Chapter 7 Plate Tectonics
Earthquakes Earthquake = vibration of the Earth produced by the rapid release of energy.
Seismic Waves Focus = the place within the Earth where the rock breaks, producing an earthquake. Epicenter = the point on the ground surface directly above the focus. Energy moving outward from the focus of an earthquake travels in the form of seismic waves.
1. Body waves a. P-waves b. S-waves Types of Seismic Waves
Types of Seismic Waves 1. Body waves a. P-waves Primary, pressure, push-pull Fastest seismic wave (6 km/sec in crust; 8 km/sec in uppermost mantle) Travel through solids and liquids b. S-waves
Types of Seismic Waves 1. Body waves a. P-waves b. S-waves Secondary, shaking, shear, side-to-side Slower (3.5 km/sec in crust; 5 km/sec in upper mantle km/sec) Travel through solids only
Types of Seismic Waves 1. Body waves a. P-waves b. S-waves 2. Surface waves Love and Rayleigh waves Slowest Complex motion Up-and-down and side-to-side Causes damage to structures during an earthquake
Seismogram showing Seismic Wave Arrivals
Seismographs Earthquakes are recorded on an instrument called a seismograph. The record of the earthquake produced by the seismograph is called a seismogram.
Determining the Earth's Internal Structure Earth has a layered structure. Boundaries between the layers are called discontinuities. Mohorovičić discontinuity (Moho) between crust and mantle Gutenberg discontinuity between mantle and core
Determining the Earth's Internal Structure The layered structure is determined from studies of how seismic waves behave as they pass through the Earth. P- and S-wave travel times depend on properties of rock materials through which they pass. Differences in travel times correspond to differences in rock properties.
Determining the Earth's Internal Structure Seismic wave velocity depends on the density and elasticity of rock. Seismic waves travel faster in denser rock. Speed of seismic waves increases with depth (pressure and density increase downward).
Determining the Earth's Internal Structure Curved wave paths indicate gradual increases in density and seismic wave velocity with depth. Refraction (bending of waves) occurs at discontinuities between layers.
Place where no S- waves are received by seismograph. Extends across the globe on side opposite from the epicenter. S-waves cannot travel through the molten (liquid) outer core. Larger than the P-wave shadow zone. S-wave Shadow Zone
Place where no P-waves are received by seismographs. Makes a ring around the globe. Smaller than the S-wave shadow zone. P-wave Shadow Zone
The Earth's Internal Layered Structure Crust Mantle Outer core Inner core
Crust Continental Crust (granitic) Oceanic Crust (basaltic) Basaltic crustal rocks are more dense than granitic crustal rocks. The Mohorovicic (Moho) discontinuity, determined by seismic reflection is the boundary between the crust and upper mantle.
Oceanic Crust Basaltic composition 5-12 km thick More dense (about 3.0 g/cm 3 ) Has layered structure consisting of: Thin layer of unconsolidated sediment covers basaltic igneous rock (about 200 m thick) Pillow basalts - basalts that erupted under water (about 2 km thick) Gabbro - coarse grained equivalent of basalt; cooled slowly (about 6 km thick)
Lithosphere 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 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.
Isostasy Buoyancy and floating of the Earth's crust on the mantle. Denser oceanic crust floats lower, forming ocean basins. Less dense continental crust floats higher, forming continents. As erosion removes part of the crust, it rises isostatically to a new level.
Isostasy is Isostatic adjustment to erosion and gravity.
The Earth's Internal Layered Structure
Mantle Composed of oxygen and silicon, along with iron and magnesium (based on rock brought up by volcanoes, density calculations, and composition of stony meteorites). Peridotite (Mg Fe silicates, olivine) Kimberlite (contains diamonds) Eclogite 2885 km thick Average density = 4.5 g/cm 3 Not uniform. Several concentric layers with differing properties.
Core Outer core Molten Fe (85%) with some Ni. May contain lighter elements such as Si, S, C, or O. 2250 km thick Liquid. S-waves do not pass through outer core. Inner core Solid Fe (85%) with some Ni 1220 km radius (slightly larger than the Moon) Solid
Core and Magnetic Field 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.
Crustal Structures - Faults A fault is a crack in the Earth's crust along which movement has occurred. Types of faults: Dip-slip faults - movement is vertical Normal faults Reverse faults and thrust faults Strike-slip faults or lateral faults - movement is horizontal.
Faults
Fault terminology
Crustal Structures - Folds During mountain building or compressional stress, rocks may deform plastically to produce folds. Types of folds Anticline Syncline Monocline Dome Basin
A. Anticline B. Syncline C. Monocline D. Dome E. Basin Folds
Anticline
Aerial view of an anticline
Syncline
Folded strata, Switzerland
Strike and dip
Measuring strike and dip with a Brunton compass
Plate Tectonics Plate Tectonic theory was proposed in late 1960s and early 1970s. It is a unifying theory showing how a large number of diverse, seemingly-unrelated geologic facts are interrelated. An outgrowth of the old theory of "continental drift," supported by much data from many areas of geology.
The Data Behind Plate Tectonics Geophysical data collected after World War II provided foundation for scientific breakthrough: Echo sounding for sea floor mapping discovered patterns of midocean ridges and deep sea trenches. Magnetometers charted the Earth's magnetic field over large areas of the sea floor. Global network of seismometers (established to monitor atomic explosions) provided information on worldwide earthquake patterns.