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.