Travel-time versus Distance Curves Distanced measured in degrees from epicenter.

P-wave Velocities Crust 6-7 km/sec Mantle 8-13 km/sec Outer Core Inner Core 10-11 km/sec

VP = P-wave velocity VS = S-wave velocity D = density

Granite (and Average Continental Crust) 6.0 km/sec 2.5 km/sec 3.5 km/sec 3.0 km/sec Differences in density and other physical properties affect the velocity of seismic waves.

Density of Water 1 gm/cm3 Crust 2.6-3.1 gm/cm3 Mantle 3.3-5.7 gm/cm3 Outer Core 10-12 gm/cm3 Inner Core 13-14 gm/cm3

K = bulk modulus = resistance of material to squeezing D = density G = shear modulus = resistance of material to shearing

Seismological Results Values of K, G and D at a given point in the Earth’s interior. Search for materials with appropriate values of K, G and D Estimates of composition of different parts of the Earth’s interior

Outer Core of Earth is liquid G = 0 for liquids Outer Core of Earth is liquid

Basalt Granite Peridotite Peridotite = a dense rock that is thought to be a major component of the Earth’s Mantle.

Composition of Earth’s Interior Crust: Granite/Basalt Mantle: Peridotite Outer Core: Liquid Iron Inner Core: Solid Iron

Why have geophysicists concluded that the Earth’s core is primarily composed of iron?

Why have geophysicists concluded that the Earth’s Core is primarily composed of iron? Iron is an abundant element in the Solar System (meteorites). The Core has the right density for iron (10 to 13 gm/cm3). The Core has the right VP and VS for iron. Liquid iron in the Outer Core - Earth’s magnetic field.

Solar System

Meteorites Meteor Crater, Arizona (about 0.75 miles across and about 560 feet deep). The crater was created approximately 50,000 years ago.

Meteorites Meteorites are the remains of rocky and metallic material that fall to the Earth from inter-planetary space. These meteorites are thought to be representative samples of material that formed the early solar system, and they are believed to have solidified at about the same time as the Earth.

Origin of the Solar System

Meteorites provide information about the types of materials that were available to form the Earth and the other planets when the Solar System formed 4.6 billion years ago. Thus, by combining information about meteorites, with results of seismic studies, we can estimate the composition of the Earth’s interior.

Types of Meteorites Iron Meteorite Stony Meteorite

Composition of Meteorites Iron Meteorites – Composed of iron and nickel. Stony Meteorites – Composed of dense rock similar to “peridotite”.

The Earth’s crust contains a much smaller % of iron than meteorites do. Heavy minerals (iron) sank to the center of Earth during the early history of the Earth to form the Core, and lighter minerals floated to the top forming the Crust.

Heavy minerals (iron) sank to the center of Earth to form the Core, and lighter minerals floated to the top forming the Crust. Therefore, the Core of the Earth is believed to be mainly iron (similar to iron meteorites). and The Mantle is believed to be mainly peridotite (similar to stony meteorites).

What causes the Earth’s magnetic field?

Dynamo Theory of Earth’s Magnetic Field Rapid motion of liquid iron circulating in the outer Core

Venus, our nearest neighbor in space, may be structurally similar to Earth, also having an iron core.

Rock Metallic Core

But, Venus is spinning very slowly (once in 243 days). So, one of the important ingredients of the dynamo theory is absent. Thus, Venus would not be expected to have a magnetic field (and none has been observed).

Composition of Earth’s Interior Crust: Granite/Basalt Mantle: Peridotite Outer Core: Liquid Iron Inner Core: Solid Iron

Variations in the seismic velocity are depicted by warm (low velocity) or cool (high velocity) colors. Large scale patterns correlate with features at Earth's surface. Lower velocities are seen beneath mid-ocean ridges, high velocities are detected beneath regions of past or present subduction.

Computer simulation of P-waves from underground nuclear explosion in Nevada, propagated westward under the Sierra Nevada to stations in California.

Environmental Geophysics Depth to Bedrock Subsurface Structure Subsurface Paths for Flow of Groundwater 250 - 600 m/sec 300 - 900 m/sec 2000 - 6000 m/sec VP

Image of upper Mantle seismic structure at 100 km depth beneath North America. Blue is high velocity Mantle and red is low velocity Mantle.