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TITLE: Explore the Unreachable Frontier

In 1990, the world's deepest drill hole penetrated to a depth of 12.3 km (7.6 mi) beneath Russia's Kola Peninsula. More than 99 percent of the distance to Earth's center still lay beneath the drill bit. If the inner Earth is so remote and inaccessible, how can we learn anything about it? Geologists gather clues from meteorites, rocks, diamonds, earthquake waves, and Earth's magnetic field.


Clues from Meteorites

A flash of light in the midnight sky announces the arrival of another messenger from space: a meteorite. These extraterrestrial rocks testify to the origin of the Solar System and of the inner Earth. Asteroids — the parent bodies of most meteorites — originated at the same time as the Sun, Earth, and other planets. When fragments of asteroids land here as meteorites, we glimpse the raw materials that formed our planet and the secrets of its core.

  [Photo: Meteor striking the earth]

Clues from Rocks

We can learn about the inner Earth from special rocks called peridotites, which come from the upper mantle. Besides telling us what the upper mantle is made of, peridotites record geological processes that take place in this inaccessible layer of Earth. It's no coincidence that most peridotites are olive green. Some of their color comes from olivine, the most abundant mineral in Earth's upper mantle.

  [Photo: Source of peridotite xenoliths]

Chondrites — The Right Stuff

Chondrites — meteorites composed of the Solar System’s original dust — provide clues to Earth's core. These seven elements make up 97 percent (by weight) of both chondrites and Earth. But, compared to chondrites, Earth’s crust and mantle are poor in iron. The iron must be concentrated in the core.

  [Photo: Iron Meteorite and Pallasite Meteorite]

Interiors of Lost Worlds

These two meteorites probably came from the core and mantle of the same asteroid. The pallasite came from the asteroid's core-mantle boundary, where olivine from the mantle mixed with iron-nickel metal from the core. Earth's rocky mantle and iron core may be separated by a similar hybrid zone. The iron meteorite came from the asteroid's core. Notice the large crystals of metal. Some scientists have proposed that Earth's core is a single crystal of iron!

  [Photo: Carbonaceous Chondrite]

Special Delivery from the Mantle


These dense chunks of Earth's upper mantle hitched a ride to the surface in rapidly rising magma. They are called xenoliths (meaning "stranger rocks"). The large egg-shaped peridotite is a single xenolith, one of the largest ever found. The basaltic lava is chock-full of green and dark reddish-brown peridotite xenoliths.

  Photo: Garnet Peridotite   Photo: Peridotite Xenoliths in Basalt  
  Garnet Peridotite   Peridotite Xenoliths
in Basalt

Peridotites Under Pressure


Peridotite, the upper mantle's most abundant rock, is named for peridot — the gem variety of olivine. These two specimens also contain two other minerals that are clues to how deep the rocks were formed.

Garnet (present in right specimen) can take a lot of pressure: It is stable at depths greater than 60 km (37 mi). Spinel (present in left specimen) is stable at depths shallower than 60 km (37 mi).

  Photo: Spinel Peridotite   Photo: Garnet Peridotite  
  Spinel Peridotite   Garnet Peridotite  

The basaltic magmas that erupt at Earth's surface come from partial melting of peridotite. The more diopside a peridotite contains, the more basalt it can produce. You can see apple-green flecks of diopside in the garnet peridotite. The spinel peridotite also contains tiny diopside grains.


Mantle Rocks Go With the Flow

Under mantle temperatures and pressures, solid rocks can flow like a glacier. The textures of these two specimens reflect motion in the mantle.

Photo: Eclogite  


The eclogite was originally a basalt erupted at Earth’s surface. It was subducted into the mantle, changed to eclogite, caught up in a kimberlite magma, and blasted back to the surface.

Photo: Deformed Spinel Peridotite  

Deformed Spinel Peridotite

The spinel peridotite contains bands of strongly sheared and deformed crystals.

Mantle Rocks Store Water

The mantle contains more water than all the world’s oceans. Where is it? Locked up within the structures of minerals.

