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 II. Mechanical (Rheological) Layer Depth Composition Density Characteristics (km) (g/cm3) Crust Continental 20-80 Granitic 2.5 cool and rigid Oceanic 5-10 Basaltic 3.0 cool and rigid Mantle Upper to 670 ultramafic 3.5 partially 1-2% molten Lower 670-2900 ultramafic 5.5 high pressure minerals Core Outer 2900-5150 iron + nickel 10 liquid Inner 5150-6370 iron + nickel 13 solid II. Mechanical (Rheological) Lithosphere to 125 varies 2.5-3.3 cool, rigid, brittle Asthenosphere 125 - 670 ultramafic 3.5-4.0 hot, weak, plastic

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.

Force = constant [(massA x massB) / distance2] 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!)