3. Geochemical Cycles

Gravity pulled our planet together by the process of accretion. Similar to a star, the huge number of particles colliding and squeezing together means that most of the energy is in the center. The energy within a planet is not great enough to cause atoms to fuse but heavier atoms do sometimes split apart, releasing energy that can be reabsorbed by surrounding atoms. Our planet was so hot when it first formed that most of the material was in a semi-liquid semi-solid phase allowing the atoms to flow around.

If you have ever seen oil and water mixed together, you know liquids can have different densities. The denser water sinks to the bottom of the glass and the less dense oil floats on top. If you add an even denser copper penny, it will sink to the bottom of the water layer. The same phenomenon occurred when our planet was first forming.

Elements with more protons in the nucleus, usually called “heavier” elements, were denser and sank to the center of the planet as they experienced a greater force from gravity. The “lighter” elements raised to the top of the mixture. This is the reason the surface of our planet is mostly solid with liquid water atop the solid rock and gaseous atmosphere atop the rock and water. The atmosphere refers to the layers of gaseous molecules surrounding our planet.

A quick aside: our planet was not large enough to have enough gravity to hold onto very light elements like hydrogen and helium, but it could hold onto heavier oxygen, carbon dioxide, and nitrogen gases. The larger planets in our solar system, called the gas giants, are able to hang onto these super light particles. The gas giants are too far from the sun to have many heavy elements because the elements with more mass did not travel far away after the first fusion reaction in the sun.

Back to our planet! The heaviest elements, mostly iron (26 protons), sunk to the center and the gravity of the surrounding atoms squeezed the iron into a solid inner core. Surrounding that core is a liquid iron layer; the heat was great enough to allow the solid to melt but there was not enough pressure to keep it in a solid form. The churning of the magnetic, liquid iron forms a magnetosphere around the planet to keep it safe from high energy radiation from other stars. 

Earth's magnetosphere protects the planet from high energy photons from our sun and foreign stars
Earth’s magnetosphere protects the planet from high energy photons from our sun and foreign stars

Surrounding the liquid outer core is a large chunk called the mantle and here is where we’ll take a look at a single atom. Let’s say our atom is near the bottom of the mantle and it absorbs some energy that diffuses from the core. The energy our atom absorbed will allow our atom to break free from the grips of gravity and begin its ascent through the mantle towards the surface.

When our atom has used all of its absorbed energy to rise high up in the mantle, gravity will take over and pull the atom back down. This is happening constantly as millions of atoms absorb enough energy from the core to rise up, only use up that energy and fall back down. Because so many atoms are doing this, a cyclical current is formed. Diffusion helps the process along, pushing atoms to fill in where others just left.

Arrows indicate direction of current
Arrows indicate direction of current

This same process is also happening on the surface of our planet. You have probably heard of the Water Cycle and you may even remember some of the names that you were told to memorize in school. Let’s look at the path of a single water molecule in liquid form.

Liquid water at the surface absorbs sunlight and our molecule breaks free from the liquid bonds, greatly lowering its density. Our water molecule will rise to the top of the dense air in the atmosphere but will settle at the bottom of the less dense air above it. Yes, the atmosphere has multiple levels of density, you may have heard of these levels called the troposphere or the stratosphere, etc.

Our water molecule will then join other water molecules that made the same journey. The bunching up of water molecules allows a few liquid bonds to reform, causing the water to become denser. We call this semi-liquid semi-gaseous patch of water a cloud. The denser liquid will then be pulled back toward the center of the planet by gravity, we call this rain. The water will fall and trickle to the lowest point possible in order to rejoin the liquid to repeat the cycle.

Clouds behave similarly to liquid water
Clouds behave similarly to liquid water

Earth’s liquid water also experiences a gravitational force from our satellite body, the moon. The moon’s gravitational pull lifts some of the water toward it. However, the Earth is much more massive than the moon so the water will not lift much before it falls back down. This process repeating on a large scale across the oceans of the planet creates the tides and waves. When the moon is directly overhead a part of the Earth, the oceans experience an upward force that pulls the water back from the coast slightly, otherwise called “high tide.”

The cyclical process will occur with many other elements on our planet like carbon, nitrogen, sulfur, and phosphorus. The main difference between these cycles and the water cycle is that water maintains its molecular form while the other elements undergo reactions throughout the cycle.

Let’s examine the Carbon Cycle specifically. Carbon exists in the form of gaseous carbon dioxide in the air, it can be stored and escape from the mantle through hydrothermal vents on the ocean floor, it sometimes exists as solid calcium carbonate called “limestone.” Carbon also exists as us! We, along with all other living things, are made mostly of carbon. We are part of the carbon cycle, we can only borrow our carbon for so long before we have to give it back to the Earth. Another word for giving your carbon back is dying.

Remember, nothing is created nor destroyed, only converted between different forms. Our planet demonstrates sustainability by merely obeying the laws of gravity and density. I find the natural world is even more beautiful when you understand the basic rules that make it work.

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