How Does The Structure Of Earth’S Interior Affect Seismic Waves?

How Does The Structure Of Earth
Evidence for Internal Earth Structure from Seismology Geological Waves When an earthquake happens, seismic waves (P and S waves) propagate through the Earth’s core in all directions. Seismic stations positioned at varying distances from the epicenter of an earthquake will capture seismic waves that have traveled through increasing Earth depths.

  • Seismic velocities rely on the material qualities of the medium through which seismic waves travel, such as composition, mineral phase and packing structure, temperature, and pressure.
  • Seismic waves travel faster through denser materials and, thus, move faster increasing depth.
  • Abnormally warm regions retard seismic vibrations.

Through a liquid, seismic vibrations travel more slowly than through a solid. Molten regions inside the Earth delay P waves and prevent S waves from propagating because shearing motion cannot be communicated through a liquid. Partially molten regions may delay P waves and diminish or weaken S waves.

  1. When seismic waves traverse across geologic strata with opposing seismic velocities (when any wave passes through medium with markedly different velocities), reflections, refraction (bending), and the generation of new wave phases (e.g., a S wave generated from a P wave) frequently occur.
  2. Seismic discontinuities refer to abrupt changes in seismic velocity across a barrier.

The Mohorovicic Seismic Discontinuity of the Crust Seismic stations within 200 kilometers of a continental earthquake (or other seismic disturbance, such as a dynamite blast) report that transit durations rise regularly with distance. However, beyond 200 kilometers, seismic waves reach earlier than anticipated, causing a break in the travel time versus distance curve.

This was interpreted by Mohorovicic (1909) to suggest that seismic waves recorded beyond 200 kilometers from the earthquake source had traveled through a layer with much higher seismic velocity. This seismic discontinuity is currently referred to as the Moho (much easier than “Mohorovicic seismic discontinuity” ) It is the border between the felsic/mafic crust, which has a seismic velocity of around 6 km/sec, and the denser ultramafic mantle, which has a seismic velocity of approximately 8 km/sec.

The typical depth of the Moho under the continents is from 20 to 70 kilometers deep. The Moho is typically roughly 7 kilometers below the seafloor (i.e., ocean crust is about 7 km thick).

Properties of the Crust Continental Crust Depth to Moho: 20 to 70 km, average 30 to 40 km Composition: felsic, intermediate, and mafic igneous, sedimentary, and metamorphic rocks Age: 0 to 4 b.y. Summary: thicker, less dense, heterogeneous, old Oceanic Crust Depth to Moho: ~7 km Composition: mafic igneous rock (basalt & gabbro) with thin layer of sediments on top Age: 0 to 200 m.y. Summary: thin, more dense, homogeneous, young

The Mantle Low Velocity Zone Seismic velocities tend to steadily rise with depth in the mantle due to the increased pressure, and hence density, with depth. However, seismic waves recorded at distances corresponding to depths of roughly 100 km to 250 km arrive later than predicted suggesting a zone of low seismic wave velocity.

Furthermore, while both the P and S waves travel more slowly, the S waves are attenuated or weakened. This is understood to represent a zone that is partly molten, presumably one percent or less (i.e., higher than 99 percent solid) (i.e., greater than 99 percent solid). Alternatively, it may simply signify a zone where the mantle is extremely close to its melting point for that depth and pressure that it is highly “soft.” Then this reflects a zone of weakness in the upper mantle.

This zone is called the asthenosphere or “weak sphere.” The asthenosphere divides the strong, solid rock of the highest mantle and crust above from the remainder of the strong, solid mantle below. The combination of topmost mantle and crust above the asthenosphere is termed the lithosphere,

The lithosphere is free to travel (glide) across the weak asthenosphere. The tectonic plates are, in reality, lithospheric plates,670 km Seismic Discontinuity Below the low velocity zone are a few of seismic discontinuities at which seismic velocities increase. Theoretical analysis and laboratory experiments demonstrate that at these depths (pressures) ultramafic silicates will shift phase (atomic packing structure or crystalline structure) from the crystalline structure of olivine to tighter packing structures.

A discontinuity at around 670 kilometers deep is quite prominent. The 670 km discontinuity occurs from the transition from spinel to perovskite crystal structure, which is stable to the mantle’s base. Perovskite, which shares the same chemical formula as olivine, is the most prevalent silicate mineral on Earth.

