Both swaves and pwaves can travel easily through liquids in carry
These are believed to be reflected off the surface of a solid inner core , of radius km, lying at the center of the liquid outer core. Because earthquakes occur often and at widespread places across the globe, geologists have accumulated a large amount of data about shadow zones and seismic-wave properties. They have used these data, along with direct knowledge of surface rocks, to build mathematical models of Earth's interior.
Earth's outer core is surrounded by a thick mantle and topped with a thin crust. The mantle is about km thick and accounts for the bulk 80 percent of our planet's volume. Density and temperature both increase with depth. Much of the mantle has a density midway between the densities of the core and crust: Note the sharp density discontinuity between Earth's core and mantle. The high central density suggests to geologists that the inner parts of Earth must be rich in nickel and iron.
The sharp density increase at the mantle—core boundary results from the difference in composition between the two regions. The mantle is composed of dense but rocky material, compounds of silicon and oxygen.
The core consists primarily of even denser metallic elements. The model suggests that the core must be a mixture of nickel, iron, and some other lighter element, possibly sulfur. Without direct observations, it is difficult to be absolutely certain of the light component's identity.
All geologists agree that much of the core must be liquid. The existence of the shadow zone demands that and, as we will see, our current explanation of Earth's magnetic field relies on it. Despite the fact that no experiment has yet succeeded in piercing Earth's crust to recover a sample of the mantle, we are not entirely ignorant of the mantle's properties.
In a volcano, hot lava upwells from below the crust, bringing a little of the mantle to us and providing some inkling of Earth's interior. The chemical makeup and physical state of newly emerged lava are generally consistent with predictions based on the model sketched in Figure 7. The composition of the upper mantle is probably quite similar to the iron—magnesium—silicate mixtures known as basalt. You may have seen some dark gray basaltic rocks scattered across Earth's surface, especially near volcanoes.
Basalt is formed as mantle material upwells from Earth's interior as lava, then cools and solidifies. Granite is richer than basalt in the light elements silicon and aluminum, which explains why the surface continents do not sink into the interior. Their low-density composition lets the crust "float" atop the denser matter of the mantle and core below. Earth, then, is not a homogeneous ball of rock.
Instead, it has a layered structure, with a low-density crust at the surface, intermediate-density material in the mantle, and a high-density core. Such variation in density and composition is known as differentiation. Why isn't our planet just one big, rocky ball of uniform density? The answer appears to be that much of Earth was molten at some time in the past. As a result, the higher-density matter sank to the core, and the lower-density material was displaced toward the surface. A remnant of this ancient heating exists today: Earth's central temperature is nearly equal to the surface temperature of the Sun.
What processes were responsible for heating the entire planet to this extent? To answer this question, we must try to visualize the past. We will see in Chapter 15 that when Earth formed, it did so by capturing material from its surroundings, growing in mass as it swept up "preplanetary" chunks of matter in its vicinity.
As the young planet grew, its gravitational field strengthened and the speed with which newly captured matter struck its surface increased. As Earth began to differentiate and heavy material sank to the center, even more gravitational energy was released, and the interior temperature must have increased still further. Later, Earth continued to be bombarded with debris left over from the formation process. At its peak some 4 billion years ago, this secondary bombardment was probably intense enough to keep the surface molten, but only down to a depth of a few tens of kilometers.
Erosion by wind and water has long since removed all trace of this early period from the surface of Earth, but the Moon still bears visible scars of the onslaught. These elements emit energy as their complex, heavy nuclei decay into simpler, lighter ones.
While the energy produced by the decay of a single radioactive atom is tiny, Earth contained a lot of radioactive atoms, and a lot of time was available. Rock is such a poor conductor of heat that the energy would have taken a very long time to reach the surface and leak away into space, so the heat built up in the interior, adding to the energy left there by Earth's formation.
British Broadcasting Corporation Home. Vibrations from an earthquake are categorised as P or S waves. They travel through the Earth in different ways and at different speeds.
They can be detected and analysed. A wave is a vibration that transfers energy from one place to another without transferring matter solid, liquid or gas. Light and sound both travel in this way. Energy released during an earthquake travels in the form of waves around the Earth. Two types of seismic wave exist, P- and S-waves.
They are different in the way that they travel through the Earth. P-waves P stands for primary arrive at the detector first. They are longitudinal waves which mean the vibrations are along the same direction as the direction of travel. Other examples of longitudinal waves include sound waves and waves in a stretched spring. Your web browser does not have JavaScript switched on at the moment.
For information on how to enable JavaScript please go to the Webwise site. S-waves S stands for secondary arrive at the detector of a seismometer second.