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What is the Earth made of?
Earth is the only planet full of living creatures – or to be safe, it is the only planet we know that supports life. Although the planet itself hasn’t any systems like metabolism and homoeostasis etc that are found in living organism, Earth is pretty much alive. The evolution and the processes of Earth are fascinating and allow us to understand and appreciate the planet.
Scientists are still unravelling the question of where, when and how it all started and these questions cannot be fully addressed in just one article. There is a general acceptance that Big Bang (formation of matter from energy), which happened 13.7 billion years ago, was the starting point of the still expanding universe.
Formation and differentiation of Earth
According to Victor Safranov, the planets formed out of cosmic dust grains that collided to form larger bodies. These larger bodies attracted each other through their gravity and formed planetesimals. Eventually the planetesimals collided to each other and a planet such as Earth was born. Planetesimals tend to be cold, but the impact of meteorite, heat of compression and radioactive disintegration caused the heating up of Earth’s interior. The composition of earth mainly consisted of silicon, iron, magnesium oxides and some other natural chemical elements. As the planet started to heat up, iron started to meld and fell towards the centre and a liquid core was formed. The temperature rose to 2000° C and a large fraction of Earth started to melt. Eventually about 1/3 of planet’s material sank to the center. The differentiation of earth started, as the molten material produced was lighter than the parent form. It floated upwards and cooled to form a primitive crust. Before, Earth had a homogeneous body, which means that it has the same structure. Now, with iron core, crust of lighter materials and between them the remaining mantle divided the earth in layers. It is probably the differentiation process that caused the gases to escape from the interior and form the atmosphere and oceans. But the question is how do we know this? We were surely not there to observe all these changes. We cannot drill a hole to find out what the interior of the Earth is made of. However, there is an indirect way that can help us to find an answer to this question: seismic waves.
Firstly, we need to understand what these are and how they travel through the Earth. Rocks have energy stored in them. Seismic waves or Earthquake shock waves are waves of energy that are sent out when a rock suddenly breaks. These cause the shaking and trembling of the ground during an earthquake. The center of the earthquake is called focus. In other words, focus is the point within the Earth where seismic waves originate. There are two types of seismic waves: body waves and surface waves. Body waves travel through the Earth’s interior, while surface waves travel on the surface of the Earth away from the epicenter. Epicenter is the point on the Earth’s surface directly above the focus. To find out about the Earth’s interior, we need to focus on the body waves. There are two types of body waves: primary (p) waves and secondary (s) waves. P waves are compressional and move faster. Think of the wave movement of a spring, when one end of the spring is pushed it produces longitudinal waves. S waves are caused by the shearing motion and move slower. Think of a rope when one end of a rope is displaced vertically (transverse waves). Another difference between these waves is that, although both can travel through solid rock, P waves can travel through fluids, but S waves cannot. Both waves can be refracted (like light rays) as they encounter denser rocks.
Now you are thinking how these waves are going to tell us about the composition of Earth and its core. Well, P and S waves don’t travel in straight line through Earth. This allows us to come to the conclusion that Earth is not uniform. Scientists have found that P and S waves travel at different velocities in different rocks. The Earth materials become denser as we go deeper. So, as the density of the rocks increases, the velocities of P and S waves increase. There are certain areas on the opposite side of the Earth that seismic waves cannot reach. This creates S waves shadow zone (area beyond 103 – 105 from an epicenter of an earthquake where no S waves arrive) and P waves shadow zone (area between 103/105 – 142 of arc from an epicenter where no P waves arrive). Also, at more than 142 from an epicentre, P waves do reappear on seismogram but their velocities have been slowed down. What does this all mean? It means that because S waves cannot pass through a fluid, the Earth has a core and the outer part of which, at least, is liquid in composition. P waves, however, can still pass through this area, as they are slowed down Moreover, the slowing down of P waves and the refracted pathway they take tell us that outer part of the core is liquid, but there is an inner solid core. These findings confirm the idea that the composition of earth is not uniform and there is a core of liquid and solid material.
The core is surrounded by mantel. It is generally a solid, rocky, part. Although the velocities of seismic waves increase with depth, there is a low-velocity layer in the upper mantel. This layer is called asthenosphere and can be thought of as a plastic layer. Because there is partial melting going on, this layer slows down both P and S waves (S waves the most). It is also the main source of magmas. Above the asthenosphere lies lithosphere that is strong and brittle. It should be noted that crust and lithosphere are not the same, because lithosphere is the crust plus the upper most solid part of the mantel (not to confuse with asthenosphere which is plastic).
The crust is thick beneath the continents (continental crust) and thin beneath the oceans (oceanic crust). They also differ in density and composition. Oceanic crust has a higher density (3.0 t/m3) than continental crust (2.7 t/m3). It is also basaltic in composition, while continental crust is granitic. Oceanic crust is high in silicon and aluminium and oceanic crust is high in silicon and magnesium. Let’s think about ice and water at this point. Ice floats in water, because it is less dens. Also, if you think about ice cube in a glass of water or iceberg in ocean, there is just a certain part visible. The rest of it (the root) is below the waterline. Similarly, continental crust is less dense and basically ‘floats in higher density oceanic crust at equilibrium level and has a root zone. This idea is also called isostasy.
The Earth is much more than just the basic characteristics that are mentioned above. However, these findings and discoveries are crucial to expand our understanding of Earth. This article is mere a glance at the composition and interior of Earth, but hopefully it helps you understand the planet a bit better.