Essential Science for Teachers: Earth and Space Science
Journey to the Earth’s Interior Journey to the Earth’s Interior | A Closer Look
A Closer Look
Look for the following topics in the video, indicated by the onscreen icon, and click below to learn more.
A Closer Look: Igneous Rocks
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?
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.
A Closer Look: Mapping Earth's Interior
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 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?
- 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.
Session 1 Earth’s Solid Membrane: Soil
How does soil appear on a newly born, barren volcanic island? In this session, participants explore how soil is formed, its role in certain Earth processes, its composition and structure, and its place in the structure of the Earth.
Session 2 Every Rock Tells A Story
How can we use rocks to understand events in the Earth's past? In this session, participants explore the processes that form sedimentary rocks, learn how fossils are preserved, and are introduced to the theory of plate tectonics.
Session 3 Journey to the Earth’s Interior
How do we know what the interior of the Earth is like if we've never been there? In this session, participants examine the internal structure of the Earth and learn how it is possible for entire continents to move across its surface.
Session 4 The Engine That Drives the Earth
What drives the movement of tectonic plates? In this session, participants learn how plates interact at plate margins, how volcanoes work, and the story of Hawaii's formation.
Session 5 When Continents Collide
How is it possible that marine fossils are found on Mount Everest, the world's highest continental mountain? In this session, participants learn what happens when continents collide and how this process shapes the surface of the Earth.
Session 6 Restless Landscapes
If almost all mountains are formed the same way, why do they look so different? In this session, participants learn about the forces continually at work on the surface of the Earth that sculpt the ever-changing landscape.
Session 7 Our Nearest Neighbor: The Moon
Why is the Moon, our nearest neighbor in the solar system, so different from the Earth? In this session, participants explore the complex connections between the Earth and Moon, the origin of the Moon, and the roles played by gravity and collisions in the Earth-Moon system.
Session 8 Order out of Chaos: Our Solar System
Why do all the planets orbit the Sun in the same direction and why are the planets closest to the Sun so different from the gas giants farther out? In this session, participants gain a better understanding of the nature of the solar system by examining its formation.