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Chapter 17: Problem 55

The ocean floor is underlain by a layer of basalt that constitutes the crust, or uppermost layer, of the Earth in that region. Below this crust is found denser periodotite rock, which forms the Earth's mantle. The boundary between these two layers is called the Mohorovicic discontinuity ("Moho" for short). If an explosive charge is set off at the surface of the basalt, it generates a seismic wave that is reflected back out at the Moho. If the speed of this wave in basalt is \(6.50 \mathrm{km} / \mathrm{s}\) and the two-way travel time is \(1.85 \mathrm{s}\) what is the thickness of this oceanic crust?

Short Answer

Expert verified
The thickness of the oceanic crust is approximately \(6.01 \mathrm{km}\).

Step by step solution

01

Understand the given information

It's given that the speed of the seismic wave in basalt is \(6.50 \mathrm{km} / \mathrm{s}\) and the two-way travel time is \(1.85 \mathrm{s}\). This means that the wave travels down to the boundary and then back up again.
02

Calculate the total distance travelled

Given that speed is distance over time, we can rearrange this formula to find distance by multiplying speed by time. Distance \( s \) is therefore \( s = vt \), where \( v = 6.50 \mathrm{km} / \mathrm{s} \) and \( t = 1.85 \mathrm{s} \). Therefore, \( s = 6.50 \times 1.85 = 12.025 \mathrm{km} \).
03

Calculate the thickness of the oceanic crust

The total distance travelled by the wave is twice the thickness of the oceanic crust, as the wave has travelled down and back up again. Therefore, the thickness \( d \) of the oceanic crust is half the total distance travelled: \( d = s / 2 = 12.025 / 2 = 6.0125 \mathrm{km} \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Oceanic Crust
The Oceanic Crust forms a fundamental part of our Earth鈥檚 structure. It is the portion of the Earth's surface that is under the ocean. This thin layer comprises primarily a type of rock known as basalt, which is less dense than the rocks found deeper down in the Earth's interior. Unlike the thicker continental crust, which supports the land we live on, the oceanic crust is relatively thin, usually only about 5 to 10 kilometers thick.
During seismic studies, such as the one described in the exercise, the oceanic crust plays a key role. By understanding how seismic waves reflect and travel through this layer, scientists can map its thickness and composition.
  • Thin layer compared to continental crust
  • Composed mainly of basalt
  • Critical in seismic studies
Basalt
Basalt is a type of igneous rock that makes up the majority of the oceanic crust. It forms from the cooling of lava and is known for its dark color and fine grain. This volcanic rock is significant because it is denser than granite, which makes up most continental crust. Therefore, basalt plays a crucial part in how seismic waves travel through the Earth's crust.
In the context of the exercise, the basalt layer is where seismic waves begin their journey. As they travel at speeds of around 6.50 km/s, measuring their travel time provides data on the thickness of the basalt layer.
  • Dark-colored igneous rock
  • Makes up oceanic crust
  • Denser than continental rocks
Mohorovicic Discontinuity
The Mohorovicic Discontinuity, often called the "Moho," represents the boundary between the Earth's crust and the underlying mantle. This discontinuity is named after the Croatian seismologist Andrija Mohorovi膷i膰, who discovered it in 1909. It signifies a significant change in rock composition and density, as the relatively less dense basaltic rock of the crust meets the denser periodotite rock of the mantle.
Seismic waves reflect off this boundary, allowing scientists to measure their travel times and hence determine the depth of the Moho. Understanding this boundary is crucial, as it can give insights into tectonic activities and the dynamics of our planet's interior.
  • Boundary between crust and mantle
  • Discovered by Andrija Mohorovi膷i膰
  • Important for understanding tectonics

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Most popular questions from this chapter

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