10. Earth's Interior
I.
II.
III.
Early ideas of the Earth's Interior
A. Early people imagined the interior as an underground world of vast caverns, intense heat, and
sulfur gases with demons and a hell like place
B. Jules Verne's Journey to the Center of the Earth captured the imagination where they went
140 km deep into a cavern containing the central sea, prehistoric animals like the mastodon
C. Scientists knew that the pressures and temperatures increased with depth and there could not
be caverns deep within the earth
1. The deepest mines in Africa only go 3 km below the surface
2. The deepest drill holes only go about 15 km deep, but with a crustal depth of 150 km,
it was barely 10% of the way to the mantle
3. The Mohole project in 1958 attempted to drill to the Moho or the boundary between
the crust and mantle, but ran out of funds
D. At great depths the pressure and temperature on rocks is so great that they act like a plastic
and flow even though they are solid
Scientists have known for 200 years that the earth was not homogeneous throughout
A. Sir Issac Newton in 1650 AD noted that the Earth's average density is 5-6 gm/cm3
1. In 1797 Henry Cavendish calculated 5.5 gm/cm3
2. This is much greater than the rocks at the surface of 2.5 - 3.0 gm/cm3
B. Generally accepted that the Earth has concentric layers differing in composition and density
with distinct boundaries between them - known as the crust ,mantle , and core
1. The crust is a thin layer
2. The mantle makes up about 80% of the Earth's volume
3. The core divided into the inner and outer core
C. But no direct measurements of the interior have been made, this information is derived from
the study of seismic waves
Seismic waves - the clues to the interior
A. Waves form a wave front that moves outward from the center
1. Only those that release large amounts of energy can travel completely through the
Earth
2. Only seismic waves generated by large earthquakes, volcanic eruptions, nuclear
explosions, or large meteorite impacts can travel completely through the earth
B. The main tool is the fact that P and S waves have different travel times and properties
C. Velocity of P and S waves are determined by the density and elasticity of the material
1. Wave's velocity is slowed by increasing density but increases with greater elasticity
2. A general increase of wave velocity occurs with increased depth
3. P-waves travel faster than S-waves
4. S-waves do not travel through liquids because they have no shear strength and tend to
flow instead of rebound elastically
D. If Earth was homogeneous, P and S-waves would travel straight paths through the planet
1. But as the waves travel through material of different density they are bent from
refraction, much like light is refracted going from air into a more dense medium like
water
2. Since waves pass through materials of differing densities and elasticity, they are
continually being refracted and have curved paths
3. If the waves pass perpendicular to a boundary they are not refracted but go straight
through
E. Waves are also reflected when they encounter a boundary layer separating material of
different density and composition
1. Much like light encountering water, some of the waves are reflected
IV.
2. By knowing the waves velocity and the time taken to travel to the boundary layer and
back to the surface, we can calculate the distance and then the depth of the boundary
3. Seismic reflection is a common tool used in geophysics to determine depths of
objects below the surface like oil or metals
F. P waves velocity has been found to
1. Increases suddenly at the base of the crust
2. This marks a change and is a discontinuity or boundary
3. Decreases rapidly at 2900 km
4. Marks the difference of the mantle and the core
G. Seismology has been used extensively to understand the Earth's interior and the internal
structure was worked out by 1936
Seismic waves help to discover the Earth's core
A. R.D. Oldham in 1906 discovered that seismic waves arrived later than expected in India
1. He postulated that a core existed that slowed the seismic waves
2. We now know that this represents a major discontinuity the mantle-core boundary
B. The sudden decrease of P-wave velocity at the core - mantle boundary caused the waves to
be refracted in such a way to leave a shadow zone in an area 103 and 143 from the earthquake
focus
1. Also noticed that at 130 the P-waves arrived much slower than expected
2. Postulated that there existed a core of liquid material
C. But the P-wave shadow was not complete and some waves made it through
1. It was then thought that the core may not be all liquid but have a solid core
2. Inge Lehmann in 1936 then demonstrated that a refraction from a solid inner core
could account for the arrival of weak P-waves in the P-wave shadow
3. This was quickly accepted by other seismologists
D. The S-waves were not just refracted by the core but were completely blocked by it
1. This left a more complete shadow called the S-wave shadow zone
2. Locations 103 and greater there were no S waves
3. Since S-waves cannot travel through liquids, the outer core must be liquid or behave
as a liquid
E. Core Density and Composition
1. Lies below the mantle, 2900 km deep and has a diameter of 6960 km (about the same
diameter as Mars)
2. Constitutes 16.5% of the volume and 33% of the mass
3. Can estimate the density by seismic evidence
4. The diamond anvil pressure cell is used to test samples at high pressures and
temperatures like the core
5. Metallic meteorites are thought to be the core of differentiated asteroids and are
probably the same material as the core
a) The density of the outer core varies from 10to 12 gm/cm3
b) The inner core ranges from 12.6 -13.0 gm/cm3
c) Pressures are 3.5 million times atmospheric pressure on the surface
6. Outer Core cannot be made of pure iron - it is too dense and is probably made of iron
sulfur, silicon, oxygen, nickel, and potassium
7. Pure iron is not dense enough to account for the inner core- mixture of nickel and iron
F. Core model
1. Must explain why the inner core is solid and outer core is liquid
2. How a magnetic field is generated within the core
G. When the core was formed in the early history of the Earth it was probably all molten
1. It has cooled to the point that the inner core has crystallized and continues to grow
V.
VI.
2. Evidence indicates that the inner core rotates at a rate faster than the outer core
moving at 20 km/yr faster
3. Temperature of the core mantle boundary is estimated to be 3500 to 5000 , but the
high pressure within the inner core prevents melting
4. The outer core has less pressure but has different composition that prevents melting
with the sulfur content preventing solidification
The Mantle
A. In 1909 Andrija Mohorovicic discovered a discontinuity at 30 km depth
1. While studying the seismic profiles from earthquakes in the Balkans he found two
distinct set of P and S-waves
2. He postulated that some seismic waves traveled directly through the upper layer while
others traveled through the deeper layer with some refracted back to the surface
3. Waves traveling through the deeper layer travel further but arrive at a seismic station
sooner than the waves in the shallower layer
4. It is present everywhere except at the spreading ridges and is deeper under the
continental rocks (20-90 km) than the oceanic rocks(5-10 km)
5. 3.3 - 5.5 gm/cm3
B. Structure and Composition of the Mantle
1. Beneath depths of 100 to 250 km both P and S waves decrease due to partial melting
and local variations
a) This is a low velocity zone corresponding to the athenosphere-where rocks
are near their melting point and are less elastic
b) The athenosphere is an important zone because this is where magmas are
generated
c) It is also a plastic like layer that is thought to be the layer over which the rigid
plates of the lithosphere move
d) This low velocity zone of the athenosphere is poorly defined and even absent
beneath the oldest continental rocks
2. There are several indistinct discontinuities in the mantle that represent structural
changes in the minerals rather than compositional changes
a) At 440 km the structures of minerals gets denser
b) At 660 km minerals break down into metal oxides like FeO and MgO and
SiO2
c)
3. Composed largely of peridotite- olivine and pyroxene
a) It's physical properties would account for the density
b) It forms parts of rock sequences thought to be the lower parts of the crust or
mantle
c) Peridotite is found in deep igneous features like kimberlite pipes
C. Seismic tomography
1. This is a three dimensional way of looking at the interior of the Earth by looking a
seismic reflections from many different directions
2. Has found hotter molten areas beneath volcanoes and spreading centers
3. Has found colder spots under the ancient continents
4. Also there are other exciting features found
a) Core-mantle interface is not smooth
b) Broad depressions and rises extending several kilometers into mantle
sometimes termed antimountains
c) These variations maybe the result of sinking and rising of mantle material or
convection produced by the subduction of crustal material
The Crust
A. Along with the upper mantle constitutes the lithosphere of the plate tectonics theory
B. Two types of material- oceanic and continental crust
C. Continental crust is more complex- containing igneous, metamorphic, and sedimentary rock
types varying greatly vertically and horizontally
1. Closely corresponds to granodiorite in composition
2. Densities range from 2.0 to 3.0 gm/cm3 , with avg. density being 2.7 gm/cm3
3. Generally it is 35 km thick but much thinner in Basin and Ranges and rift valleys
4. Beneath the Himalayas it is close to 90 km thick due to crustal thickening but only 20
km thick in rift valleys
D. Oceanic Crust is more uniform
1. 5-10 km thick, thinnest at the spreading centers
2. Denser than continental crust, averaging 3.0 gm/cm3
3. Mostly composed of basalt, as determined by P-wave velocities
VII. Internal Heat
A. Geothermal gradient of 25 C/km
1. Temperature rises with depth
2. Rises faster near spreading ridges and near volcanoes
B. Most heat generated by radioactive decay
1. Because rock is a poor conductor of heat, it adds up over time
2. Uranium and thorium isotope decay produces a lot of heat from the emission of
gamma and alpha rays
C. The geothermal gradient must decrease with depth
1. If we were to extrapolate the gradient downward the temp at 100km would be so hot
to melt all known rocks
2. Temperatures at the crust-mantle boundary is between 800 - 1200 C
3. We know this is not so and there seems to be an equilibrium at depths of 300 km at
about 1200 C
4. Temperature the core mantle boundary the temp is estimated to be between 3500 and
5000 C
5. Estimates of the core temperature are close to 6500 C- with the inner core outer core
boundary near 7500 C, close to the temp of the surface of the Sun!
D. Heat flow is the movement of heat from the interior to the exterior and is small through the
crust
1. Heat flow is greatest at volcanic areas and spreading ridges
2. Lower than average heat flow is found at the subduction ridges
3. Higher than average areas are places like Yellowstone Nat. Park or the Cascades
4. More than 70% of the heat flow is through ocean floor
5. The Earth produces some heat from radioactive decay but this diminishes through
time
6. The Earth had more internal heat in its early history and is continuously cooling from
the core to the surface
7. Ultramafic lava flows estimated to be 1600 C are known from rocks older than 2.5
billion years ago indicating that the Earth had more internal heat at that time
VIII. Gravity and it measurement
A. The universal law of gravitation is F= G m1 x m2 / D2
B. This means a gravitational force exists between stars, planets, the moon
C. The force decreases by the square of the distance
1. As the distance doubles between objects, the force is diminished by four
2. We say the force is inversely proportional to the square of the distance between
objects
IX.
X.
D. Generally we refer to the gravitational force between the Earth and an object as its weight of
a mass
E. Gravitational force would be equal everywhere if
1. The earth was perfectly spherical
2. Homogeneous
3. Not rotating
F. Rotation produces centrifugal forces that counteract the gravity force
1. An object at the equator weighs slightly less than the same object at the poles
2. The force of gravity varies as to the center of the object so an object would weigh
slightly less above the Earth
G. A gravimeter is used to measure the force of gravity over certain areas
1. Basically a weighted object suspended from a spring
2. This responds to variations in gravity of the Earth
H. Anomalous gravity measurements have been found over
1. Ore deposits
2. Metal bearing strata or mafic regions (like oceanic basalt) located within siliceous
ones like accreted terrain
3. Salt domes buried in the earth
I. Gravity anomalies are unexpected values of gravity at certain areas
1. A positive gravity anomaly would be an ore body or buried mafic rock
2. A negative gravity anomaly would be a mass deficiency
3. Large negative anomalies exist at subduction zones
4. This principal has been used to view the subsurface and locate buried metallic objects
Principal of Isostasy
A. A lighter or less dense material will float or rise above a more dense material
B. Easily seen in the example of an iceberg floating in water
1. According to the idea of buoyancy it will sink until it displaces an equal volume of
water that equals it total weight
2. Essentially 10% of the iceberg will be above the water
C. Earth's crust responds the same way- it sinks into the mantle to reach its equilibrium level
1. Where it is thickest, as in mountain ranges, it sinks further into the mantle
2. Where it is thinnest it rises as in spreading rides
D. Continental crust being thicker and lighter than oceanic crust stands higher than oceanic crust
E. If the crust is loaded with weight as with glaciers it tends to sink into the mantle
1. When the glaciers are melted off, the weight is lessened and the land rises or
rebounds
2. The crust also responds to widespread erosion and deposition
3. Also responds to removal or addition of rock material by low angle faulting
4. This is called isostatic rebound
5. Greenland and Antarctica have the surface depressed below the sea level
6. The land in Canada has also rebounded and has risen as much as 100 m in the last 600
years
F. This implies that the mantle acts as a liquid but the S-waves travel through it like a solid
1. The time it takes to rebound or reach equilibrium is long and the mantle acts as a
plastic material
2. The S-waves travel through quickly and the mantle responds to the rapid motion as a
solid
G. The principal of Isostasy can have wide ranging effects on all parts of the crust
The Magnetic Field
A. The Earth has a magnetic field that resembles a bar magnet with a north and south pole
B. How is it generated?
1. Magnetite loses its magnetic properties when it reaches the Curie Point at 580
2. Also the locations of the poles change indicating that it is not a buried source
3. It must be generated by the liquid outer core by electrical currents developing a
magneto effect
4. This is still not completely understood
C. Inclination and Declination
1. Deviation of the magnetic lines of force is called the magnetic inclination
a) This effects compasses and a small weight is placed on the south end of the
needle in the north
b) As you travel further north the needle will point downward
c) This principal is important to determine where the ancient poles were and the
geographic locations of the wandering tectonic plates
2. magnetic poles do not correspond to the geographic poles-called magnetic
declination
a) An 11.5 at present differs between the two
b) Consequently everywhere on the Earth the deviation is different
c) Navigation has to take this into account (also surveying)
D. Magnetic anomalies
1. Variations in the strength of the magnetic field result in differences
2. Regional variations result from variations in the core
3. Local variations result from different rock types
4. A magnetometer can detect slight variations in the field
5. A positive magnetic anomaly is where the rocks are more iron bearing
6. Areas with large amounts of basalt rock can create positive anomalies
7. Places with little or no iron in the sedimentary rocks can have a negative anomaly
compared to the surrounding iron bearing rocks
E. Magnetic Reversals
1. When a rock or magma cools through the Curie point the iron bearing minerals align
themselves to the earth's magnetic field
2. This records both the direction (inclination, declination) and strength
3. If the rock is subsequently reheated it loses its signature and takes on another setting
when it passes through the Curie point again
4. Iron bearing rocks all record the magnetic field at that time
a) Oceanic basalts record the magnetism and location when they were formed
b) Some sedimentary and metamorphic rocks can be used to determine the
location of the deposits
c) This is called Paleomagnetism - the study of the ancient magnetic field
recorded in rocks
5. As early as 1906 these studies have shown that the magnetic field has reversed itself
many times
a) This is when the north pole exchanges polarity with the south pole
b) These magnetic reversals show when the north pole has changed to a south
pole
c) it has weakened 5% in the last century
d) Rocks with a polarity the same as today's are said to be normal polarity
e) Rocks with opposite magnetism have reverse polarity
f) This has been important and instrumental in developing the Theory of Plate
Tectonics