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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!)