Important: – Earth’s interior has boundary alterations. At the boundary, seismic waves are refracted and reflected before returning to the surface. Arrivals of seismic waves at distant sites defined the limits. Seismic shadow zones have taught us a great deal about the earth’s interior. This demonstrates how P waves flow through solids and liquids, but S waves are impeded by the outer liquid core. Animation Freshmen The wave qualities of light are utilized as an example to help us comprehend the behavior of seismic waves.
Animation Freshmen The shadow zone is the region of the earth between 104 and 140 degrees angular distance from an earthquake that does not receive direct P waves. The various phases illustrate how the original P wave evolves as it encounters Earth’s limits. Animation Freshmen The shadow zone is the result of S waves being completely blocked by the liquid core.
Three distinct S-wave phases illustrate how the original S wave is halted (damped) or altered as it encounters Earth’s limits. Animation Freshmen Due to variations in composition, pressure, and temperature inside the Earth’s strata, seismic waves travel along a curved course within the planet.
Animation Freshmen Seismic waves move at various rates through various types of materials. In this two-layer model, two wave fronts hit simultaneously, but the lower layer moves quicker. Animation Freshmen Due to variations in composition, pressure, and temperature inside the Earth’s strata, seismic waves travel along a curved course within the planet.
Animation Freshmen Animated depiction of the competition between the direct seismic wave and the deeper, longer-path significantly refracted seismic wave. Graph documents the times of arrival. Animation Freshmen In this concept of rising velocity with depth, the severely refracted seismic rays accelerate as they travel through five distinct velocity barriers.
- Animation Intermediate In this concept of rising velocity with depth, the severely refracted seismic rays accelerate as they travel through five distinct velocity barriers.
- Intermediate Animation Intermediate Animation Students compare anticipated seismic wave travel periods, derived from a scaled Earth model, with observed seismic data from recent earthquakes, first in small groups and subsequently as a complete class.
This exercise employs models and actual data and stresses the scientific method. Lesson Novice Seismic waves, like other waves, obey the rules of physics. In this project, Physics students have the chance to apply their knowledge of fundamental wave principles (such as reflection, refraction, and energy transmission) as they analyze seismic data to calculate the distance between the surface and bedrock.
Lesson of Intermediate Level To view the distinction between P- and S-wave seismic trajectories as well as their corresponding shadow zones, roll over the buttons. Interactive Freshmen View seismograms of big earthquakes from stations across the world with ease. Plots may be utilized for a variety of purposes, including determining the diameter of the Earth’s outer core as a teaching activity.
Software-Web Application Novice View seismograms of big earthquakes from stations across the world with ease. Plots may be utilized for a variety of purposes, including determining the diameter of the Earth’s outer core as a teaching activity. Software-Web Application Novice Seismic Waves is a browser-based application for visualizing the propagation of earthquake-generated seismic waves through the Earth’s interior and over its surface.
How do various body waves interact with the interior of the Earth?
Internal waves within the earth Earthquakes create two types of waves that go through solid rock: With P or compressional waves, the rock vibrates in the propagation direction. The earthquake’s P waves move the fastest and are the first to arrive. Rock oscillates perpendicular to the propagation direction of S or shear waves.
- In rock, S waves move at approximately 60% the speed of P waves and always arrive after P waves.
- For instance, sound waves are P waves with a sufficiently high frequency to be audible.
- Wiggling or shaking a rope that is secured at one or both ends is an illustration of a S wave.
- Both P and S waves radiate from the epicenter of an earthquake within the ground.
Seismographs frequently record the waves as independent arrivals at great distances from the earthquake. The direct P wave reaches sooner because its path is via denser, faster-moving rocks located deeper inside the ground. The PP (one bounce) and PPP (two bounces) waves move more slowly than the straight P as a result of passing through shallower, slower-moving rocks.
- The S waves arrive following the P waves.
- Surface waves, such as the L wave, are the slowest (and last to arrive on seismograms).
- L waves are called after the Cambridge mathematician who initially characterized them, A.E.H. Love.
- Generally, the surface waves are the biggest recorded during an earthquake.
As they move away from the epicenter of an earthquake, body waves in the earth’s interior lose amplitude fast due to their dispersion inside the earth’s volume. However, surface waves propagate more slowly and exclusively on the earth’s surface. The surface confines the energy of surface waves to a smaller volume, hence the wave amplitude required to transport this energy is greater than that of body waves.
