{"id":96,"date":"2017-01-23T16:35:20","date_gmt":"2017-01-23T16:35:20","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/introduction-to-oceanography\/chapter\/3-3-determining-the-structure-of-earth\/"},"modified":"2021-10-25T21:06:14","modified_gmt":"2021-10-25T21:06:14","slug":"3-3-determining-the-structure-of-earth","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/introduction-to-oceanography\/chapter\/3-3-determining-the-structure-of-earth\/","title":{"raw":"3.3 Determining the Structure of Earth","rendered":"3.3 Determining the Structure of Earth"},"content":{"raw":"
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\r\n\r\nThe previous section described the properties and composition of Earth's interior, which begs the question: how can we know what conditions are like deep in the Earth? It's easy to sample the [pb_glossary id=\"670\"]crust[\/pb_glossary] through drilling, and [pb_glossary id=\"930\"]mantle [\/pb_glossary] material often comes to the surface as magma, but the farthest we have been able to drill into the crust so far is only about 12 km; this for a planet with a radius of 6370 km! So to understand the composition and structure of the Earth's deep interior, we need to use indirect methods such as seismology.\r\n\r\n[pb_glossary id=\"1130\"]Seismology [\/pb_glossary]<\/strong> is the study of vibrations within the Earth. These vibrations are caused by various events, including earthquakes, extraterrestrial impacts, explosions, storm waves hitting the shore, and tidal effects. Of course, seismic techniques have been most widely applied to the detection and study of earthquakes, but there are many other applications, and arguably seismic waves provide the most important information that we have concerning Earth\u2019s interior. Before going any deeper into Earth, however, we need to take a look at the properties of seismic waves. The types of waves that are useful for understanding Earth\u2019s interior are called [pb_glossary id=\"596\"]body waves[\/pb_glossary]<\/strong>, meaning that, unlike the surface waves on the ocean, they are transmitted through Earth materials.\r\n\r\nImagine hitting a large block of strong rock (e.g., [pb_glossary id=\"786\"]granite[\/pb_glossary]) with a heavy sledgehammer. At the point where the hammer strikes it, a small part of the rock will be compressed by a fraction of a millimeter. That compression will transfer to the neighboring part of the rock, and so on through to the far side of the rock, from where it will bounce back to the top \u2014 all in a fraction of a second. This is known as a compression wave, and it can be illustrated by holding a loose spring (like a Slinky) that is attached to something (or someone) at the other end. If you give it a sharp push so the coils are compressed, the compression propagates (travels) along the length of the spring and back (Fig. 3.3.1). You can think of a compression wave as a \u201cpush\u201d wave \u2014 it\u2019s called a [pb_glossary id=\"1060\"]P-wave[\/pb_glossary]<\/strong> (although the \u201cP\u201d stands for \u201cprimary\u201d because P-waves are the first to arrive at seismic stations). In a P-wave the motion of the particles is parallel to the direction of wave propagation.\r\n\r\nWhen we hit a rock with a hammer, we also create a different type of body wave, one that is characterized by back-and-forth vibrations (as opposed to compressions). This is known as a shear wave ([pb_glossary id=\"1202\"]S-wave[\/pb_glossary]<\/strong>, where the \u201cS\u201d stands for \u201csecondary\u201d), and an analogy would be what happens when you flick a length of rope with an up-and-down motion. As shown in Figure 3.3.1, a wave will form in the rope, which will travel to the end of the rope and back. In this case, the motion of the particles is perpendicular to the direction the wave travels.\r\n\r\n[caption id=\"attachment_90\" align=\"aligncenter\" width=\"700\"]\"A<\/a> Figure 3.3.1<\/strong> Representations of a compression wave (P-wave, top) and a shear wave (S-wave, bottom) (Steven Earle, \"Physical Geology\").[\/caption]\r\n\r\nCompression waves and shear waves travel very quickly through geological materials. As shown in Figure 3.2.2, typical P-wave velocities are between 0.5 km\/s and 2.5 km\/s in unconsolidated sediments, and between 3.0 km\/s and 6.5 km\/s in solid crustal rocks. Of the common rocks of the crust, velocities are greatest in [pb_glossary id=\"570\"]basalt [\/pb_glossary] and [pb_glossary id=\"786\"]granite[\/pb_glossary]. S-waves are slower than P-waves, with velocities between 0.1 km\/s and 0.8 km\/s in soft [pb_glossary id=\"1126\"]sediments[\/pb_glossary], and between 1.5 km\/s and 3.8 km\/s in solid rocks.\r\n\r\n \r\n\r\n[caption id=\"attachment_91\" align=\"aligncenter\" width=\"400\"]\"Typical<\/a> Figure 3.2.2<\/strong> Typical velocities of P waves (red) and S waves (blue) in sediments and in solid crustal rocks (Steven Earle, \"Physical Geology\").[\/caption]\r\n\r\n[pb_glossary id=\"930\"]Mantle [\/pb_glossary] rock is generally denser and stronger than [pb_glossary id=\"670\"]crustal [\/pb_glossary] rock and both P- and S-waves travel faster through the mantle than they do through the crust. Moreover, seismic-wave velocities are related to how tightly compressed a rock is, and the level of compression increases dramatically with depth. Finally, seismic waves are affected by the [pb_glossary id=\"1022\"]phase [\/pb_glossary] state of rock. They are slowed if there is any degree of melting in the rock. If the material is completely liquid, P-waves are slowed dramatically and S-waves are stopped altogether.\r\n\r\n \r\n\r\n[caption id=\"attachment_92\" align=\"aligncenter\" width=\"800\"]\"Wave<\/a> Figure 3.3.3<\/strong> Wave velocities through the different layers of the Earth (left). Enhanced view of wave velocities in the crust and upper mantle (right) (Steven Earle, \"Physical Geology\").[\/caption]\r\n\r\nAccurate seismometers have been used for earthquake studies since the late 1800s, and systematic use of seismic data to understand Earth\u2019s interior started in the early 1900s. The rate of change of seismic waves with depth in the Earth (Fig. 3.3.3) has been determined over the past several decades by analyzing seismic signals from large earthquakes at seismic stations around the world. Small differences in arrival time of signals at different locations have been interpreted to show that:\r\n