Geology: The Earth

I studied Geology up until A-Level, nearly 20 years ago now and recently I spent five years with amnesia, so I’m not going to claim my knowledge is complete, infallible or up to date. But it is a subject I adore and which, to my mind is vital. I thought I would cover some basics.

Geology means ‘study of the earth’. To most people it means ‘looking at boring rocks’. Neither of these definitions cover the subject well.

The subject of Geology is more or less observing history, sometimes as it happens. As far as I am concerned, it is the most vital of all sciences. Geology also manages to offer the greatest of philosophies. Geologists have been around for centuries, but the science is one of the youngest at barely 150 years old.

Yes, at root, Geology is looking at rocks and studying the earth. But today I’m going to cover the basics of the Earth.

The Earth consists of four major layers:

  1. A solid inner Core, mostly of iron.
  2. A liquid outer Core.
  3. The Mantle, also liquid.
  4. The Crust

We know this because of earthquakes. We think of earthquakes as localised movements of rock that occur in the crust, but they are events so powerful that they send waves of seismic energy throughout the entire planet so an earthquake in Napoli can be felt by a seismograph in Moscow or New York. But earthquakes cannot be felt on the opposite side of the planet to where they occur. It turns out that the seismic waves are stopped dead by the metal inner core. Additionally, with each transition between planetary layers, reflection and refraction of these waves occur. A system of seismographs around the world and Geologists with much better maths than I, were able to identify exactly where those layers are, and have a good idea of what their properties are.

That said, we know relatively very little about the deep internal workings of the planet. We know that the core rotates and that this gives us our magnetic field. We know that the magnetic field reverses every few million years or so, so presumably something happens to the core during those times. We know that convection currents exist and we know that sometimes plumes of extra hot material (magma when it’s below ground, lava when it hits the surface) rises from the outer core to become a volcano. I will come back to all of these.

We do however know lots about the crust. The crust is separated into ‘plates’. These plates essentially float on the magmatic mantle. As the mantle is subject to convection currents, the plates are dragged around the globe. This has all sorts of interesting consequences. Firstly, they grind against each other. This causes earthquakes. The path of grinding is known as the fault line and you’ve probably heard of the most famous fault, the San Andreas faultline. Some of these faults move so quickly that the effect can be seen almost in real time.

Pete, Going Postal

Some move slowly and then snap.

Pete, Going Postal

It’s important to note that earthquakes happen all the time. Everywhere. Even in places nowhere near a plate boundary. Most of them are so small you cannot feel them. At the time of writing, Britain has suffered 27 earthquakes in the past 50 days.

Earthquakes around the British Isles in the last 50 days

The Richter scale of magnitude that measures earthquakes that we all have heard of, is actually no longer in use, but the media hasn’t noticed and you will hear ‘Richter scale’ often. Regardless, all the scales used are logarithmic, which means that a 1.0 increase represents a tenfold increase in amplitude but a 0.2 increase represents a doubling of energy released. This is why anything under 2.0 will not be felt by humans, but by the time you get to 6.0+ we are talking about cities being levelled.

So plates grind against each other. But they can also move away from each other, as seen in the second picture, where East Africa is gradually being split off from the main continent, and plates can also move into one another. This last movement has two different effects.

There are in fact two different types of plate. There is continental plate and there is oceanic plate. Continental plate consists of many different rocks and is less dense, so it rises, Oceanic plate consists of Basalt rock and is denser and usually relatively flat. The difference between Ocean and Sea is that Sea is continental plate covered by water. Ocean is water over oceanic plate.

When two continental plates bash into each other, they both try to occupy the same space and so rise up to form mountains. India is currently bashing into Asia and forming the Himalayas. The Himalayas are full of fossil fish because they used to be under water. Most mountains are formed this way, if they are currently active due to plate pressure then they are rising. Otherwise they are under erosive pressure and will gradually fall away.

When oceanic plate meets continental plate however, something different occurs. The oceanic plate is subducted beneath the continental plate. We have identified these places on Earth as Oceanic Trenches, you have probably heard of the Marianas Trench, the deepest place on the planet. The oceanic plate dives deep under the continental plate, and melts. The resulting magma then rises up through the continental plate to create a volcano. You may have heard of the Ring of Fire, the series of volcanoes that rings the Pacific Ocean.

There are four types of volcano: I mentioned earlier that volcanoes can form from a super hot magma plume from the Earth’s outer core. This is known as a hot spot volcano, or shield volcano, because it forms a shield pattern. They are Earth’s biggest volcanoes and not much to worry about unless you live on them (and people live on and around volcanoes because volcanic ash is the most fertile soil known to man), they erupt fairly continuously and slowly, allowing the shield pattern to build up. The most famous volcano of this type is Hawaii. Kiluaea is the current volcano, but the rest of the Hawaiian islands are where the volcano used to be. The ocean plate has moved, but the hot spot plume has remained stationary. Mount Olympus on Mars is the largest known volcano in the solar system and is also a shield volcano, but it is now extinct, so we can see that Mars was once a geologically active planet like Earth but that something happened to shut down Mars’ core (current theory suggests massive asteroid strike).

