earth 2: What goes on deep inside the earth (v1.0)

 Collected from many sources. 

Understanding what goes on deep inside the earth is hard. Quite clearly, we cannot drill a hole deep enough to actually investigate. So, it is useful to explore our current understandings, and the tools and techniques earth scientists use to study the earth. 

The structure of the earth is divided into four major components by composition: the crust, the mantle, the outer core, and the inner core. Each layer has a unique chemical composition, and physical state, and can impact life on Earth's surface. If you look at mechanical properties on the other hand, we have the lithosphere, and the asthenosphere as two layers of the mantle. Earth's inner core is dense, metallic, and solid. The outer core is molten metal. The mantle is predominantly solid and is the middle layer of the Earth. It is made of hot rock located between the crust and the core. The mantle constitutes 67% of the mass of the Earth and 84% of its volume. The crust is the rocky outer layer of the Earth's surface; and the two types of crust are continental and oceanic.

The earth's core is mostly Iron and Nickel. Elements that dissolve in iron, called siderophiles, are also found in the core. Because these elements are found much more rarely on Earth’s crust, many siderophiles are classified as “precious metals.” Siderophile elements include gold, platinum, and cobalt. Another key element in Earth’s core is sulfur—in fact 90 percent of the sulfur on Earth is found in the core. Earth’s core is the furnace of the geothermal gradient. The geothermal gradient measures the increase of heat and pressure in Earth’s interior. The pressure in the inner core is nearly 3.6 million atmospheres (atm). The geothermal gradient is about 25° Celsius per kilometer of depth (1° Fahrenheit per 70 feet). The primary contributors to heat in the core are the decay of radioactive elements, leftover heat from planetary formation, and heat released as the liquid outer core solidifies near its boundary with the inner core. Although we know that the core is the hottest part of our planet, its precise temperatures are difficult to determine. The fluctuating temperatures in the core depend on pressure, Earth's rotation, and the varying composition of core elements. In general, temperatures range from about 4,400° Celsius (7,952° Fahrenheit) to about 6,000° Celsius (10,800° Fahrenheit).

The magnetosphere around the earth is the region around the planet dominated by the planet's magnetic field. Other planets in our solar system have magnetospheres, but Earth has the strongest one of all the rocky planets: Earth's magnetosphere is a vast, comet-shaped bubble, which has played a crucial role in our planet's habitability. Life on Earth initially developed and continues to be sustained under the protection of this magnetic environment. The magnetosphere shields our home planet from solar and cosmic particle radiation, as well as erosion of the atmosphere by the solar wind - the constant flow of charged particles streaming off the sun.  The magnetosphere is generated by the convective motion of charged, molten iron, far below the surface in Earth's outer core. The liquid metal of the outer core has very low viscosity, meaning it is easily deformed and malleable. It is the site of violent convection. The churning metal of the outer core creates and sustains Earth’s magnetic field. While the study of the outer core helps better understand the magnetosphere, the converse is also true - A study of the magnetosphere gives us one lens to look at this earth's outer core. 

One technique used to study the earth is to use a variety of tools in a lab to subject rocks/metals to the kinds of temperatures and pressures experienced deep inside the earth and study how they behave and transform from one form to another. Studying volcanoes and the material that is emitted by volcanoes also gives another lens to examine the processes and materials inside the earth. Studying earthquakes offers another powerful lens. One type of seismic wave is Body waves which are the waves that can travel through the layers of the earth. Body waves can move through all states of matter including rocks and molten lava. Surface waves can only travel on the surface of the earth. Their frequency is lower than body waves. Studying body waves (for example P-waves or S-waves) and the way it travels is another lens into earth's inner workings. P waves, or Primary waves, are the first waves to arrive at a seismograph. P waves are the fastest seismic waves and can move through solid, liquid, or gas. They leave behind a trail of compressions and rarefactions on the medium they move through. P waves are also called pressure waves for this reason. S waves, or secondary waves, are the second waves to arrive during an earthquake. They are much slower than P waves and can travel only through solids. It is after studying the trajectory of S waves through the layers of earth, scientists were able to conclude that the earth’s outer core is liquid. Studying tectonic plates is another lens to earth's inner workings.