Rocking Out

In this article, we'll be looking into:

1.

• What are rocks?

• Geology

• The elemental content of rocks

• What can we do with rocks?

• The structure of the Earth

Photo by Marius Venter

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What are rocks?

This may not seem a very intelligent question. We all know what rocks are, right? They're the hard things found on the ground. Stones are the smaller versions of rocks and pebbles are smaller still. Certainly, rocks are not as intricate and complex as living things or man-made artefacts (a word used for anything that humans make, which could be a vase, book, telephone, car or even a building). However, in science, just because something is simple to comprehend in terms of what it is, it doesn't mean there's nothing else to understand about it. We could look into what makes rocks? why do we have rocks? why do some of them break off from larger boulders or mountains? and when you think about it, rocks come from the Earth. They're broken up pieces of our planet, and there is a lot to learn about our planet.

Geology

The scientific study of geology is concerned with the physical structure of the Earth and all the bits that break off from it. Geologists are also interested in how landscapes form: mountains, valleys, canyons etc and how processes like the weather and rivers wear down the rocks and move them about. The Earth is not as stable as we may be lead to believe:- it can move about and cause earthquakes, some of which are powerful enough to split open great long cracks in the ground. Volcanoes erupt and spew lava. Earthquakes and volcanoes also fall into the realm of geology.

Photo by Joeri Romer

What are rocks made of?

There are, as you might imagine, many different types of rocks. The elements that they are made from affect their colour, thickness, level of hardness and other properties.

Typically, rocks aren't made from single elements but are compounds of two or more elements. I'll go into more detail about rock composition (or the ingredients and their amounts) in another geology article. However, basically, the way to look at the composition of most rocks is to consider how much there is of silicate and how much there is of everything else. Silicate is a compound of silicon and oxygen and has the formula SiO2 and is the most abundant constituent, or ingredient in rocks. Silicate usually makes up around 40% to 80% of the composition of rock but it can be as low as 0% and as high as 100%. The rocks that have 0% silicate usually contain a carbonate instead, which is a carbon atom and an oxygen atom together with a metal, usually calcium or magnesium.

As I said, I'll go into more detail about this in another article, which will cover the different types of rock. This is just to give you an idea about the elemental content of rock. It's something I wonder about, anyhow.

What can we do with rocks?

Rock is useful to us humans and serve a variety of purposes. We can use it to build with and we can construct buildings on top of rock. Depending on the elemental composition of rocks, we can find many different metals, gemstones and other things when we dig inside the Earth. We also use soil, which is made of ground up rocks, or sand, together with material from dead organisms, as the ground from which food crops and other useful plants are grown.

The four layers of planet Earth

The planet is spherical or ball-shaped. Well, almost. It actually bulges out a bit in the middle. If you were to imagine that you dug all the way into the Earth, the deepest you could go would be the centre of the planet as after that you would be digging through to the other side. On the way, you would pass through four main layers. Just like a cake has different layers, with perhaps icing forming the top layer, then maybe a hard layer followed by a soft one then hard again, the Earth has layers with different rocks and forms of rock along the way.

The four layers are the crust on the outside, the mantle in the middle and the outer core and inner core in the centre, as shown in the diagram below:

Image from www.proprofs.com

The crust

The crust is the outermost layer and is made up of solid rock. It is not one solid mass but is broken up into several large pieces called tectonic plates. When you look at a map of these, you'll notice that there are continental plates for the large landmasses and oceanic plates for the world's oceans. Since the surface of the Earth is mostly sea, the oceanic plates make up 71% of the Earth's crust while the continental plates have the other 29%.

The crust is the thinnest layer with the continental crust typically being 30 to 50 km thick and the oceanic crust a mere 5 to 10 km. We haven't been able to dig all the way through the crust as the deepest hole ever dug was about 12 km (which would have been enough through oceanic crust, but this was through continental). These layers move (usually very slowly) upon the mantle, the middle layer.

The mantle

The mantle consists of mostly solid rock with some semi molten rock (partly melted rock), which is constantly moving, with hot material flowing up and cooler rocks flowing down. The same thing happens with air in the atmosphere and water in oceans and rivers (but much faster) and it's referred to as convection or a convection current. This can be hard to get your head around as we're used to rocks not moving. But imagine a box filled with small stones and then you shake the box vigorously. The stones will move around and change places. That should help you visualise this process, except that with very hot temperatures, the rocks move up if they're hotter than the others around them and down if they're cooler.

At 2,890 km, the mantle is the thickest layer and makes up about 84% of the Earth. The temperature ranges from about 1000 degrees Celsius at the boundary with the crust to about 3700 degrees at the outer core boundary.

The outer core

This layer is made of liquid metal: mostly iron and nickel to be precise. It's not liquid like water, but more viscous, or thick. If you've seen lava flowing out of a volcano, it's like that except it's metallic.

The outer core is the second thickest layer, at about 2,400 km and temperatures range from 3700 degrees Celsius by the mantle to 7700 degrees near the boundary with the inner core.

The inner core

Lastly we have the inner core. Although this layer is also made primarily of iron and nickel, the extreme pressure from all the earth above it makes it solid. The temperature is around 5,400 degrees Celsius.

There we have it. You now know the basic layout of the Earth and what rocks are.