* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download presentation source
Survey
Document related concepts
Ionospheric dynamo region wikipedia , lookup
Spherical Earth wikipedia , lookup
Post-glacial rebound wikipedia , lookup
Geochemistry wikipedia , lookup
Earth's magnetic field wikipedia , lookup
History of Earth wikipedia , lookup
Geomagnetic reversal wikipedia , lookup
Schiehallion experiment wikipedia , lookup
History of geology wikipedia , lookup
Age of the Earth wikipedia , lookup
Future of Earth wikipedia , lookup
Plate tectonics wikipedia , lookup
Large igneous province wikipedia , lookup
Mantle plume wikipedia , lookup
Transcript
Earth’s Interior “Seeing into the Earth” Structure of the Earth I. From Seismic/Chemical classification – Crust • Continental • Oceanic – “Moho” - seismic wave velocity discontinuity at crust--mantle boundary – Mantle – Core • Outer liquid core • Inner solid core Crust • Earth’s outermost layer • 5-80 km thick • Composed of relatively low-density silicate rocks Continental crust • • • • Makes up the continents Predominantly granitic composition 20-80 km thick Less dense than oceanic crust Oceanic crust • • • • Makes up most of ocean floors Predominantly basaltic composition 5-15 km thick Denser than continental crust Mantle • Mostly solid (few % melt in upper mantle) • Extends from crust to core (~2900 km depth) • Represents ~80% of total volume of the earth • Upper Mantle – Extends to 670 km seismic discontinuity • Lower Mantle – 670 km to 2900 km Core • Innermost region of earth • Believed to be composed of iron, nickel and sulfur • Outer core – Liquid portion of core extending from mantle to ~5150 km • Inner core – Solid portion of core extending from inner core to center (~6370 km) Structure of the Earth II. From Mechanical Behavior – Lithosphere • Cool, rigid (brittle) outer layer of the Earth • Extends to ~125 km • Includes crust and part of upper mantle – Asthenosphere • Hot, ductile, weak portion of upper mantle • Extends from base of lithosphere to ~350 km depth LAYERS OF EARTH I. Seismic/Chemical Layer Depth (km) Composition Density (g/cm3) 20-80 5-10 Granitic Basaltic 2.5 3.0 cool and rigid cool and rigid Mantle Upper Lower to 670 670-2900 ultramafic ultramafic 3.5 5.5 partially 1-2% molten high pressure minerals Core 2900-5150 5150-6370 iron + nickel iron + nickel 10 13 liquid solid Crust Continental Oceanic Outer Inner Characteristics II. Mechanical (Rheological) Layer Depth (km) Composition Density (g/cm3) Characteristics Lithosphere to 125 varies 2.5-3.3 cool, rigid, brittle 125 - 670 ultramafic 3.5-4.0 hot, weak, plastic Asthenosphere How do we know? • Direct Sampling – – – – Deepest mines ~3 km (2 miles) Deepest oil wells ~8 km Deepest research drilling ~ 12-15 km Mantle nodules and diamond pipes provide samples from upper mantle (100-200 km). – But, radius of Earth ~6370 km How do [we think] we know? • Indirect Sampling – – – – – – Seismic waves Density (inertia) studies Isostacy Gravity Magnetics Heat Seismic Waves – Velocity of seismic waves increases with increasing density of the rock. Hence, seismic wave velocities show general increase with depth into the Earth. – When seismic wave crosses a boundary between rock layers of different density it may be: • Reflected off of the boundary and back toward the surface, and/or • Refracted or bent due to the slowing down of the wave. Seismic Wave Shadow Zones • P-wave shadow zone – No direct P-waves are detected at 103° to 142° from earthquake focus. – ‘Shadow’ zone is due to refraction of P-waves at the core-mantle boundary. – Indicates dramatic increase in density Seismic Wave Shadow Zones • S-wave shadow zone – No direct S-waves are detected at >103° from earthquake epicenters. – ‘Shadow’ zone indicates that S waves do not travel through the core at all. – Implies that Earth’s core is liquid (or acts as a liquid). Density – Overall density of Earth = 5.5 g/cm3 • Calculated from speed of earth’s revolution about the sun and the speed of its rotation on its own axis. – Density of crustal rocks = 2.7 to 3.0 g/cm3 • From direct measurements – Density of mantle rocks = 3.3 g/cm3 upper mantle to 5.5 g/cm3 at base of mantle. • Some direct measurements and calculations from experimental data. – Density of core must be 10 to 13 g/cm3 • By mass balance calculation. Isostacy – Isostacy (“equal standing”) refers to the balance of the height of a less dense mass floating in a more dense material to acieeve hydrostatic equilibrium. – Isostatic adjustment - vertical movement of masses to achieve hydrostatic equilibrium. – Isostatic rebound - uplift of the crust in response unloading by erosion or melting of glaciers. • Provides a measure of the viscosity of the asthenosphere. Gravity • Gravitational forces are dictated by the masses of the objects and the distance between them. Force = constant [(massA x massB) / distance2] • The gravitational force is greater for equal volume of denser rocks. Gravity Anomalies • Deviations from normal regional gravity. • Positive when greater than normal, negative when less than normal. • Useful for: – Determining isostatic equilibrium – Minerals exploration (ie, dense metal ores) Magnetics • The Earth has a dipole magnetic field. • Strength of the field is greatest at the poles. • Source of Magnetic Field: – Earth’s magnetic field is generated in the liquid outer core by convection currents. – Convection of metal generates an electrical current that creates magnetic field. Magnetism in Rocks • Many rocks contain the direction and strength of the magnetic field present at the time they formed. • When iron-rich minerals like magnetite crystallize and cool through Curie point (`580°C), they acquire the direction of earth’s magnetic field at that time. Magnetic Reversals • Studies of the magnetic field of stacked lava flows on continents show that some of the lava flows have a magnetic orientation opposite to the current magnetic field. • Explanation is that the direction of earth’s magnetic field has change through time: – Normal - aligned with current field direction – Reverse - aligned opposite to current field direction Heat flow • Small but measurable amount of heat is gradually being lost through the earth’s surface. • Sources of Thermal Energy – Acquired during planetary accretion (“original” heat). – Decay of radioactive isotopes. Heat flow • Average heat flow from continents is the same as from the seafloor. – Seafloor’s heat due to rising hot mantle rock. – Continent’s heat from great concentration of radioactive material. • Regional heat flow varies: – High heat flow at mid-ocean ridges – Low heat flow at deep ocean trenches. Temperatures in the Earth • Geothermal Gradient – ~25°C/km depth near the Earth’s surface – Drops to ~1°C in the mantle. • At core-mantle boundary T = 4800°C • At inner core - outer core boundary T = 6600°C • At center of Earth T = 6900°C (~1000°C hotter than surface of the sun!)