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LECTURE 10. MATERIALS OF THE EARTH'S CRUST Structure of the Earth Studies of seismic waves have revealed that the Earth has four internal layers - the solid inner and molten outer core, consisting of iron and nickel; the mantle and crust, consisting of various types of rock. Harry Williams, Earth Science 1 Harry Williams, Earth Science 2 Geography is mainly concerned with the CRUST, because that is the zone of human habitation. Compared to the size of the Earth, the crust is small; oceanic crust averages only about 5 miles thick; continental crust about 20 miles thick. The base of the CRUST is marked by a sharp increase in density - known as the MOHO DISCONTINUITY - this marks the transition from lower density rocks of the crust to higher density rocks of the mantle. The upper part of the mantle is rigid and solid together with the overlying crust this forms the LITHOSPHERE (to about 70 km depth). Below this is the ASTHENOSPHERE: a weak part of the mantle extending to about 250 km depth, within which the rock is partially molten and capable of flowing under pressure. Harry Williams, Earth Science 3 MINERALS. As mentioned previously, the crust is made of rock - but what are rocks made of? The answer is "MINERALS"; a combination of atoms of various elements. Relatively few elements make up most of the Earth's crust. Harry Williams, Earth Science 4 Atoms of these elements become bonded together in specific 3-dimensional arrangements to form crystalline substances. The packing/arrangement of atoms depends primarily on their size. Therefore, the type of atoms, type of atomic bonding and size of atoms determines; MINERAL COMPOSITION (type of atoms), STRENGTH (bond strength) and STRUCTURE (arrangement of atoms) (In other words, the PHYSICAL AND CHEMICAL PROPERTIES of the mineral). Harry Williams, Earth Science 5 Atomic structure of the mineral halite (chemical name = sodium chloride; common name = table salt). The sodium and chloride ions (charged atoms) are bonded by opposite electrical charges. The size of the ions determines how they pack together. Harry Williams, Earth Science 6 Most minerals are quite small, but millions of mineral particles join together to form ROCKS. Fluorite Halite Harry Williams, Earth Science Calcite 7 Harry Williams, Earth Science 8 ROCKS Rocks are classified by their origin: 1. IGNEOUS: formed from magma (molten rock). 2. SEDIMENTARY: formed from the accumulation of rock and mineral fragments or precipitation of dissolved minerals. 3. METAMORPHIC: rocks altered by heat, pressure and/or chemical action. Harry Williams, Earth Science 9 Igneous rocks are formed by cooling of molten rock, which is referred to as MAGMA (below surface in magma chambers) or LAVA (on surface). Classification of igneous rocks is based on TEXTURE (crystal sizes) and MINERAL CONTENT. Extrusive Intrusive Harry Williams, Earth Science 10 TEXTURE: The size of mineral crystals within an igneous rock depends on the rate of cooling of the magma or lava. The rate of cooling is usually very fast for extrusive rocks, producing fine grains; and very slow for intrusive rocks, producing coarse grains. Harry Williams, Earth Science 11 Intrusive Extrusive Harry Williams, Earth Science 12 Granite is a very common intrusive igneous rock, while BASALT is the most abundant extrusive igneous rock on Earth, for the simple reason that all the ocean floors of the world are made of basalt (the reason why will be explained later in the section on plate tectonics). Harry Williams, Earth Science 13 Basalt is EXTRUSIVE - formed from LAVA erupting onto the surface of the earth (the ocean floor and land). Harry Williams, Earth Science 14 It should be noted that although intrusive rocks form below the surface, many are later exposed by erosion; for example, the Sierra Nevada mountains of Yosemite Park are granite. Half Dome Mountain Harry Williams, Earth Science 15 An example of basalt = the Columbia Plateau of Washington/ Oregon; and Hawaii Harry Williams, Earth Science 16 The Hawaiian Islands are large basaltic volcanoes that have grown from the ocean floor to rise above sea level in the Pacific. Harry Williams, Earth Science 17 SEDIMENTARY ROCKS Rock exposed at the surface of the Earth is subject to disintegration and solution by the elements of the weather (rain, ice, wind etc.). This WEATHERING produces rock and mineral fragments and dissolved minerals called SEDIMENT (eg. boulders, gravel, sand, mud, clay). Harry Williams, Earth Science 18 Sediment is often carried by rivers into lakes and seas where it accumulates to form layers. After some time, these layers are compressed by the weight of overlying sediment and dissolved minerals precipitate into spaces between particles forming a cement - the sediment layers are thereby LITHIFIED into solid sedimentary rock strata. Harry Williams, Earth Science 19 Common examples of such CLASTIC sedimentary rocks are sandstone and shale. Sandstone (cemented sand particles). Harry Williams, Earth Science 20 Shale (cemented clay particles). Harry Williams, Earth Science 21 There are also organic sedimentary rocks made from the remains of organisms; for example many marine organisms have calcium carbonate shells, the fragments of which form limestone. These shells can also dissolve and be precipitated as limestone without shell fragments. Harry Williams, Earth Science 22 The white cliffs of Dover are made of CHALK, which is a type of LIMESTONE. These cliffs formed as thousands of layers of calcium carbonate mud on the ocean floor - the layering is visible. Harry Williams, Earth Science 23 Most of Dallas County is on Austin Chalk, shown here at Tenmile Creek, DeSoto. Harry Williams, Earth Science 24 West Dallas County contains Eagle Ford Shale, shown here near Mountain Creek Lake. Harry Williams, Earth Science 25 The Dallas/Tarrant county border is on Woodbine sandstone, shown here at Arlington. The sandstone contains wood and oyster fossils. Harry Williams, Earth Science 26 Enchanted Rock, in the Hill Country, is a granite dome. Harry Williams, Earth Science 27 METAMORPHIC ROCKS A metamorphic rock is any rock that has been changed by heat and/or pressure (resulting from earth movements), or chemical action. Heat and pressure causes rock to become "PLASTIC", meaning that atoms and minerals can move around and slowly slide past each other. The atoms can join together in new ways to form different minerals and often the pressure produces a FOLIATED STRUCTURE. Harry Williams, Earth Science 28 1. Granite -> Gneiss Harry Williams, Earth Science 29 2. Shale + low intensity metamorphism -> clay minerals change to tiny mica flakes = slate. Shale + medium intensity metamorphism -> clay minerals change to larger mica flakes = phyllite. phyllite slate Harry Williams, Earth Science 30 3. Shale (and other rocks) + high intensity metamorphism -> schist. Accessory minerals, unique to metamorphic rocks, may form e.g. below is a garnet mica schist. Harry Williams, Earth Science 31 4. Sandstone -> quartzite. Harry Williams, Earth Science 32 5. Limestone -> marble. Harry Williams, Earth Science 33