Education.com
Try
Brainzy
Try
Plus

Earth's Inner Structure Help (page 3)

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Mantle

The mantle, also known as the mesosphere , lies just beneath the crust. It forms over 83% of the Earth’s volume and about 58% by mass.

In the mantle, earthquake waves jump suddenly in speed (velocity) from the way they travel through the crust. This jump is determined by the change in density between the crustal rock and the denser mantle rock. This discontinuity between high and low velocities is called the Mohoroci On the Inside i On the Inside discontinuity or Moho for short. It is named after a Croatian seismologist and meteorologist, Andrija Mohoroci On the Inside i On the Inside , who first noticed its strange behavior after looking at seismic waves from the Kulpa Valley, Zagreb, Croatia earthquake in 1909.

Mohoroci On the Inside i On the Inside discovered that seismic waves came in two separate sets. Naming these wave sets, P- and S-waves, he noticed that one set arrived earlier than the other during the course of the quake. It was Mohoroci On the Inside i On the Inside ’s thought that one set had traveled through denser material than the other and was slowed by it. He proposed a theory that the Earth’s outer rocky crust is about 30km thick and rides on top of a denser mantle beneath it.

The Mohoroci On the Inside i On the Inside discontinuity is the boundary between the crust and the mantle.

When geologists began to track this wave change, they weren’t sure what it meant and they tested a lot of different theories. As they gathered more and more measurements, they found that the speed of the P- and S-waves followed the variations in the thickness of the crust. This was seen in crust measurements from about 35 to 40km below the continents and about 10km below the oceans. However, below some high mountain ranges like the Andes, it can be as deep as 70km in places.

This boundary between the crust and mantle became known as the Mohoroci On the Inside i On the Inside discontinuity . Research has fine tuned this slowing and found the seismic waves travel nearly 20% slower below the Moho than above it. Most seismologists consider the Mohoroci On the Inside i On the Inside discontinuity, where it meets the upper mantle, to be the bottom or deepest limit of the Earth’s crust.

Practice problems of this concept can be found at: Earth's Structure Practice Test

View Full Article
Add your own comment