Inferences about the early solar
system as well as the earth's interior come from meteorites
which encounter
the earth and pass through the earth's atmosphere.
Meteorites
which are large objects such as asteroids, fragments of asteroids,
and comets are classified on the basis of their textures and compositions.
All meteorites seem to be about the same age which is 4.5 billion
years old or nearly the same age as the earth (4.6 b.y.):
Iron meteorites
- principally an iron-nickel alloy. These comprise 10% of all
meteorites and they may tell us something about the composition
of the core of the earth.
Stony-iron meteorites
- silicate minerals and nickel-iron alloy in approximately equal
amounts.
Stony meteorites
- plagioclase and iron-magnesium silicates such as olivine and
pyroxene. Terrestrial rock most similar in composition to chondrites
is the ultramafic rock called peridotite which is the major component
of the earth's mantle. About 90% of all stony meteorites contain
round silicate grains called chondrules and are referred to as
chondrites.
Carbonaceous chondrites
- a stony meteorite containing up to 5% organic materials including
carbon, hydrocarbon compounds and amino acids.
Deep parts of the earth
are also studied indirectly, however, largely through the branch
of geology called geophysics. Information on the earth's
interior comes from four types of measurements: gravity, magnetic,
seismic, and heat flow.
Seismic waves
- sound waves from a large earthquake or nuclear bomb explosion
which pass entirely through the interior of the earth. The speed
with which these seismic waves move is called seismic velocity.
Two major seismic waves are the compressional (P) waves and shear
(S) waves. Some seismic waves return to the surface after bounding
off of (reflect from) rock boundaries. A study of such waves
of called the study of seismic reflection. Seismic waves
also bend (refract) as they pass from one material to another.
A study of such waves of called the study of seismic refraction.
A seismic wave going from a low velocity layer to a high velocity
layer will bend so the angle between the wave and the interface
is less in the higher velocity material. This same bending occurs
if seismic velocity increases with depth in the earth. The reflection
and refraction of seismic waves has told us much about the earth's
internal structure.
The Earth's Internal Structure
-- The outer layer of rock forms a thin skin call the crust
of the earth. Below the crust lies the mantle which is
around the core of the earth.
The crust consists of two types.
In the thicker continental crust P-waves travel at about
6 km/sec whereas in the thinner oceanic crust P-waves travel
at about 7 km/sec. The continental crust consists of granite,
other plutonic rocks, schist, gneiss, and a sedimentary cover
with an average density of 2.7 gm/cm3 and a thickness of 30 to
50 km. The oceanic crust consists of basalt and gabbro in the
lower crust with an average density of 3.0 gm/cm3 and a thickness
of 7 km. The boundary between the crust and mantle is called
the Moho.
The mantle, which is composed
of ultramafic rocks such as peridotite, consists of several concentric
layers. The ultramafic rocks through to make up the mantle probably
has a density of 3.3 gm/cm3 in the upper mantle although rock
pressure should raise this value to about 5.5 gm/cm3 at the base
of the mantle. The upper most mantle plus the crust form the
lithosphere (the rock sphere) which is about 70 km thick
beneath the oceans and 125 km thick beneath the continents. The
next layer down in the mantle is called the asthenosphere
(the weak sphere) which is marked by a low-velocity seismic zone
where rocks are closer to melting than those both above and below.
The base of the asthenosphere is between 200 and 300 km down
in the earth. The mesosphere (middle sphere) is found
below the asthenosphere and extends down to the mantle-core boundary
at 2830 km. Seismic reflection and refraction indicate prominent
boundaries at 400 km and 670 km in the mesosphere. At a depth
of 400 km the mineral olivine collapses into a denser mineral
called spinel..
The core, composed of mainly
iron plus silicon, sulfur, and nickel, has two parts. The outer
core is liquid whereas the inner core is solid. The density of
the core is between 12 and 13 gm/cm3 thus giving the earth an
average density of 5.5 gm/cm3. Evidence for an iron-nickel core
come from a combination of meteorites, the earth's density, and
the seismic velocity of the earth's interior.
Isostasy
is a balance or equilibrium between adjacent blocks of less dense
brittle crust "floating" on a more dense plastic upper
mantle. Rocks of the crust will move vertically to reach isostatic
equilibrium in a concept called isostatic adjustment. As
a consequence tall mountain ranges sink deeper into the mantle
than adjacent lowland areas. If we imagine that each small segment
of crust and mantle as a column, then at some depth of equal
pressure each column is in balance with other columns for
each column of rock has the same weight. This is so because the
weight of each column of crust is equal to the weight of the displaced
mantle.
Gravity
- The force of gravity between two objects varies with the masses
of the objects and the distances between them. A gravity meter
measures the gravitational attraction between the earth and a
mass within the gravity meter. If the earth is in isostatic equilibrium,
then each column of rock in the earth will have the same mass.
More mass may be present is tectonic forces are holding a region
up out of isostatic equilibrium which leads to a positive gravity
anomaly. A mass deficiency in a region produces a negative
gravity anomaly.
Magnetic Anomalies
- A region of magnetic force surrounds the earth. The strength
of the earth's magnetic field varies from place to place because
some rocks contain more magnetic minerals than other rocks. As
with gravity, a deviation from average readings is called an anomaly.
A positive magnetic anomaly is caused by a body of magnetite
ore in a fracture in limestone which is nonmagnetic. A negative
magnetic anomaly is caused by a downdropped fault block (a
graben) in igneous rock. The graben may be filled with nonmagnetic
sediments where as the crystalline rocks on either side of the
graben may be highly magnetic.
Heat within the earth
- The temperature increase with depth into the earth is called
the geothermal gradient. The average temperature increase
is about 25°C/km. Geologists believe that the geothermal
gradient must taper off sharply a short distance into the earth,
otherwise the mantle would be at temperatures above the melting
point. Seismic evidence seems to indicate a solid, not molten,
mantle, so the geothermal gradient must drop to 1°C/km within
the mantle.
Heat flow
- A small but measurable amount of heat from the earth's interior
is being lost gradually through the earth's surface in a process
called heat flow. Two sources of heat: One - the heat
may be original if the earth formed as a hot mass that is not
cooling down. Two - the heat could be a by-product of
the decay of radioactive isotopes inside the earth. Oddly, the
average hear flow from the continents is the same as the average
heat flow from the sea floor. Yet, there is a greater concentration
of radioactive material in continental rock which would suggest
that the continental rocks should have a higher heat flow. The
unexpectedly high average heat flow under the ocean crust may
be due to hot mantle rock rising slowly by convection under parts
of the ocean crust.