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10.4 Studying Earth’s Interior. Key Idea: Scientists use data from seismic waves to learn about the structure of Earth’s interior. Remember:. The velocity of the seismic waves increases as the density of the rocks increases. (The more compact the rock, the higher the velocity of P and S waves)
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10.4 Studying Earth’s Interior Key Idea: Scientists use data from seismic waves to learn about the structure of Earth’s interior.
Remember: • The velocity of the seismic waves increases as the density of the rocks increases. (The more compact the rock, the higher the velocity of P and S waves) • The P waves travel through solids (rocks), and fluids (magma, or water). The P waves move faster than the S waves. • The S waves can only travel through solids. When the S are not received at some point, it is a sign that they traveled through a fluid material.
Changes in Behavior of Seismic Waves Scientists observed that: • P waves are greatly slowed around 2900 Km below the surface of the earth. • S waves are stopped at the same depth. Knowing that the velocity of seismic waves changes with the density of the rock, and that S waves do not travel through liquids, then the scientists concluded that the material below 2900 Km, the outer core, must be liquid.
Then… • P wave velocity increases again at a depth of about 5200 Km. This increase in speed suggests that the inner core is solid.
The Shadow Zone • Seismograph stations in what is called the earthquake’s “shadow zone” cannot detect P or S waves from the earthquake. • The Shadow zone is wide belt around the side of Earth opposite the focus of the earthquake.
Why Does the Shadow Zone Exists? • P waves passing through the mantle are refracted (bent) in smooth arcs back to the surface. • When P waves travel deep enough to enter the outer core, they are refracted again as they enter the outer core and yet again when they reenter the mantle. • As a result, seismograph stations that are between the earthquakes epicenter and the shadow zone receive both P and S waves. • Stations within the shadow zone receive neither P nor S waves, because P waves have been refracted away and S waves do not pass through the liquid outer core. • Stations Beyond the shadow zone, on the opposite side of the Earth receive only P waves.
Why Do Shadow Zones Exist? • Notice the refraction of the P waves and S waves. S waves do not pass through the core
The Moho Discontinuity • An increase in the velocities of P waves and S waves occurs between the crust and the mantle. This transition zone, which is denser then the crust was discovered by a Croatian scientist, Andreja Mohorovicic. • The discontinuity is named after him (today people use the short name: Moho) • The Moho discontinuity is about 32 Km under the continents, and between 5-10 km under the oceans.
Who Was Mohorovichic? • As a boy of 15 he spoke Italian, French, and English as well as his native Croatian, later added German, Czech, Latin, and old Greek. He studied physics at the University of Prague under some famous professors including E. Mach and did his graduate work at the University of Zagreb, from which he obtained a Ph.D. In 1894 Dr. Mohorovicic became Director of the Institute for Meteorology and Geodynamics and Professor at the University of Zagreb in 1897, where he remained until his retirement in 1921. His special interest was the precise measurement of time for both astronomical and seismical events, but his reputation mainly rests on his classic paper in the field of seismology, The Earthquake of October 8, 1909, which contains the news of his discovery of a major discontinuity at a depth of 55 kilometers. This discontinuity, now generally known as the Moho in his honor, defines the crust of the earth. Professor Mohorovicic died in 1936 in circumstances approaching poverty.
The Transition Zone • The P waves and S waves increase their velocities at depths between 400 and 670 Km. These depths mark a region in the middle of the mantle, which scientists call “the transition zone”. • This is because the material deeper within the mantle is under greater pressure from the overlaying material; the material is more compressed, the particles are closer packed together; in other words the material is more dense, and the P and S waves travel more quickly through it.
The Inner Structure of the Earth The direct scientific investigation of the deep layers of the Earth is basically impossible; the scientists use indirect methods explore the depth of our planet. • Based on the behavior of the seismic waves, the scientists were able to identify certain discontinuities which mark the transition from one inner layer to another. • Each layer has different chemical composition, density, and state.
