If you like geology, but you aren't much of an expert, then a lot of things can seem far to complicated and scary. Richard Hammond has a new programme on the BBC discussing how the Earth was formed. I would definitely recommend that you watch it as it's very funny and very interesting, but if you don't have the time, then not to worry, I shall outline the key points below.
The first thing that he looked at was what the Earth is made of. The Earth weighs 6 x 1024kg (that's 6 with 24 0's after it!) and has four basic components: iron (32%), oxygen (30%), silicon (16%) and magnesium (15%). The remaining 7% of the planet is composed of things like hydrogen, aluminium, salt and calcium. These components started off as tiny dust particles as a cloud in space 4.5 billion years ago.
Richard then looked at how all of these components came together to form the Earth. 4.5 billion years ago there was no planet, and hence there was no gravity (if you want to see Hammond floating around in a zero gravity simulator, I suggest you go to around 15 minutes in the video!). This means that something else would have had to have brought the dust particles together, and this something else is an electrostatic force. As the particles were floating around they would have rubbed together, which would have given individual ones positive or negative charges, allowing several of them to stick together. However, this is only affective over a tiny difference, but it does mean that the slightly larger grains would have some gravitational force (which is the attraction of one object to another). This means that a clump of dust would attract another clump, which would in turn attract another clump, and so on and so on, eventually forming rocks, and then larger rocks, and eventually a whole planet. This whole process only takes a few million years.
Gravity is still affecting and shaping the planet today. Meteorites are drawn to the Earth's surface, causing potentially large impact craters. A well known, relatively recent (50,000 years ago) impact caused the Barringer Crater to form in Northern Arizona. The meteor itself was only 30m in size, whilst the crater that it left behind is over 1km in diameter. Scientists have calculated that it would have impacted at 26,000mph, giving it a relative weight of 2 mega tonnes. We still have a lot of meteorite matter reaching the Earth today (around 40,000 tonnes per year), although most of it is dust. When the Earth was forming, around one impact happened every few minutes! This onslaught helped to build the planet.
Hammond then looked at how the surface of the Earth became habitable, as when it formed it would have been molten and magma rich. For magma to cool and become solid, it needs to flow so it can form a crust on top (there's an amazing bit at around 37 minutes where he shows how it cools). This didn't work when the Earth was forming, as it was being constantly bombarded with meteorites, which transferred their kinetic energy into heat energy when they hit, meaning that the Earth's surface couldn't cool down. 4 billion years ago, the meteorites slowed down in intensity, giving the magma time to cool and solidify.
He then went on to explain how water and gas came into existence. The volcanic activity released water into the atmosphere as steam, which consequently formed clouds, which in turn filled the very first oceans. Asteroids and comets also brought some water to the Earth. This volcanic activity also pumped out toxic gases into the atmosphere, making it impossible for life as we know it to develop. Luckily, 3 billion years ago, stromatolites (primitive blobs of bacteria) dominated the Earth, which live off sunlight and carbon dioxide from the water, and they also release oxygen, much like modern plants. This allowed complex life to have a chance.
Richard finally explained about how the moon formed, and it's vital effects on the Earth. As the Earth was forming, a large planet sized rock crashed into the Earth, causing large fragments of rock to be hit off out into space. These fragments clumped together thanks to gravity and eventually formed the moon. The moon helps to give the Earth gravitational stability, as without it there would be a large amount of wobble of the Earth which would cause drastically variable seasons, making it nearly impossible for life to form. The moon also affects the tides, which allowed life to gradually leave the sea, leading to the land animals that we have now. These tides would have been much larger than they are today, but the moon has been gradually moving further away from us (to its current position of 239,000 miles), allowing the tides to decrease and flooding to be reduced.
I found this programme very interesting as it outlined the key bits of information about how the Earth formed, which to someone like me is common knowledge, but to other people without a geological background can be quite complex. A good video to watch regarding this is on YouTube, where you can see how little people really know about the formation of the Earth (although I imagine that anyone who did know something more complex was cut out!). As geologists, we tend to forget that people don't know all of this, as a lot of it is basic to us, but ask us about say basic medical care, or how to programme a computer, or how to write a book and we won't have a clue! I hope you enjoy the programme and/or my overview of the geology!
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