An introduction to geology
An introduction to geology

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An introduction to geology

1.1 What is a rock?

What is a rock? This might seem like a simple question – rocks are hard things that are found in the ground and that mountains are made of – but unsurprisingly, it is more complicated than that. Basalt isn’t just made up of stuff called basalt and, believe it or not, there are lots of different things that can make up a sandstone. The following video explores this in more detail.

Download this video clip.Video player: Video 1.2 Three types of rock
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Transcript: Video 1.2 Three types of rock

MARCUS BADGER:
So here we have the three fundamental types of rock. We’ve got a sedimentary rock, an igneous rock, and this metamorphic rock.
GAIL:
And how do I tell the difference?
MARCUS BADGER:
Well, the easiest place to start is to tell the difference between the sedimentary rock and the other two. And the way we can do that is look really carefully at the grains, the little bits that were in the rocks. So if you look at this one, it looks kind of like lots of little bits of stuff stuck together, and that’s because that’s basically what a sedimentary rock is, whereas if we look at the other two, they both look like they’ve got crystals – individual crystals which look like they’ve kind of grown together, and they’re all interlocking.
The next thing is, how do we tell the difference between the igneous rock and this metamorphic? And in these two examples, I hope it’s pretty obvious that here, in the metamorphic rock, we’ve got this banding. We’ve got really obvious layering, whereas in this igneous rock, there’s no obvious orientation of the grains. There’s no layering or banding that we can see.
GAIL:
And how did those differences come about?
MARCUS BADGER:
Well, the difference between these two is the banding that we see here in this metamorphic rock. And that difference has come about because when this one was formed, it was exposed to some kind of pressure. And in this case, the pressure probably came from the sides like this – that kind of direction. And that causes the alignment of the minerals and that banding that we see.
GAIL:
Cool.
ANNE JAY:
Learning how to differentiate between these three different types of rock is a key skill for geologists. And you’re going to learn how to do that in the next part of the course.
End transcript: Video 1.2 Three types of rock
Video 1.2 Three types of rock
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So, there are three different types of rock: igneous, sedimentary and metamorphic. These three types are formed in different ways. This means that they have different properties that, to the trained eye, usually make them easy to tell apart. But, what is a rock? A rock is an aggregate, or combination, of minerals. These can be all the same mineral, as seen in limestone, or more often, lots of different minerals, as seen in granite. So what is a mineral? Well, a mineral is defined as any naturally occurring element or compound which has a characteristic chemical composition, physical properties and, importantly, crystal form.

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