Chapter 20 Lecture Metamorphic Rocks Chemical Sedimentary Rocks Coal: Different from other rocks, because it is composed of organic material. Stages in coal formation (in order): Plant material Peat Lignite Bituminous coal Anthracite coal Sedimentary Rocks Fossils = remains of life forms Found in sedimentary rocks Permineralization Impression (cast) Replacement Carbonization L04 1
Sedimentary Rocks CHECK YOUR KNOWLEDGE The most characteristic feature of sedimentary rocks is A. they contain fossils. B. the lithification and cementation of sediments. C. the layered sequence of strata. D. the fusing of unconsolidated sediments into solid rock. Metamorphic Rocks Metamorphic rocks are produced from: Igneous rocks Sedimentary rocks Other metamorphic rocks Metamorphism occurs via recrystallization and mechanical deformation. Metamorphic Rocks Contact metamorphism: a body of rock is intruded by magma typically associated with: high temperatures and high water content lots of chemical activity, not much, or no, mechanical deformation. L04 2
Metamorphic Rocks Regional metamorphism: The alteration of rock by both heat and pressure over an entire region rather than just near a contact between rock bodies. Appalchian Mtns. In central Pennsylvania Classifying Metamorphic Rocks Metamorphic rock is defined by appearance and mineral content. Classified into two groups: Foliated layered in sheets Nonfoliated not layered Classifying Metamorphic Rocks Foliated metamorphic rock: Layered in sheets (a) Slate (b) Schist (c) Gneiss L04 3
Classifying Metamorphic Rocks Nonfoliated metamorphic rock: Not layered (a) Marble (b) Quartzite Metamorphic Rocks CHECK YOUR NEIGHBOR Marble is an example of: A. Crystalline, metamorphosed limestone. B. Foliated metamorphic rock. C. Recrystallized sandstone. D. Nonfoliated mica. The Rock Cycle Molten rock rises from the depths of Earth, cools, solidifies, and eventually returns to become magma again. L04 4
Chapter 20 Lecture Chapter 21: Plate Tectonics and Earth's Interior This lecture will help you understand: Seismic Waves Earth's Internal Layers Continental Drift Seismic Waves When rock under Earth's surface moves or breaks, energy travels in the form of seismic waves, which cause the ground to shake and vibrate an earthquake. Analysis of seismic waves provides geologists with a detailed view of Earth's interior. L04 5
Seismic waves Study of seismic waves has led scientists to understand that Earth is a layered planet consisting of: Crust Mantle Outer core Inner core Seismic Waves Two main types of seismic waves: Body waves travel through Earth's interior Primary waves (P-waves) Secondary waves (S-waves) Surface waves travel on Earth's surface Rayleigh waves Love waves L04 6
Seismic Waves Body Waves: Primary Waves Longitudinal waves: They compress and expand the material through which they move. Compression/expansion occurs parallel to the wave's direction of travel. Travel through any type of material. Are the fastest of all seismic waves. http://media.pearsoncmg.com/aw/aw_hewitt_cps_4/interactive_figures/cps_4e_if_22_03.swf Seismic Waves Body Waves: Secondary Waves are transverse: They vibrate the rock in an up-and-down or side-to-side motion. Transverse motion occurs perpendicular to a wave's direction of travel. travel through solids, but unable to move through liquids. slower than P-waves. L04 7
Seismic Waves Surface Waves slowest seismic waves Rayleigh waves - rolling-type of motion: They roll over and over in a tumbling motion, similar to ocean wave movement. Ground moves up and down. Love waves have similar motion to S-waves: Horizontal motion side to side. Motion - perpendicular to the wave's direction of travel. Seismic Waves CHECK YOUR NEIGHBOR The most destructive earthquakes are caused by the passage of surface waves, because A. they travel faster than other seismic waves. B. they occur in the crust, the densest layer of the Earth. C. they occur at the surface where the ground shakes up and down and from side to side. D. they travel deep into the Earth's interior. Seismic Waves: Earth's Interior Abrupt changes in seismic-wave velocity reveal boundaries between different materials within the Earth. The densities of the different layers can be estimated by studying the various seismic-wave velocities. L04 8
Earth's Internal Layers: Core Mantle Boundary P-waves and S-waves travel together for a distance, then encounter a boundary where the S- waves stop and the P-waves refract (Richard Oldham,1906). This limit is at 2900 km inside the Earth (Beno Guttenberg, 1913) Crust Mantle Boundary Sharp increase in seismic velocity at a shallow layer within Earth (Andrija Mohorovičić, 1909). Earth is composed of a thin, outer crust that sits upon a layer of denser material, the mantle. Earth's Internal Layers: Core Mantle Boundary When P-waves reach the depth of 2900 km, they refract so strongly that the boundary casts a P-wave shadow (where no waves are detected) over part of the Earth. Inner Core - Outer Core Boundary The core, or part of it, must be liquid (Sir Harold Jeffries, 1926). P-waves also refract at a certain depth within the core. At this depth, P-waves show an increase in velocity, indicating higher density material (Inge Lehmann, 1936). Earth's Internal Layers: Taken together, the discoveries of Oldham, Mohorovičić, Gutenberg, and Jeffreys indicate that Earth is composed of three layers of different compositions: the crust, mantle, and core. Lehmann discovered that the core has two parts: a liquid outer core and a solid inner core. L04 9
