Journey to the Earth’s Interior Journey to the Earth’s Interior | A Closer Look

What are igneous rocks?

Igneous rocks are the most common rocks on Earth. All of Earth’s ocean floor, its entire mantle, and much of the continental crust consists of igneous rock. Igneous rock forms as molten (liquid) rock cools and solidifies. Most of this molten rock originates under the Earth’s surface in a zone within the upper mantle where it is extremely hot but the pressure is not great enough to keep the rock solid. There are two major types of igneous rock: extrusive and intrusive.

How are different types of igneous rocks formed?

Basalt

Extrusive, or volcanic igneous rock forms when magma erupts, or extrudes, onto the surface of the Earth as lava. This occurs along active plate margins, such as mid-oceanic spreading ridges and subduction zones, as well as at intraplate settings, like hot spots. Lava cools and hardens quickly at the surface of the Earth, as finely grained rock with many tiny crystals. In some cases, the lava cools so quickly that the molten material does not have enough time to arrange itself into crystalline structures. This kind of igneous rock is called volcanic glass. Another product of volcanic eruption is pyroclastic debris, which are particles formed from the gas and lava that explode into the atmosphere. Pyroclastic debris includes fine particles of volcanic glass called ash, and larger pieces of rock called, depending on their size, cinders and bombs. The most common extrusive igneous rock is basalt, which is the rock that comprises oceanic crust. Basalt accounts for more than 90% of all volcanic rock on the planet.

Intrusive, or plutonic, igneous rock forms when magma beneath the Earth’s surface rises upward and pushes its way, or intrudes, into pre-existing crustal rocks. Features associated with this ascension of magma include sills (nearly horizontal intrusion of magma that is injected between layers of rock); dikes (nearly vertical injection of magma that cuts across layers of rock); laccoliths (when a sill domes upward, looking like a blister); and batholiths (immense, deep, dome-shaped intrusions of igneous rock). Magma cools and solidifies more slowly underground than at the Earth’s surface, which produces igneous rocks with coarse crystals that can easily be seen with the naked eye. One of the most well known intrusive igneous rocks is granite, which comprises much of the continental crust.

In addition to being categorized as extrusive or intrusive, igneous rocks are also classified in other ways:

• Mafic igneous rocks are dense, and rich in iron- and magnesium-bearing minerals and are usually dark in color.
• Felsic igneous rocks are rich in less dense minerals, such as quartz, and are often light in color.
• Granular igneous rocks consist of crystals that are large enough to be easily seen, such as granite.
• Aphanitic igneous rocks are made of tiny crystals that cannot be seen with the naked eye, such as basalt.
• Glassy igneous rocks are composed mostly of volcanic glass. Obsidian is one example.
• Porphyritic igneous rocks have larger crystals embedded in a finer-grained matrix.
• Pyroclastic igneous rocks are volcanic rocks that form from explosive eruptions that shatter magma and can be either cemented together or unconsolidated fragments or slivers. Pumice and ash are examples.

How do we know the nature of Earth’s interior structure?

Much of what we know about Earth’s interior comes from seismic waves. Seismic waves are waves of energy that can be caused by earthquakes. The two main types of seismic waves are body waves and surface waves. Body waves travel through the Earth’s interior in all directions. Surface waves travel only along the surface of the Earth, like ripples on water. It is the behavior of body waves that gives us clues about the nature of Earth’s interior. There are two types of body waves: primary waves (P waves) and secondary waves (S waves).

What are P and S waves?

P and S waves.

P waves stand for “primary waves.” They’re considered to be primary because they travel faster than S waves and, after any given earthquake, will reach a seismic recording station first. In the video, two children simulate P waves by holding opposite ends of a Slinky on the floor. One child pushes the end of the Slinky towards the other child. As a wave moves down the Slinky, the coils can be seen to push forward and compress, then pull back and open up again. This simulates the action of P waves. P waves are compressional waves that exert a force in the direction that the wave travels. These waves push through rock in the same way that sound waves push through air.

S waves stand for “secondary waves.” In the video, two children hold opposite ends of a Slinky on the floor and one child moves the Slinky from side to side. In this case, as the wave moves down the Slinky, the coils can be seen to shake side to side, elastically springing back. S waves are shear waves that exert a force perpendicular to the direction that the wave travels.

What are the differences between P and S waves?

Scientists have learned about Earth’s internal structure by studying how these waves travel through the Earth. The technique is straightforward — it involves measuring the time it takes for both types of waves to reach seismic stations from the epicenter of an earthquake. Since P waves travel faster than S waves, they’re always detected first. The farther away from the epicenter, the larger the time interval between the arrival of P and S waves — and if the Earth were built of a uniform substance, that would be the only variation measured.

Scientists, however, noticed variations that could not be accounted for based simply on the distance traveled from the epicenter. For instance, they noticed places in the Earth through which S waves didn’t travel. Geologists inferred that these sections of the Earth were liquid, through which S waves (which, remember, are shear waves) cannot travel. You may not know it, but you are probably already familiar with this phenomenon. In a bathtub, if you submerge your arm underwater and push your hand straight out from your body, you can see a wave arrive as it hits the edge of the tub. Consider this an example of a P wave. If you then move your hand side to side in the water, you should notice that the wave does not hit the edge of the tub in front of you. Consider this to represent an S wave. What happened to it?

Solids and liquids both transmit P waves because their particles transfer energy in the direction of the wave as they compress and elastically spring back along its length. As with the child at the end of the Slinky, the seismograph at the end of a P wave detects a “push” — the energy of this action. Although S waves don’t compress, they still travel through solids because the particles in solids elastically spring back even when moved only from side to side. This is not a property of liquids. In liquids, the energy of an S wave simply dissipates. So technically, the second Slinky described above represented an S wave traveling through a solid.

What were scientists able to learn from P and S waves?

Simulation of an Earth cross section.

The absence of S waves in certain places along with an understanding of S wave behavior in solids and liquids led scientists to conclude that the outer core is liquid and effectively absorbs S waves. As the number of seismic readings increased along with their precision, a worldwide community of scientists uncovered patterns that indicated a much more complicated picture of the Earth’s interior than was previously believed. Scientists have been able to distinguish the layers of the Earth that are made of different materials that transmit waves at different speeds. Based on these seismic observations, the Earth’s interior has been divided into the following layers:

• Crust: A very thin, solid outer layer. The oceanic crust is about 5 km (3 miles) thick. The continental crust is from 30–40 km (18–24 miles) thick.
• Moho: The boundary between the crust and the mantle.
• Mantle: The layer beneath the crust. The mantle is about 2885 km (1790 miles) thick.
• Upper mantle: Includes a solid layer fused to the crust. This layer combined with the crust is called the lithosphere. Beneath this is the asthenosphere, which is a partly molten layer. The asthenosphere is thought to be the layer upon which tectonic plates ride. The upper mantle is about 700 km (420 miles) thick.
• Lower mantle: Is composed of solid rock under conditions of extremely high temperature and pressure. This layer is about 2,185 km (1,370 miles) thick.
• Outer Core: A layer about 2,270 km (1,400 miles) thick, having the properties of a metallic liquid.
• Inner Core: A solid, metallic, spherical layer about 1,216 km (755 miles) thick.