  • It is believed that the 670 km discontinuity constitutes a key barrier between the less dense upper mantle and the more dense lower mantle.
  • Gutenberg Discontinuity of Seismic Waves / Core-Mantle Boundary Recorded seismic waves at increasing distances from an earthquake demonstrate that seismic velocities rise steadily with mantle depth (exceptions: see Low Velocity Zone and 670 km Discontinuity above).

However, between around 103° and 143° arc distance, no P waves are reported. In addition, no S waves are recorded beyond around 103°. Gutenberg (1914) attributed this to a molten core originating at a depth of around 2900 kilometers. This molten layer would prevent shear waves from penetrating, and P waves would be significantly delayed and refracted (bent).

  1. Lehman Seismic Disruption / The Inner Core Between 143° and 180° after an earthquake, there is a second refraction (Lehman, 1936) caused by a dramatic rise in P wave velocities at a depth of 5150 km.
  2. This acceleration is compatible with the transition from a liquid outer core to a solid inner core.
  3. The above illustration depicts seismic ray trajectories (perpendicular to seismic wave fronts) through the Earth.
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What does the Core Consist of? This material must be thick: it must be denser than the mantle and dense enough to account for the remainder of Earth’s mass. Since the core comprises approximately one-third of the Earth’s mass, it must be composed of a common solar system substance.

  • It must explain the reported seismic velocity measurements.
  • In order to account for the Earth’s magnetic field, it should likewise possess magnetic characteristics.
  • Iron is the most apparent option.
  • There are several types of meteorites discovered on Earth.
  • Differentiated meteorites are one classification.

They are believed to depict planetesimals that formed alongside the Earth and other planets. The planetesimal became large enough to become partially molten and to separate into a silicate mantle and a metallic core, which subsequently cooled and solidified slowly.

  • Contradictory gravitational pulls from the Sun and Jupiter caused the planet to fragment as it grew.
  • The remnants are located between Mars and Jupiter in orbit.
  • Some of the fragments that fall to Earth are composed of silicates (mafic and ultramafic) and iron.
  • Presumably, iron meteorites are the remnants of the planetesimal’s core.

What causes the magnetic field of the Earth? Earlier theories on the source of the compass needle pointing north included divine attraction to the polestar (North Star) or attraction to massive iron ore deposits in the arctic. According to a more credible theory, the Earth or a solid layer within the Earth is composed of iron or other magnetic material that forms a permanent magnet.

There are two significant issues with this idea. Initially, it became evident that the magnetic field wanders with time; magnetic poles migrate. Second, magnetic materials only retain permanent magnetism below their Curie temperature (e.g., magnetite’s Curie temperature is 580 °C). The majority of the Earth’s core is hotter than all known Curie temperatures, and the magnetic field cannot be explained by the magnetic content of cooler crustal rocks, which are also highly heterogeneous.

The finding of the outer core’s liquid state enabled the geodynamo theory. Iron, whether liquid or solid, is an electrical conductor. Therefore, electric currents would flow through molten iron. Flowing electric current creates a magnetic field at a right angle to the direction of the electric current (basic physics of electromagnetism).

How do seismic waves connect to the interior of the Earth?

Seismic waves indicate that the interior of the Earth is composed of a series of concentric shells, including a thin outer crust, a mantle, a liquid outer core, and a solid inner core. P waves, or primary waves, move the quickest and hence reach at seismic stations first. The S waves, or secondary waves, follow the P waves.

We may utilize our understanding of Earth to comprehend planets in our solar system and those orbiting faraway stars. Importance of researching the earth’s interior

How are seismic waves utilized to measure the position and thickness of the inner layers of the Earth?

Notes for Chapter 19 Earth’s (Interior) The Earth’s Interior Paths of Seismic Waves in Earth Waves Penetrating the Earth’s Interior If the composition of the entire planet were homogeneous, P and S waves would move across the planet in virtually straight lines.

The makeup of the earth, however, is stratified, and the density of rocks, particularly in the mantle, often rises with depth (Figure 19.2). As a result, seismic waves that flow through the earth bend and reflect. Seismic waves from the epicenter of an earthquake move through the ground via curved trajectories, finally being detected by distant seismograph stations.

The nature of the waves and the time it takes for them to reach a given spot provide crucial information about the interior of the planet. Due to the increasing density of mantle rocks with depth, P-waves often bend outward as they travel into the mantle, as seen in Figure 19.2a.

