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Where Giants Roam

 
 

We are living on a planet like no other. Much of what exists on Earth is unique and occurs nowhere else. First of all, Earth is the only planet we know of which has life on it. Relatively primitive life, such as single-cell Archaea and Bacteria is already amazing, but here we’ve gone beyond that. We encounter complex webs consisting of different types of life-forms in any niche we can test. We see organisms exhibit a variety of properties - everything from flying, digging, swimming and even just standing in the sun all day. And among all that there is even a species of primates that can actually communicate, cooperate and coordinate to achieve unimaginable things. That is the human species, of course - the only known species that builds modern cities, drives cars and continuously buys Michael Jackson albums to this day.

Earth is also the only place we know of that has stable bodies of liquid water on its surface, let alone the only place that is mostly covered with liquid water. Should we hold a universal swimming contest, put your money on Earth’s swimmers. Earth has a unique atmosphere, climate, magnetic field and many other properties that make this uniqueness possible. When you think about all of this, it’s easy to conclude that all of these conditions are so specific that it could not have been a coincidence. But then again, the universe is unfathomably big; the number of galaxies, stars, and planets it contains is beyond our capabilities to understand - it only makes sense that one of these planets would have this set of conditions.

An often overlooked and underappreciated feature of the Earth is its plate tectonics. The movement of plates that are made of the crust and upper mantle of the Earth is something that I have learned about in high school. I bet most people have heard and know about plate tectonics. The thing is, I kind of took it for granted - I thought it was a regular thing that happens on many planets. I recently found out that I was wrong, and that it is, like many other properties, unique to Earth.

Plate tectonics carry with them a staggering force right beneath our feet, all around the planet, all the time. The more I thought about it the more I realized that it must be one of the major players in the dynamics of the Earth. I took a deep dive into the subject, and I’m going to try and explain what we know about plate tectonics, tell the story of how it was discovered, and why it happens here on Earth, but nowhere else that we know about.

Plate tectonics is a scientific theory describing the movement, or drift, of a set of separate plates that make up our lithosphere, the outermost layer of the Earth. The solid mantle of the Earth is made up of seven or eight large plates (depending on the definition) and around 24 smaller plates that all drift and move, slowly but surely. The movement is caused by processes on the surface and also deep inside the Earth.

We still don’t know everything there is to know about plate tectonics. When I learned about it in high school, I figured that it has been common knowledge for ages, but again, I was wrong. The idea that led to the discovery of plate tectonics arose near the beginning of the 20th century, but the theory did not become a consensus until the late 1960’s. To put in perspective how new the accepted theory is, keep in mind that we had a man, Yuri Gagarin, in outer space and orbiting the Earth in 1961, before we looked closer to home and accepted that the ground we are standing upon is drifting.

The idea of continental drift was floated (pun intended) by Alfred Wegener, a meteorologist, in 1912. He and a few others before him have noticed that the eastern outline of South America fits with the western outline of Africa, like two pieces of a puzzle. There are quite a few places that fit together like this around the world, and that has led Wegener to think that the continents were once connected, forming one big continent - Pangea. Another point that supports this idea is the fact that fossils of the same species of plants and animals were found on different continents, which would have been much less possible if these continents were not once connected.

This concept became well known, but it was surrounded by much debate for a few decades. The mechanism that we now know makes continental drift possible was suggested and it was even proposed that rocky material was rising in the middle of the oceans, but there was no proof. Later, in the 1950s-60s, oceanographers have started mapping out the ocean floor. They used a tool named a Fathometer, that was previously used in World War II to help ships maneuver in shallow waters. They were immensely surprised to find that the ocean floor is not flat, but that it is split in two by the mid-ocean ridge, which is part of the longest mountain range in the world stretching over 40,000km underwater. In comparison, the second longest, the Andes, is 7,000km long. People who were laying ocean crossing cables have noticed that something must be there. They did not figure that they were laying cables on what is somewhat of a crack in the crust of the Earth, splitting it in the middle.

The last bit of evidence is what made this theory a reality and it has allowed us a glimpse at the drift itself. Near the beginning of the 1900s, scientists discovered that when molten rocks that contain magnetic materials cool down and harden, these materials freeze in a position of pointing along the Earth’s magnetic field. That means that if you test the age of these minerals and measure the direction at which they point, you can tell the angle of the land at the time of the rock formation. Using that knowledge, scientists were able to create maps that show the drift of the continents. Testing minerals that were found in England and in America have shown results that support each other. This all made sense.

One of the main forces that move the plates is mantle convection. The mantle is one of the layers of the Earth, and many other planets have a layer quite similar to ours. The mantle holds great heat (up to 4,000oc/7,230of) and immense energy, which make the mantle act somewhat like a fluid. The rocks that make up the mantle may melt and thus material circulation that is similar to liquid flow is occurring. Hot material wells up and cooler, heavier material sinks down.

