By Robert Hazen, Ph.D., George Mason University
One of the great mysteries in science is the origin of Earth’s magnetic field. It probably takes place in the Earth’s core, specifically its outer core, the liquid core, as currents and fluids circulate deep within the Earth. This sets up electric fields and magnetic fields, reflected in the North and the South poles of the Earth.
Core-Mantle Boundary
The core-mantle boundary is a fascinating zone for seismologists; indeed, it may be one of the most complex and fascinating of all the inner zones of the Earth. We see the scattering of seismic waves at the core-mantle boundary that is quite chaotic and suggests that there might be vast mountains of dense material sitting on this metal-rock interface, which is the core-mantle boundary, deep under our feet.
There may in fact be giant mountains, 100 kilometers tall, of dense minerals sitting on the very base of the mantle, and then lighter minerals surround it. It’s a neat idea, and seismology is the one way we’re going to be able to find out if that’s true.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Layers of the Mantle
The Earth’s mantle contains, by far, the largest volume of the Earth. It’s divided into three layers, each of which is bounded by a seismic discontinuity.
The lower mantle extends from about 700 kilometers down to almost 3,000 kilometers deep, and it contains more than half the Earth’s volume.
Although the mantle is composed of what we call “solid rock,” it’s an extremely dynamic region that drives most of the geology at the Earth’s surface because, at those high temperatures and pressures, the rock becomes plastic, like taffy. It actually, over millions of years, moves and flows and changes, and that drives many of the activities at the surface of the Earth.
Between 400 kilometers and 670 kilometers deep is the Earth’s transition zone, the transition zone of the mantle. This is a region distinguished by dramatic phase transitions. You see seismic velocities increasing tremendously from the top to the bottom of the transition zone. That corresponds to denser and denser mineral combinations, as atoms rearrange and reform themselves into these denser minerals by phase transformations.
Then we have the upper mantle, the upper 400 kilometers of the Earth, which features relatively simple composition of the entire mantle; that is, oxygen, magnesium, silicon, and iron, but in minerals that are very similar to the low-density minerals at the Earth’s surface.
Learn more about the mechanism of plate tectonics.
Gift of Seismology
Seismology, coupled with field studies and deep drilling, reveals that the Earth’s crust is an extremely heterogeneous place, a much more complex place than the core or the mantle—or at least so we think.
If you look at the crust, you see that it’s compositionally varied. While in some places, it’s volcanic rock, like the basalt lavas that pour out of Hawaii’s great volcanoes, in other places, we find miles and miles of thickness of sediments of various kinds: sands and silts, muds, sometimes limestones, layer upon layer, and these are very different in composition from the basalts.
The crust also has an extremely complex structure. It has faults and folds; it has mountains and valleys, earthquakes, volcanoes.
Learn more about the rock cycle.
Story of Our Home Planet
To summarize, our home planet formed in a solar nebula of dust and gas, and as the Earth formed, it differentiated. The molten Earth separated into layers. The iron metallic core, with some nickel: an inner core that’s solid; and the liquid outer core of molten nickel and iron.
Then the Earth’s mantle separated into three layers: the lower mantle, the transition zone, and the upper mantle. All are composed primarily of just four elements: oxygen, magnesium, silicon, and iron. And finally comes the crust: the dynamic, the varied, the heterogeneous crust.
Common Questions about the Fascinating Characteristics of the Earth’s Core
The scattering of seismic waves at the core-mantle boundary is quite chaotic and suggests that there might be vast mountains of dense material sitting on this metal-rock interface, which is the core-mantle boundary, deep under our feet.
Starting from the top, there is the upper mantle, the transition zone, and the lower mantle. Then we reach the Earth’s core. There we have the liquid outer core and the solid inner core.
The lower mantle extends from about 700 kilometers down to almost 3,000 kilometers deep, and it contains more than half the Earth’s volume.