  1. However, when P-waves contact the outer core, they bend downward as they go through and bend again as they depart.
  2. This suggests that the outer core has a considerably different composition than the mantle and is perhaps liquid.
  3. This bending in the outer core produces a P-wave shadow zone where P-waves cannot be detected.

The term for the bending of seismic waves is refraction. S-waves do not go through the outer core, generating an even larger shadow zone for S-waves (figure 19.2b). The absence of S-wave propagation through the outer core shows that it is liquid. Reflected Waves on the Earth Some seismic waves reflect when they reach the boundary between two distinct materials, as seen in Figure 19.3.2.

  1. A PcP wave is a P-wave that has returned to the surface after bouncing off the mantle-core barrier.3.
  2. PP and SS waves are reflected at the surface and returned to the mantle without reaching the core.
  3. A PKP wave travels through the outer liquid core whereas a PKIKP wave crosses the inner solid core.
  4. Composition and Structure of the Interior of the Earth Its Crust Seismic tests of the earth’s outermost layer reveal that the thickness of the crust varies considerably.
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The oceanic crust is thin (5 km on average) and mostly consists of basalt. (b) The continental crust is significantly thicker (40-65 km) and composed predominantly of granite. Therefore, continental crust is less dense (more buoyant) than oceanic crust.

Figure 19.7: Seismic waves increase sharply under the crust, showing a distinct barrier between the crust and upper mantle. This is because the composition of the upper mantle changes from granite or basalt to peridotite. Moho refers to the barrier between the crust and upper mantle. Figure 19.7: The lithosphere consists of the crust and the very top section of the upper mantle, which contains the Moho.

The lithosphere comprises the lithospheric plates and is the hard outermost layer of the Earth.1. The Mantle Illustration 19.7: Within a zone immediately under the lithosphere, the speed of S-waves drops. This indicates that the peridotite inside this zone has a small amount of partial melting, but not enough to entirely prevent the S-waves from propagating.

This area is consequently known as the asthenosphere or zone of low velocity. From 200 to 400 kilometers in depth, the velocity of S-waves progressively rises until they reach the 400-kilometer transition zone, when it accelerates abruptly. This rise may be the result of a transition in the crystal structure of olivine to the spinel structure, characterized by a tighter atomic packing.

At the 670 km transition zone, the S-wave velocity increases abruptly, indicating a second change to an even tighter atomic packing as the spinel structure transforms into that of perovskite.4. Below the transition zone at 670 km depth, the S-wave and P-wave velocities grow less dramatically until they reach the mantle-core barrier at 2900 km deep.

Figure 19.5: The slowing of P-waves in the outer core and the inability of S-waves to travel through the outer core indicate that the outer core is liquid. The outer core is formed of molten iron, based on experimental observations of seismic waves through various materials and the fact that the core holds one-third of the Earth’s mass.

P-waves accelerate once more into the inner core, and S-waves also move within it, indicating that the inner core is formed of solid iron and nickel. The increase in temperature with depth in the ground is shown by the geotherm curve in Figure 19.10.4.

In general, the geotherm lies below the mantle melting curve until 2900 km deep, where the two curves intersect at the mantle-core border. The geotherm is above the iron melting curve in the outer core.5. At the border between the outer and inner cores, the two curves cross again, and the geotherm is once more below the iron melting curve, therefore the inner core is formed of solid Fe.

The liquid iron in the outer core is agitated into convective motion by heat created by radioactivity in the core, as seen in Figure 17.C. The circulation of liquid iron in the outer core generates electric currents, which in turn form the magnetic field of the earth.

How does this internal heat impact the Earth’s surface?

Heat generated within the Earth by the radioactive decay of atoms and residual heat from the planet’s birth. This heat is the driving force behind plate tectonics and portions of the rock cycle. Plate tectonic processes can result in the opening and shutting of ocean basins and the separation and reunification of continents over millions of years.

CHAPTER 19 NOTES Earth’s (Interior) (Interior) CHAPTER 19: The Earth’s Interior Paths of Seismic Waves in the Earth Waves Traveling Through the Earth 1. If the entire globe was of homogeneous composition, then P and S waves would flow through the earth in nearly straight lines.2.

