Earth’s Layers (1st upper layer crust)

Combining the data obtained from seismological studies and mineral physics experiments has given us a layer-by-layer understanding of Earth’s composition. The variations in seismic velocities with depth are shown in Fig 7

By examining the behavior of a variety of rocks at the pressures corresponding to these depths, geologists have made important discoveries about the compositions of Earth’s crust, mantle, and core.

Earth’s Crust: (1st layer of earth)

Earth’s crust consists of two distinct types—continental crust and oceanic crust. Continental crust and oceanic crust have very different compositions, histories, and ages. In fact, oceanic crust is compositionally more similar to the mantle than to the continental crust.
Oceanic Crust:
The ocean crust averages about 7 kilometers (4.5 miles) thick and forms at mid-ocean ridges, which separate two diverging tectonic plates. Ocean crust has a density of about 3.0 g/cm3, which compares to measured values for the rocks basalt and gabbro.
Continental Crust:
While oceanic crust is fairly uniform, no two continental regions have the same structure or composition. Continental crust averages about 40 kilometers (25 miles) thick but can be more than 70 kilometers (45 miles) thick in mountainous regions such as the
Himalayas and the Andes. The thinnest crust in North America is beneath the Basin and Range region in the western United States, where the crust is as thin as 20 kilometers (12 miles). The thickest North American crust, beneath the Rockies, is more than 50 kilometers (30 miles) thick. Seismic velocities within continents are quite variable, suggesting that the composition of continental crust must also vary greatly. Continents have an average density of about 2.7 g/cm3, which is much lower than the densities of both oceanic crust and mantle rock. This low density explains why continents are buoyant—acting like giant rafts, floating atop tectonic plates—and why they cannot be readily subducted into the mantle.

Discovering Boundaries: The Moho
The boundarybetween the crust and mantle, called the Moho, was one of the first features of Earth’s interior discovered using seismic waves. In 1909, Croatian seismologist Andrija Mohorovicic discovered this boundary that now bears his name. At the base of the continents, P waves travel about 6 kilometers per second (km/s) but abruptly increase to 8 km/s at a slightly greater depth.Mohorovicic cleverly used this large jump in seismic velocity to make his discovery. He noticed that two different sets of seismic waves were recorded at seismographs located within a few hundred kilometers of an earthquake. One set of waves moved through the ground at about 6 km/s, while the other set of waves traveled about
8 km/s—allowing Mohorovicic to correctly determine that the different waves were traveling through two different layers.During a shallow earthquake, direct waves travel along a nearly straight path through the crust, as shown in Fig 8.

Other seismic waves follow a path through the crust and along the top of the mantle. These are called refracted waves because they are bent, or refracted, as they enter the mantle. Seismographs near the
epicenter record the direct waves first. However, seismographs further from the epicenter record the refracted waves first. The point at which both waves arrive at the same time, called the cross-over, can be used to determine the depth of the Moho. Thus, using data from these two sets of waves and seismographs at various distances from an earthquake’s epicenter, the thickness of the crust
for any location can be calculated. The difference between travel times for direct and refracted waves is comparable to driving to a destination on local roads versus on interstate highways. For short
distances, you will typically arrive sooner if you drive the most direct route using local roads. For long distances, the trip may take less time if you take a less direct route that involves mostly interstate highways. The cross-over point, where both routes take an equal amount of time, is directly related to how far you must drive before reaching the interstate highway. Applied to determining the depth of the Moho, the cross-over is related to how far seismic waves travel through the crust (slow layer) before they reach the mantle (fast layer): The greater the cross-over distance, the deeper the Moho. The Moho lies about 25 to 70 kilometers (15 to 45 miles) beneath the continents and about 5 to 10 kilometers (3 to 6 miles) below the ocean floor.