Notice that during this process, atoms that were originally in the minerals of
the igneous rock are ending up in class of sediment and
later in the metamorphic mineral submerge.
In other words, atoms are moving among different minerals,
different locations as a consequence of the rock cycle.
Now there's some caveats to this.
You can go straight from metamorphic to igneous if you have the metamorphic rock
buried so deeply that it starts to melt, forms magma, and rises in the crust.
Or you can have uplift and exposure of the metamorphic rock, so
the metamorphic rock transforms into sediment.
In fact, you can have sedimentary rocks uplifted and
broken apart to form new sediment that then forms another sedimentary rock.
So there are many different paths and roots through the rock cycle.
But the rock cycle exists on our planet because of how dynamic our planet is.
We don't have a rock cycle on the moon.
There's sediment on the surface formed from
impacts of meteorites over billions of years.
But that sediment is never forming into sedimentary rock, and
it's never being carried down deeply enough to form metamorphic rock.
And for at least the last few billion years,
there has not been igneous activity forming new igneous rocks on the moon.
It's just sitting there, static.
Now, I'm going to finish this discussion with one last topic.
And that is how do we get rocks that were formed deep in the Earth up and
exposed to the surface of the Earth?
Well, this process is called exhumation, and there are two components to it.
First, we have uplift.
Uplift may happen during mountain building processes,
because the rock is being squeezed together horizontally, and
therefore is rising vertically, or because one rock is being thrust over another.
In any case, we can have tectonic phenomena that lead to the uplift of rock,
bringing it up towards the surface.
While this is happening, at the surface, weathering processes are taking place.
We can have wind blowing rock away.
We can have streams grinding it away.
We can have glaciers grinding it away as well.
So as the rock is being uplifted, it's being removed at the top, and
the result is that rock that was once at depth ends up at the surface.
So, in this topic, we've seen how rocks change through Earth history.
We're now going to leave our general description of how rocks form and
how rocks change behind and turn our attention to rocks and you.
In other words, what kinds of useful materials can we get from rocks,
and how do we get them?
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