Figure 19.2 : The earth, however, is compositionally stratified and the density of rocks, particularly in the mantle, often increases with depth. As a result, seismic waves bend and reflect as they travel through the ground.3. Seismic waves from an earthquake’s focus move through the earth along bent routes and are finally detected by distant seismograph stations.

The character of the waves and the time it takes for them to reach a given spot gives crucial information as to the nature of the earth’s interior.4. Figure 19.2a : P-waves often bend outward as they move into the mantle due to the increased density of mantle rocks with depth.

When P-waves contact the outer core, however, they bend downward when going through the outer core and bend again when they depart. This suggests that P-waves slow down at the outer core, implying that this layer has a considerably different composition from the mantle and may really be liquid. This bending in the outer core causes a P-wave shadow zone where no P-waves are recorded.5.

The bending of seismic waves is termed refraction,6. Figure 19.2b : S-waves do not flow through the outer core, providing an even greater shadow zone for S-waves. The fact that S-waves do not go into the outer core shows that the latter is liquid. Waves Reflected in the Earth Some seismic waves reflect when they reach the boundary between two distinct materials, as seen in Figure 19.3.2.

  • A PcP wave is a P-wave that has returned to the surface after bouncing off the mantle-core barrier.3.
  • PP and SS waves are reflected at the surface and returned to the mantle without reaching the core.
  • A PKP wave travels through the outer liquid core whereas a PKIKP wave crosses the inner solid core.
  • Composition and Structure of the Interior of the Earth Its Crust Seismic tests of the earth’s outermost layer reveal that the thickness of the crust varies considerably.
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The oceanic crust is thin (5 km on average) and mostly consists of basalt. (b) The continental crust is significantly thicker (40-65 km) and composed predominantly of granite. Therefore, continental crust is less dense (more buoyant) than oceanic crust.

  • Figure 19.7: Seismic waves increase sharply under the crust, showing a distinct barrier between the crust and upper mantle.
  • This is because the composition of the upper mantle changes from granite or basalt to peridotite.
  • Moho refers to the barrier between the crust and upper mantle.
  • Figure 19.7: The lithosphere consists of the crust and the very top section of the upper mantle, which contains the Moho.

The lithosphere comprises the lithospheric plates and is the hard outermost layer of the Earth.1. The Mantle Illustration 19.7: Within a zone immediately under the lithosphere, the speed of S-waves drops. This indicates that the peridotite inside this zone has a small amount of partial melting, but not enough to entirely prevent the S-waves from propagating.

This area is consequently known as the asthenosphere or zone of low velocity. From 200 to 400 kilometers in depth, the velocity of S-waves progressively rises until they reach the 400-kilometer transition zone, when it accelerates abruptly. This rise may be the result of a transition in the crystal structure of olivine to the spinel structure, characterized by a tighter atomic packing.

At the 670 km transition zone, the S-wave velocity increases abruptly, indicating a second change to an even tighter atomic packing as the spinel structure transforms into that of perovskite.4. Below the transition zone at 670 km depth, the S-wave and P-wave velocities grow less dramatically until they reach the mantle-core barrier at 2900 km deep.

Figure 19.5: The slowing of P-waves in the outer core and the inability of S-waves to travel through the outer core indicate that the outer core is liquid. The outer core is formed of molten iron, based on experimental observations of seismic waves through various materials and the fact that the core holds one-third of the Earth’s mass.

P-waves accelerate once more into the inner core, and S-waves also move within it, indicating that the inner core is formed of solid iron and nickel. The increase in temperature with depth in the ground is shown by the geotherm curve in Figure 19.10.4.

  1. In general, the geotherm lies below the mantle melting curve until 2900 km deep, where the two curves intersect at the mantle-core border.
  2. The geotherm is above the iron melting curve in the outer core.5.
  3. At the border between the outer and inner cores, the two curves cross again, and the geotherm is once more below the iron melting curve, therefore the inner core is formed of solid Fe.

The liquid iron in the outer core is agitated into convective motion by heat created by radioactivity in the core, as seen in Figure 17.C. The circulation of liquid iron in the outer core generates electric currents, which in turn form the magnetic field of the earth.

What evidence enables us to characterize the interior of the Earth?

For this reason, scientists rely on seismic waves — shock waves created by earthquakes and explosions that travel through Earth and over its surface — to disclose the internal structure of the planet. Every year, thousands of earthquakes occur, and each one gives a brief view of the interior of the Earth.

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