This region is referred to as the asthenosphere or “weak sphere.” The asthenosphere divides the strong, solid rock of the highest mantle and crust from the remainder of the mantle. The lithosphere consists of the topmost mantle and crust above the asthenosphere.
Over the weak asthenosphere, the lithosphere is free to move (glide). In actuality, the tectonic plates are lithospheric plates. Seismic Discontinuity at 670 km Below the zone of low seismic velocity, there are a few seismic discontinuities where seismic velocities rise. At these depths (pressures), theoretical calculations and laboratory tests indicate that ultramafic silicates shift phase (atomic packing structure or crystalline structure) from the crystalline structure of olivine to structures with 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).
- 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.
- This acceleration is compatible with the transition from a liquid outer core to a solid inner core.
- The above illustration depicts seismic ray trajectories (perpendicular to seismic wave fronts) through the Earth.
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).
Why do waves travel at varying rates across the strata of the Earth?
Why do waves’ speeds vary as they pass through the earth’s layers? Question Date: 2016-02-02 The interior of the Earth is made of solid and liquid layers of various compositions. Scientists know this because s-waves (elastic waves that shake the ground perpendicular to the direction the wave is traveling) and p-waves (primary waves) created by earthquakes behave differently as they travel into the Earth’s core.
S-waves cannot flow through liquids and so cannot pass through the outer core of the Earth, which is liquid. P-waves may travel through both solids and liquids, although their speed varies as they go through layers of varying stiffness (compositions and densities). Generally speaking, p-wave velocity rises with increasing depth and material stiffness.
Therefore, p-waves move the quickest through the Earth’s inner core of solid iron. Answer 2: Waves move differently across various materials. Imagine touching the surface of water and observing how the ripples spread, and then touching the surface of Jell-O and observing how the ‘ripples’ spread; the wave that our hand creates in water is completely different from that in Jell-O, because one is a liquid and the other is a gel.
The same concepts apply to earth’s strata. Since the inner core of the planet is solid, waves move there more quickly than in the mantle, which is liquid (semi-solid, semi-liquid). Some waves may pass through solids but not liquids, allowing us to determine which portions of the planet are solid or liquid.
Scientists originally found that the inner core is solid by observing the differing behavior of waves in different places of the planet. Answer 3: Varied strata within the Earth’s interior have different densities, as they are composed of different substances at different temperatures.
Ch.17, pg.440: #1-4, 6, 7, 12 Describe how seismic reflection and seismic refraction demonstrate the existence of Earth’s layers. When seismic waves hit a layer of rock with a differing density, they may reflect or bend as they pass the barrier. By observing these patterns of reflection and refraction, the presence and depth of Earth’s strata may be determined.2.
Draw a cross-section of the entire planet, indicating the major inner divisions and providing the name, thickness, and likely composition of each. Figure 17.7 appears on page 423. What evidence suggests that the majority of the Earth’s core is formed of iron? Estimates of the core’s density, the abundance and composition of iron meteorites, and the existence of a magnetic field on Earth all indicate that iron is the predominant element.
Describe the distinctions between continental and oceanic crusts. The oceanic crust is 7 kilometers in thickness, denser, and formed of basalt-like rock. Continental crust is thicker (30-50 km), less dense, and consists of granite-like rock coated with sedimentary rock than oceanic crust.
- Discuss seismic-wave shadow zones and what they reveal about the interior of the Earth.
- For P-waves, the Earth has a shadow zone extending from 103 to 142 degrees from the earthquake’s epicenter.
- This shadow zone is the result of P waves being refracted (bent) when they meet the core barrier.
- It is known that S-waves cannot pass through liquid.
There is an S-wave shadow zone beyond 103 degrees from the epicenter where S-waves do not propagate. Describe the magnetic field of the Earth. Where is it produced? It is an area of magnetic force that encircles the Earth. Its invisible magnetic force lines encircle the planet and deflect magnetized things, such as compass needles.
At the magnetic poles, where magnetic lines of force appear to exit and enter Earth vertically, the Earth’s magnetic field is greatest. It is presumably formed in the outer core’s liquid metal. What distinguishes the lithosphere from the asthenosphere? Consisting of the crust and mantle, the lithosphere is generally solid and brittle.
The asthenosphere is comprised of ductile-appearing mantle rocks located close under the lithosphere. It is likely that the asthenosphere reflects a minute fraction of partial melting. Last update 2/24/2005 Jackson created the website. Hiram Jackson, the Geology webmanager, may be reached at [email protected]