The second type of volcano is the Subductive volcano, where oceanic plate melts and rises again. These are your classic volcanoes. Mount St Helens, Vesuvius, Krakatoa and so on. These are dangerous volcanoes. Because they do not erupt constantly, and can go dormant for centuries, they can build up pressure in their magma chambers and then explode with great force.

The fourth type of volcano is the Super Volcano, again, you have probably heard of Yellowstone being a super volcano that will one day erupt. And in doing so it will probably take out much of North America. Not to scare you, but there are at least 12 super volcanoes that we know about. These do not look like the traditional volcano. The magma build up is so massive that the entire region bulges. If a subductive volcano is an explosive whitehead pimple on the face of the earth, the super volcano is like an explosive tumour growth. Fortunately for all of us, these do not happen often, although they have happened in the life span of Homo Sapiens.

You thought I’d missed one didn’t you. I left number 3 till last because there’s more to say about it. So we know that oceanic plate is subducted and destroyed beneath continental plate. So how is it replaced? Through our third volcano – the Mid Ocean Ridge. Every ocean has a series of volcanos that run down the centre, right where the convective forces of the mantle are pulling the plates in two, allowing magma to rise directly to the surface, constantly erupting and creating new plate rock which is then pulled across the ocean to be destroyed, only to rise again as continental plate. Occasionally this volcano produces so much material that it rises above the waves and becomes Iceland.

As an aside, there are around 1,500 active volcanoes on the planet although this does not mean 1,500 currently erupting, that’s around 130+. You’d think this gives us ample study on volcanoes, but in fact there are only around 1,500 vulcanologists, many of whom are not field based. Of the field based scientists, several are killed by volcanoes every year. This was the field I always wished that I had gone into. When my friend recently told me I was mad, why would I have wanted to go into a field with such a high kill rate, I noted that I nearly died from diabetes while sitting on my arse. A volcano would have been so much cooler.

Now earlier on in this little overview, I mentioned that Earth’s magnetic field has reversed itself on many occasions. But how do we know this? Because the ocean floors of the planet are magnetically striped. From creation to subduction takes a couple of hundred million years, and leading from mid ocean ridge to ocean trench, the rock forms magnetic stripes of opposing polarity. i.e. when the rock was formed, the poles were either N North S South or S North N South and the magnetic minerals within the rock formed in magnetic alignment.

Pete, Going Postal

Not only can we know that the poles have reversed in the past, we can also approximately measure how strong Earth’s magnetic field has been at any point for up to around 200 million years. The waning of the field has been tied to extinction theory as the magnetic field protects us from many galactic threats, not least, solar radiation. So we can also see that a geologically active planet – although threatening in the form of volcanoes, earthquakes and other geological phenomena, was also vital to our evolution and the survival of life overall. Geologically dead bodies such as the moon or mars cannot protect life from constant solar radiation, even if terraformed for an atmosphere.

This also serves to show my biggest point here, to most people, a lump of rock is, well, a lump of rock. To a geologist, even an amateur one like me, it is a time machine, learn to understand it and you can understand millions of years of history, you can glimpse into the internal workings of planets and you anchor all the other sciences and our place in the universe. Nothing would have ever happened on Earth if it were not for the unique configuration of our geology. None of us would exist. Life would not exist in the universe. While Mars may once have been geologically active and therefore Earth as a model of geology is not unique, it is currently the only known rocky world with an active core.

Geology also puts us in our place. Take any piece of rock that hasn’t come straight out of a volcano. Chances are it has been around for millions of years at a minimum. Active geology happens all the time with earthquakes and volcanoes, the face of the earth can be changed in an instant, and yet the timescales for everything are in hundreds of millions of years. Humans, right up until the 20th century where we have increased our lifespans a bit, lived half a century or so. When you get your head around the geological perspective of time, you are able to get your head out of politics and realise it’s all theatre. Which is needed sometimes. Think of an ant which has a lifespan of 1-3 years. Then realise that ants have existed for hundreds of millions of years, which we know thanks to geology. How many trillions of ants have lived and died? How many billions of ant colonies have been built and destroyed, simply to get from then to now? What monuments have they built? What politics have they played? What famous ants do they remember? What have they achieved other than to survive and go on existing for the sake of existing? That may sound pessimistic to some. But when politics is so goddamn depressing, when everything seems lost, when our country goes to hell in a handcart, if you know that life in the past has survived insane forces of nature like asteroid strikes, solar radiation and super volcanoes, then politics, even evil politics seems very tame in comparison and if you can just hunker down and survive and then, just like an ant, you or your children can eventually go out and rebuild, because that is what we do; that is life. We build, it gets destroyed, we rebuild and we rebuild better. That in itself can be victory.

I am very ill and it’s taken a lot for me to finally write my first piece for GP, so I can’t right now promise a second instalment, but if I do, we shall delve into rocks. At least for now, you have ammunition if you ever get into an argument with a flat earther.
 

© The Hologram 2018
 

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