Earth's Internal Layers: The Core Composed predominantly of metallic iron. It has two layers a solid inner core and a liquid outer core. The inner core is solid due to great pressure. The outer core is under less pressure and flows in a liquid phase. Flow in the outer core produces Earth's magnetic field. Earth's Internal Layers: The Mantle Represents 82% of Earth's volume and 65% of Earth's mass. It is Earth's thickest layer 2900 km from top to bottom. Mantle rock is rich in silicon and oxygen. It also contains heavier elements, such as iron, magnesium, and calcium. It is divided into two regions upper mantle and lower mantle. http://mediaplayer.pearsoncmg.com/assets/secs-42mantlecrust Earth's Internal Layers The Asthenosphere The upper mantle has two zones: The Asthenosphere and the Lithosphere The lower part of the upper mantle is called the asthenosphere. Solid but behaves in a plastic-like manner, allowing it to flow easily. The flowing motion of the asthenosphere greatly affects the surface features of the crust. L04 10
Earth's Internal Layers The Lithosphere The upper mantle has two zones: The Asthenosphere and the Lithosphere The lithosphere includes the uppermost part of the upper mantle plus the crust. Cool and rigid. It does not flow but rides atop the plastically flowing asthenosphere. Brittle => broken up into individual plates. Movement of lithospheric plates cause earthquakes, volcanic activity, and deformation of rock. Earth's Internal Layers: The Lower Mantle From 700 km to the outer core. Solid, due to the great pressure. Earth's Internal Layers The Crust Oceanic crust basaltic rocks (dense) average thickness of 10 kilometers. Continental crust granitic rocks (less dense) thickness between 20 and 60 kilometers. L04 11
Earth's Internal Layers Isostasy The word isostasy is derived from the Greek roots "iso" (equal) and "stasis" (standing) equal standing. Isostasy is the vertical positioning of the crust so that gravitational and buoyant forces balance one another. Low-density crust floats on the denser, underlying mantle. Earth's Internal Layers Isostasy Why are continents high and oceans low? Isostasy! Variations in surface elevations result from variations in the thickness and the density of the crust. Areas of continental crust stand higher than areas of oceanic crust, because continental crust is thicker and less dense than oceanic crust. Isostasy CHECK YOUR NEIGHBOR The Earth's crust is thicker beneath a mountain, because A. the roots of the mountain are heavier than the mountain at the surface. B. mountains sink until the upward buoyant force balances the downward gravitational force. C. mantle rock is weak beneath the mountain. D. oceanic crust is thin. L04 12
Continental Drift An Idea Before Its Time Alfred Wegener (1880 1930) Continental drift hypothesis: The world's continents are in motion and have been drifting apart into different configurations over geologic time. The continents were at one time joined together to form the supercontinent of Pangaea "universal land" Continental Drift An Idea Before Its Time Wegener used evidence from many disciplines to support his hypothesis Jigsaw fit of the continents Fossil evidence Matching rock types Structural similarities in mountain chains on different continents Paleoclimatic evidence Continental Drift An Idea Before Its Time Despite evidence to support continental drift, Wegener could not explain how continents moved. Without a suitable explanation, Wegener's ideas were dismissed. L04 13
Acceptance of Continental Drift Detailed mapping of the seafloor revealed: Huge mountain ranges in the middle of ocean basins Deep trenches alongside some continental margins Acceptance of Continental Drift Seafloor Spreading Harry Hess' hypothesis of seafloor spreading provided the mechanism for continental drift: The seafloor is not permanent, it is constantly being renewed. Mid-ocean ridges are sites of new lithosphere formation. Oceanic trenches are sites of lithosphere destruction (subduction). Acceptance of Continental Drift Seafloor Spreading Seafloor Spreading Is Supported By Magnetic Studies of the Ocean Floor Lava erupted at the mid-ocean ridges is rich in iron. Magnetite crystals align themselves to Earth's magnetic field. Earth's magnetic poles flip the north and south poles exchange positions (magnetic reversal). L04 14
Acceptance of Continental Drift Seafloor Spreading Seafloor Spreading Is Supported By Magnetic Studies of the Ocean Floor The seafloor holds a record of Earth's magnetic field at the time the rocks of the seafloor cooled. The magnetic record appears as parallel, zebra-like stripes on both sides of mid-ocean ridges. The age of the ocean floor and the rate of seafloor spreading could be determined. L04 15