What evidence do we have that earth's outer core is molten?

Question

What evidence do we have that earth's outer core is molten?

Answer 1

The seismic tests revealed an anomaly. Sound and such waves travel through different densities with tell tale characteristics, the main being the speed.

Answer 2

It's the inner core

Answer 3

i guess volcanic eruptions vomitting out  lava is good proof enuf tat somewhere down there the earth is molten...

Answer 4

By studying the seismic waves as they travel through the earth.

P waves can travel through solid, liquids and gases.

S waves can travel through solids only.

When there is an earthquake, p waves can be detected by a seismograph on the other side of the earth.

However, there is a shadow zone on the other side of the earth where no s waves can be detected.

By studying the shadow zones of S waves, scientists were able to reach the conclusion that the outer core of the Earth is liquid (molten)

Answer 5

seismic waves show that there is a zone of liquid or partial liquid at the core.  The presence of a strong magnetic field is also a good indicator, the field is generated by the spinning of the solid inner core, and the inner core can only spin if it is in a liquid medium.  Planets without a partially molten core have very weak to non-existent magnetic fields.

Answer 6

Gutenberg Seismic Discontinuity / Core-Mantle Boundary

Seismic waves recorded at increasing distances from an earthquake indicate that seismic velocities gradually increase with depth in the mantle (exceptions: see Low Velocity Zone and 670 km Discontinuity above). However, at arc distances of between about 103° and 143° no P waves are recorded. Furthermore, no S waves are record beyond about 103°. Gutenberg (1914) explained this as the result of a molten core beginning at a depth of around 2900 km. Shear waves could not penetrate this molten layer and P waves would be severely slowed and refracted (bent).

Lehman Siesmic Discontinuity / The Inner Core

Between 143° and 180° from an earthquake another refraction is recognized (Lehman, 1936) resulting from a sudden increase in P wave velocities at a depth of 5150 km. This velocity increase is consistent with a change from a molten outer core to a solid inner core.

What Causes the Earth's Magnetic Field?

Early ideas about what caused the compass needle to point toward the north included some divine attraction to the polestar (North Star), or attraction to large masses of iron ore in the arctic. A more serious hypothesis considered the Earth or some solid layer within the Earth to be made of iron or other magnetic material forming a permanent magnet. There are two major problems with this hypothesis. First, it became apparent that the magnetic field drifts over time; the magnetic poles move. Second, magnetic minerals only retain a permanent magnetism below their Curie temperature (e.g., 580°C for magnetite). Most of the Earth's interior is hotter than all known Curie temperatures and cooler crustal rocks just don't contain enough magnetic content to account for the magnetic field and crustal magnetization is very heterogeneous in any case.

The discovery of the liquid outer core allowed another hypothesis: the geodynamo. Iron, whether liquid or solid, is a conductor of electricity. Electric currents would therefore flow in molten iron. Moving a flowing electric current generates a magnetic field at a right angle to the electric current direction (basic physics of electromagnetism). The molten outer core convects as a means of releasing heat. This convective motion would displace the flowing electric currents thereby generating magnetic fields. The magnetic field is oriented around the axis of rotation of the Earth because the effects of the Earth's rotation on the moving fluid (coriolis force).

ScienceDaily (Nov. 27, 2006) — For the first time, scientists have directly measured the amount of heat flowing from the molten metal of Earth's core into a region at the base of the mantle, a process that helps drive both the movement of tectonic plates at the surface and the geodynamo in the core that generates Earth's magnetic field.

The boundary between the core and the mantle lies half-way to the center of the Earth, at a depth of 1,740 miles (2,900 kilometers). Seismologists are able to probe the structure of this region by studying its effects on seismic waves generated by earthquakes. The new temperature measurements, published in the November 24 issue of the journal Science, were obtained by relating seismic observations to a recently discovered mineral transformation that occurs at the ultrahigh pressures and temperatures prevailing near the core-mantle boundary.

This gives us the temperature at two different depths right above each other, so we get not just the absolute temperature but the rate at which the temperature is changing with depth, as well as laterally," Lay said. "This temperature gradient tells us the amount of heat flowing out of the core into the base of the mantle in that location."

As heat flows from the outer core into the mantle, it drives important processes in both the mantle and the core. The mantle is a thick layer of silicate rock that surrounds a dense, predominantly iron core. The outer core is molten liquid and surrounds a solid inner core about the size of the moon. The cooling of the liquid outer core results in fluid motions in the molten metal that produce electric currents, which generate the geomagnetic field.

Heating at the base of the mantle, meanwhile, drives upwellings of hot mantle material that may rise to volcanoes at the surface and contribute to the slow shifting of tectonic plates.