Chapter 7 Earth Quakes Energy radiates out from the focus The focus is the place within the Earth where the rock breaks producing an earthquake The epicenter of an earthquake is the point on the ground s surface directly above the focus Seismic Waves P waves Primary faster Compressional S Waves Secondary Slower Transversal Earthquakes are recorded on an instrument called a seismograph The record of the earthquake produced by the seismograph is called a seismogram Seismic waves travel faster in denser rock 1 2 refraction bending of waves The boundaries between the layers are called discontinuities Mohorovicic discontinuity Moho separates the crust from the mantle Gutenberg discontinuity separates the mantle from the core Continental crust is less dense than oceanic crust Mantel Isostasy Buoyancy and floating of the Earth s crust on the mantle Lithosphere outermost 100 km of Earth Consists of the crust plus the outermost part of the mantle Divided into tectonic or lithospheric plates that cover surface of Earth Asthenosphere low velocity zone at 100 250 km depth in Earth seismic wave velocity decreases Rocks are at or near melting point Magmas generated here Solid that flows rheid plastic behavior Convection in this layer moves tectonic plates Outter Core Composition Molten Fe 85 with some Ni Convection in liquid outer core plus spin of solid inner core generates Earth s magnetic field Magnetic field is also evidence for a dominantly iron core Inner core is solid Fault A fault is a crack in the Earth s crust along which movement has occurred 1 Dip slip faults a Normal reverse 2 strike slip or lateral faults left or right Folds During mountain building or compressional stress rocks may deform plastically to produce folds Types 1 Anticline 2 Synclines 3 Monoclines 4 Domes 5 Basins Plate tectonics continental drift Echo sounding used for seafloor mapping discovered patterns of midocean ridges and deep sea trenches and provided images of rocks and sediments beneath the ocean Magnetometers charted the Earth s magnetic field over large areas of the seafloor A global network of seismometers was established to monitor atomic explosions As a side benefit it provided information on worldwide earthquake patterns Evidence to support 1 Shape of coastlines 2 Paleoclimatic evidence 3 Fossil evidence 4 Distribution of present day organisms 5 Geologic similarities 6 Rift Valleys 7 Mid ocean ridges 8 Paleomagnetic and polar wandering curves Magnetite Iron rich mineral When the lava cools to a certain temperature known as the Curie point the crystals become magnetized and aligned with Earth s magnetic field Driving force is convection Plate Boundaries Transform Mid ocean ridges Divergent rifting normal faults Convergent subduction zones volcanic island arcs The zone of convergence between the two continental plates is called a suture zone Wilson cycle The plate tectonic model for the opening and closing of an ocean basin over time is called a Wilson Cycle a Opening of a new ocean basin b seafloor spreading continues and subduction begins c continental collision Thermal plumes are concentrated areas of heat rising from near the core mantle boundary Hot spots are present on the Earth s surface above a thermal plume Calculating seafloor spreading Wide stripes long time narrow stripes short time The Benioff Zone provides evidence for subduction where one plate is sliding beneath another causing earthquakes As the plate moves across the hot spot a chain of volcanoes forms Microcontinents are moved along by seafloor spreading and may eventually arrive at a subduction zone They are too low in density and too buoyant to be subducted into the mantle so they collide with and become incorporated into the margin of a larger continent as an exotic terrane Chapter 8 The Archean Eon is the oldest unit on the geologic time scale sedimentary rocks from Australia are 4 4 billion years old The Archean and Proterozoic Eons comprise the Precambrian which spans 87 of the geologic time scale Lines of evidence that must be considered for any hypothesis on the origin of the Solar System 1 Planets revolve around sun in same direction counterclockwise 2 Planets lie roughly within sun s equatorial plane plane of sun s CCW rotation 3 Solar System is disk like in shape 4 Planets rotate CCW on their axes except for 1 Venus slowly clockwise 2 Uranus on its side 3 Pluto on its side 5 Moons go CCW around planets with a few exceptions 6 Distribution of planet densities and compositions is related to their distance from sun Inner terrestrial planets have high density 7 8 Outer jovian planets have low density 6 Age Moon rocks and meteorites are as old as 4 6 billion years Solar Nebula Hypothesis or Nebular Hypothesis 2 Cold cloud of gas and dust contracts rotates and flattens into a disk like shape 1 Roughly 90 of mass becomes concentrated in the center due to gravitational attraction 2 Turbulence in cloud caused matter to collect in certain locations 3 Clumps of matter begin to form in the disk 4 Accretion of matter gas and dust around clumps by gravitational attraction Clumps develop into protoplanets 5 Solar nebula cloud condenses shrinks and becomes heated by gravitational compression to form Sun 6 Ultimately hydrogen H atoms begin to fuse to form helium He atoms releasing energy heat and light The Sun ignites 7 The Sun s solar wind drives lighter elements outward causing observed distribution of masses and densities in the Solar System 8 Planets nearest Sun lose large amounts of lighter elements H He leaving them with smaller sizes and masses but greater densities than the outer planets Inner planets are dominated by rock and metal 9 Outer planets retain light elements such as H and He around inner cores of rock and metal Outer planets have large sizes and masses but low densities How long ago was the Solar System born Based on radiometric dates of moon rocks and meteorites the Solar System is about 4 6 billion years old Meteors are shooting stars The glow comes from small particles of rock from space being heated as they enter Earth s atmosphere Meteorites are chunks of rock from the Solar System that reach the Earth s surface The sun The Sun is a star Composition about 70 hydrogen and 27 helium and 3 heavier elements Sun s energy comes from fusion a thermonuclear reaction in which hydrogen atoms are fused together to form helium releasing energy Distance from the Sun helps maintain temperatures in the range where water is