flourite is used to make aluminum & steel, Calcite is used in microscopes, Quartz is used to make glass, watches & electronic equipment, Kaolin is used to make high-quality china &pottery, Gypsum is used to make wallboard, cement & stucco, Corundum is used in polishing & cleaning products. d. Ore: a rock that contains a metal or economically useful mineral. Most metals are not in pure form they are a combination of that metal & other elements. Smelting is the process of removing a metal from an ore. An alloy is a solid mixture of two or more metals. Rocks 1. Rocks are made of mixtures of minerals and other materials. 2. Classifying Rocks: a. Color b. Texture: the look and feel of the rock’s surface (smooth & glassy, rough or chalky). Most rocks are made up of particles of minerals or other rocks, which geologists call grains. Grain Size: Large and easy to see are called coarse-grained. Small enough to only be seen under a microscope are called fine-grained. Grain Shape: They can look like tiny particles of sand, seeds, exploding stars or they can be fragments of other rock and be smooth and rounded or jagged. Grain Pattern: Some lie in flat layers and look like a stack of pancakes, others have a swirling pattern, some occur in rows and others are random. No visable grain: Not even under a microscope. Rocks like this cooled very quickly giving them a smooth, shiny texture like glass. c. Mineral Composition: By looking at a small sliver of a rock under a microscope a geologist can observe the shape and size of the crystals in the rock and identify the minerals. Mineral tests can also be used such as Mohs hardness scale, acid test, and magnets. 3. Formation of rocks: a. Igneous rock forms from the cooling of molten rock – either magma below the surface or lava at the surface. Extrusive rocks formed from lava that erupted onto the Earth’s surface. (Ex. Basalt) Rapid cooling lava forms finegrained rock with small crystals or glassy texture with no visable crystals. Intrusive rocks formed when magma hardened below the Earth’s surface. (Ex. Granite) Slow cooling magma forms coarse-grained rock with large crystals. Uses: Most are hard, dense and durable. They have been used for tools and building materials. Obsidian – tools, Granite – buildings, bridges, paving streets, decorative stonework, Basalt – crushed to make gravel used in construction, Pumice – abrasive used for cleaning & polishing, Obsidian – heated to form perlite which is used in place of soil for starting seeds. b. Sedimentary rock forms when particles of other rocks or the remains of plants and animals are pressed and cemented together. This rock forms in layers below the surface. Forces such as heat, cold, rain, waves & grinding ice constantly break up the Earth’s surface. Erosion is when running water, wind or ice loosen and carry away fragments of rock. As the water or wind slows down is deposits its sediment. In water these materials sink to the bottom of the ocean or lake. Deposition is the process by which sediment settles out of the water or wind carrying it. These deposits may include shells, bones, leaves, stems, and other remains of living things. Compaction is the process that presses sediments together. As more and more sediment falls on top, creating new layers, the weight of the layers squeezes the sediments tightly together. During compaction minerals in the rock dissolve in water and seep into the spaces between the particles of sediment. Cementation is the process in which dissolved minerals crystallize and glue particles of sediment together. This process takes millions of years. There are three types of sedimentary rocks: Clastic, Organic and Chemical. Clastic rocks forms when rock fragments are squeezed together. Ex. Shale, sandstone, conglomerate & breccia. Organic rocks form where the remains of plants and animals are deposited in thick layers. Ex. Coal (remains of swamp plants) & Limestone (produced from the hard shells of living things). Chemical rock forms when minerals that are dissolved in a solution crystallize (limestone) or from mineral deposits left when seas or lakes evaporate (rock salt). Uses: Sandstone & Limestone have been used as building materials or for decorating the outside walls of buildings. Limestone is important in smelting iron ore and for making cement. c. Metamorphic rock is formed when an existing rock is changed by heat, pressure, or chemical reactions. Most forms deep underground. Forms from collisions in the Earth’s plates where rock is pushed down towards the heat of the mantle. Pockets of rising magma can also produce metamorphic rock. Under pressure minerals change into other minerals. Classified by the arrangement of the grains that make up the rocks. Foliated means that the grains are arranged in parallel layers or bands (Ex. Slate, schist & gneiss). Nonfoliated means the mineral grains are arranged randomly (Ex. Marble & quartzite). Uses: Marble is used for buildings & statues. Slate is used for flooring, roofing, outdoor walkways or chalkboards. 4. Rock Cycle: Forces inside Earth and at the surface produce a rock cycle that builds, destroys and changes rocks in the crust. Plate movements drive the rock cycle by pushing rocks back into the mantle, where they melt and become magma again. Plate movements also cause the folding, faulting, and uplift of the crust that move rocks through the rock cycle. Subduction zones and the collision of continental plates contribute to the rock cycle. Subducting Oceanic Plates: Sand that becomes sandstone as part of the oceanic crust drifts towards a deep-ocean trench. As the trench, subduction returns the sandstone to the mantle where is melts, forms magma and eventually becomes igneous rock. Colliding Continental Plates: This process could squeeze sandstone from the ocean floor resulting in a metamorphic rock, quartzite. This rock eventually could end up at the top of a mountain range or plateau where it will eventually erode away.