This convection and material circulation is evident at meeting points between two plates. There are two types of plate boundaries that exemplify this. Divergent boundaries are boundaries between two plates that are moving away from each other. Material from the mantle wells up between the two plates, thus creating a ridge of ‘young’ rocks. Convergent boundaries are boundaries that occur between plates that move towards each other. When the two plates meet, the denser plate will slide underneath the thinner plate. The material of the denser plate will sink down back towards the mantle and the thinner plate will slide above the other plate. The pushing force from upwelling on one side of a plate, combined with pulling from the lower pressure created by the sinking on the plate’s other side is the driving force that moves the plate.

There is another type of boundary - transform boundary. This type of boundary happens when two plates slide, or grind, next to each other in different directions. The movement of the plates is amazingly slow - only about a few centimeters a year, but they move with great force. The interaction of plates at the boundaries is strong enough to create earthquakes, raise mountains and cause volcanic activity.

The Himalayas, one of the most impressive mountain chains in the world, slowly rose to its grandeur after the Indian plate collided with the Eurasian plate, around 40-50 million years ago. In an opposing manner, the subduction between the Pacific plate and the Mariana Plate has created the deepest part of the world’s ocean, Marianas Trench, which reaches almost 11km deep. Since plate tectonic movement started (estimated at 3.2 billion years ago) it has been changing the face of planet Earth, breaking the continents apart and moving them around.

It is important to mention that the continents that you see on a globe do not necessarily coincide with the plates. For example, The North American plate starts near North America’s west coast, but goes well beyond its east coast, all the way to the middle of the Atlantic ocean, where it meets the Eurasian and African plates. Iceland is located on the meeting point of these plates, and you can actually see a divergent boundary there, on land.

Today, we believe that there are two factors that are important for initiation of plate tectonics on a planet. The first factor is the strength of the mantle convection, and the amount of pressure it exerts on the lithosphere. This factor is usually not much different between the Earth and other comparable planets. The second factor is the breakability of the lithosphere - the easier it breaks, the easier it is to separate it into plates. Well, we do have something affecting this factor that we don’t see anywhere else - liquid water.

Water, the liquid of life, important to so many processes and systems on Earth, has another important role that affects us all. Water is a huge factor on the breakability of the Earth. It starts when water seeps deep down into the crust of the Earth. There, the presence of water significantly lowers the melting temperatures of the rocks and minerals, making them act like fluids. This allows for a general weakness of the crust, the convection of the melted material and the initiation of plate movement. Then, the water acts as a lubricant between the plates. Water at great depths are isolated from surface water and are subjected to immense pressure. This makes the water lower the friction coefficient between the plates, which allows the initiated movement to continue, instead of allowing the plates to fuse back together.

When we try to study our planet, we often look at Venus for comparison. Earth and Venus are quite similar in many aspects - size, density, mantle temperature, possibly even chemical composition. The convection forces on both planets are most likely comparable, yet Venus has no liquid water, and so it has no plate movements. Instead, Venus is an example of a planet that has a stagnant lid mode of tectonics. This means that the surface of Venus is fused into one single plate. Stagnant lid is a property of all the planets we currently know of. Some material still upwells and gets to the surface, but nothing is being subducted back into the planet. The lack of subduction means that some natural processes do not occur on Venus. These processes do occur here, and they show how important tectonic plates are to the life on Earth.

Plate tectonics is an important factor in the creation of Earth’s atmosphere, for example, due to gasses released in volcanic activities. Plate tectonics is essential for the existence of the magnetic field that the Earth is enjoying, which protects us from many extraterrestrial dangers. Plate tectonics has a role in the lesser known, but not less important, carbon cycle. This cycle moves carbon around, between the Earth, atmosphere, seas and living organisms of our planet. The cycling of carbon has a massive effect on the global climate - the more carbon we have in the atmosphere, the warmer it gets. This carbon, through the cycle, gets sequestered in material on the surface of the Earth or the bottom of the oceans, and then gets subducted back into the Earth. Volcanic activity, as mentioned before, releases some of that carbon back to the atmosphere. This cycle of injecting carbon to the atmosphere and sinking carbon from the surface is important to all life on earth, as it allows for relative stability in global climate - it keeps the temperature within a fairly narrow habitable and water-conserving range.

The fact that plate tectonics might be dependant on the presence of water, while helping to regulate global climate somewhat brings us to a chicken and an egg situation. Water is very helpful in initiating plate movement, but plate movement helps keep the water on the planet. You see, without subduction, the atmosphere usually accumulates a high amount of carbon, which warms the planet up. This may lead to massive evaporation and loss of water to space. This leads to an idea that bigger planets may have better chances at having plate tectonics, due to the stronger gravity to keep the water from escaping, and a stronger convection pressure from inside the planet.

Earth has plenty of properties that grant it its uniqueness. Each is more improbable than the next. Should one of these properties slightly change, who knows where would we end up? We already know so much, but we are still only learning the basics about our planet and its complex systems. By studying other planets, we are able to deduce a lot about our own planet. Maybe one day we will find other planets with plate tectonics to compare to our own. There are great forces all around us to find out and study and I find that exciting.


As always, there's an Additional Notes piece for this article. Check it out, if you're interested in how the Earth will look in the future, or if you want to know about the tallest mountain in the solar system.

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Thanks for reading!

Semus.