Photo: Peridotite  


The golden brown crystals in this microscope photo are the mineral phlogopite, which has four percent water (by weight). Look for the brassy brown crystals.

Contains phlogopite

Making Mantle Minerals



[Photo]Laser beams pass through two diamonds, allowing the mineral sandwiched between to be heated and analyzed. Scientists use this diamond-anvil press to reproduce the inner Earth's extreme temperature and pressure conditions and to shed light on the minerals that exist there. Amazingly, it can exert pressures equivalent to those of Earth's outer core. The much larger multi-anvil press can squeeze a relatively bigger specimen at pressures that correspond to the middle of Earth's mantle.

Clues from Diamonds

To a geologist, diamonds are reminders of the tremendous temperatures and pressures within our dynamic planet. The high pressures found at depths greater than 150 km (90 mi) create diamond's compact crystal structure, making it the hardest material known. Rapidly rising magmas carry diamonds to the surface, where we can cut and polish them into gems or study them to learn more about the upper mantle.

  [Photo: Diamond]


Where Do Diamonds Grow?

At depths of at least 150 km (90 mi), in the thick lithosphere beneath the older parts of continents. By comparing the different kinds of carbon atoms in diamonds, scientists can track the carbon's source. Some carbon was once organic matter on Earth's surface and was subducted into the mantle. The rest originated in the mantle.

Compare these two rare specimens — small diamonds in eclogite and a larger one in garnet. They formed when carbon from Earth's surface was subducted into the mantle, where the eclogite and garnet also formed.

How Do Diamonds Get to the Surface?

They hitch a ride in rare magmas called kimberlite and lamproite, which form by partial melting of the upper mantle. Gas expansion propels these magmas upward at rates of 10-30 kph (6-19 mph), giving their diamond passengers a commute time of just 4-15 hours from mantle to surface!


Where Do We Find Diamonds?

Beneath cratons, the oldest parts of continents. Why there? It's too hot for diamonds to form in most of Earth's upper mantle. But the upper mantle below cratons is relatively cold, so diamonds are stable there.

Magmas that pass through cratons carry diamonds to the surface.


Clues From Earthquakes

Every earthquake releases energy that races through our planet as seismic waves. Depending on what material they travel through, seismic waves speed up, slow down, or disappear. The arrival times of different types of seismic waves around the world provide clues to the composition and structure of the inner Earth.

Strong ground motion associated with the 1964 Alaska earthquake caused the sliding that destroyed these homes.


Earthquakes Take the Mantle’s Temperature

Seismologists (scientists who study earthquakes) have learned that seismic waves slow down in hot rocks and speed up in cold ones. Knowing this, they can use the arrival times for seismic waves generated by thousands of earthquakes to infer three-dimensional images of the mantle's temperature much as a medical CAT scan reveals the body's internal organs.

This image was made using 42,000 seismic wave paths.


Earthquakes Reveal the Layered Earth

At the boundaries between Earth's layers, seismic waves are refracted and reflected. The two kinds of interior seismic waves behave in different ways:

  • P-waves (primary or pressure waves) travel through liquids and solids.
  • S-waves (secondary or shear waves) travel through solids only.

By observing the behavior of these waves, seismologists have determined the depths of the boundaries between Earth’s major layers.


Clues From Earth’s Magnetic Field

Like a bar magnet, Earth has a magnetic field with two main poles. This magnetic field not only enables us to use compasses to find our way at the surface, it also tells us about Earth's deep interior. That’s because the field is generated by motions within the outer core.

Earth contains a dynamic electromagnet. Our planet's rotation causes molten iron-nickel in its outer core to circulate, creating electrical currents and a magnetic field (right).


Are Earth’s Magnetic Poles Stable?

No. They wander over the Earth’s surface. Since 1945, they have moved at a rate of almost 12 km a year — a clue to the dynamic origin of our magnetic field. About every 500,000 years Earth's magnetic field gets progressively weaker, vanishes, then reappears with the magnetic North and South poles reversed.

If you're standing at the North Pole, you're about 15 degrees (1,670 km, or 1,035 mi) away from its slowly migrating magnetic pole.


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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits