How were Layers of the Earth found? Much of our knowledge of Earth’s insides come from monitoring the thousands of earthquakes that occur every year. Five centuries ago the world had mostly accepted that the Earth was not only a sphere but was thought to be of uniform rock throughout. Two hundred years later Sir Isaac Newton, studying our planetary system, calculated that the interior of the earth must be made of far-denser material than the surface rock. Newton’s estimate of the overall density of the Earth remains essentially unchanged today. In the early 1900 scientists discovered they could use data from earthquakes as a method for looking deep beneath the surface.
They thus determined that the Earth has three layers based on chemical composition: Crust. mantle, and core. As an analogy of relative scale, these layers can be compared to an egg with the shell representing the outermost layer, the white mantle, and the yoke core. How did scientists figure out where these layers were? They used the arrival times of seismic waves to worldwide seismic stations. Seismic waves leave the hypocenter of an earthquake and travel in all directions.
If the Earth had no change with depth, seismic waves would travel straight paths. But the earth has composition, density, and temperature changes that cause the seismic rays to reflect and refract along boundaries as velocity in the mantle increase with depth. Innovations in computer technology in concert with a steady beat of earthquakes, help scientists to continue to refine our understanding of Earth’s interior. The basic layers of the earth are grouped by their chemical composition.
Layers of the Earth:
The crust is made of chiefly 8 major elements shown by their relative abundance: Oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium At this scale the crust is too thin to show as more than a line. The crust ranges from 5–10 kilometers thick in the dense basaltic oceanic crust, and up to 75 kilometers in the less-dense granitic continental crust This difference in density and thickness of these two types of crust is the reason why the earth has oceans and continents. The crust is often mistaken for the tectonic plates.
However, the crust is just the top part of the tectonic plates. We will return to the topic in a moment but first back to the 3 layers. Below the crust is the mantle, composed of the same elements but in different proportions, with increasing amounts of the heavier elements in the rock. The chemical composition of the 2,900-km-thick mantle varies little from top to bottom, but there are distinct physical variations due to temperature pressure differences. (Wikipedia)
The uppermost mantle is relatively cool brittle and ranges from 50 to 120 kilometers thick. Below this zone, the upper mantle becomes notably more plastic malleable due to the right combination of heat pressure. That ductile zone is known as the asthenosphere and varies up to 400 kilometers deep depending mainly on temperature. The lower mantle is 55% of the planet by volume. It is denser and hotter than the upper mantle.
At the center of the Earth is the core, nearly twice as dense as the mantle because it’s metallic iron alloy rather than rock. Unlike the egg yolk analogy, Earth’s core is made up of two distinct parts: the liquid outer core a solid inner core. Although the inner core is hotter than the outer core, there is also greater pressure squeezing the atoms, changing the material from liquid to solid. The liquid outer core is convecting vigorously generates Earth’s magnetic field. But back to plate tectonics. As you recall, the cool uppermost part of the mantle is brittle. How can the top of the mantle be brittle when the same material in the asthenosphere is ductile? A Big hunk candy bar can be used as an analogy.
Like the uppermost cool mantle, when the Big Hunk is cold, it is brittle breaks when bent. When you heat it up it becomes ductile, or plastic, and can bend flow. Earlier we mention that the crust is merely the top of the tectonic plate. This uppermost brittle mantle behaves much like the overlying crust. Together they form a rigid layer of rock called the lithosphere that moves in unison The lithosphere ranges from as much as a100 kilometers thick in the oceanic plate to 200 kilometers thick in the continental plates.
It is in this brittle zone that earthquakes occur, due to compression, extension, shearing. Over billions of years the cooled surface of Earth has been broken up into the moving planes that are called lithospheric plates (commonly “tectonic plates”) Because they are mostly more buoyant than the asthenosphere, they float above it. Convection currents driven by temperature, pressure, and gravity provide the mechanism for the process we know as plate tectonics. Earthquakes, volcanoes, the Earth’s magnetic field are all the consequences of the Earth trying to lose heat as it converts some of the thermal energy into mechanical energy in the process. Without the tremendous heat being released from the interior of the earth… we would not have the mechanism to drive plate tectonics. Without the earthquakes, we may not have had a way to see so deep into the earth.