Rock Answers

by Deb on September 30, 2011


So did you know a lot about rocks? If you want another go, close your eyes and click.

1.     The three types of rocks are named for how they are formed.

  • Igneous rocks are formed in fire, it has the same root as ‘ignite.’ They are rocks that solidify out of molten rock. They can form near the earth’s surface through volcanoes, or deep underground from magma in the mantle. They are made of interlocking crystals of different minerals. Examples: basalt from lava, granite from magma.
  • Sedimentary rocks are formed of glued together grains. They range from extremely fine mudstones or siltstones up to breccia and conglomerate which can contain large rocks. They are usually formed in oceans or lakes, as sediment is eroded and carried into water where it settles to the bottom. As more builds up on top it is compressed until it is stone. conglomerate
  • Metamorphic rocks begin as either igneous or sedimentary rocks but are then changed through heat and pressure. It isn’t quite enough to completely melt them, which would turn them into igneous rocks, but enough to change the structure. They can have layers and even fossils like sedimentary rocks, but they often have crystals as well. Their form depends on what they started out as. Examples: Marble is metamorphosed limestone, Slate is metamorphosed shale – the intense pressure creates the thin planes in slate.

2.     An inselberg is literally an island mountain. And if that doesn’t give you the clue why I asked, a good example of an inselberg is Uluru. Uluru was formed from an ancient mass of sandstone that was buried then tilted so part of the edge  was at the surface. The softer stone around it has been eroded away, leaving the harder sandstone standing dramatically above the surface.

Nearby Kata Tjuta (the Olgas) are also an inselberg and were made during the same movements, but are actually a different stone. Uluru is part of the arkose sandstone formed by the alluvial fan of a river spreading into a shallow basin, as I mentioned above in ‘sedimentary rocks.’ Kata Tjuta is conglomerate, formed of larger stones that didn’t travel as far as the fine sand that got to Uluru. Both were eroded from the same original formation.


Photo courtesy of Paul Mannix.

3.     Obviously the most important thing when making a stone tool is a sharp edge! The earliest stone tools were just sharp flakes chipped off larger stones, used then discarded. If you are making something to keep, then you want to look at getting the most comfortable and useful shape and a few other things come into it.

To get sharp, straight edges you need fine grained rocks, not ones with large crystals. The most famous stone for tools is flint, and no-one is quite sure how that forms. It seems to come from a silica gel in pockets inside sedimentary rocks. Volcanic glass makes excellent tools and quartz can be used too.

Most stones used have a conchoidal fracture, which means shell-like. They break from the point of impact spreading out like a cone, they generally have lines running across just like shells. You might have seen this sort of break in glass. This type of fracture makes it easier to predict how the flake will come off and what you will be left with, which is important if you are trying to make a particular shape.

There have been three significant mental shifts in the history of stone tools, and not surprisingly they line up quite well with different fossils.

  • The first is the basic – hey this is sharp! I could cut something! I could even do this on purpose! Chimps and other animals already use some tools, but not for cutting. This is the level of ‘core tools’ and random flakes. These are all the early tools, from ancient apelike creatures to the first very early possible humans. They continued to be used alongside other tools.
  • The second is making a tool to a specific shape – knowing what you want in advance and planning how to take off the right parts rather than just bashing away. That’s quite a mental skill. The most well-known of these are the hand-axes. These are closely associated with Homo erectus, our immediate predecessors who moved out of Africa.
  • The third is flake tools, when you can’t see what you are making. While you are making your hand-axe you can see it emerging and can adjust. When you are making flakes, all the work goes into preparing the stone so that when you finally make that last tap you get a series of flakes the shape you want. It’s a bit like the difference between cooking a steak and creating a cake recipe – anyone can see how to cook a steak, but working out what ingredients to put together to rise, set and taste good is far more complicated. Only Homo sapiens has been associated with flake tools, including Neanderthals.

Making stone tools is actually very difficult, I’ve tried it! I’ve included a short video at the bottom, there are a lot on the internet but they tend to be very long. They are well worth looking at to see the details.


Photo courtesy of Luis Garcia.

4.     Firstly, a mineral is a naturally occurring chemical compound in rocks. Several different compounds with slightly different chemical formulae may appear and behave in the same way, and rocks are not made of single molecules but continuous lattices of crystals. So one mineral generally includes several different closely related chemicals.

It is difficult to know what minerals exist at great depths – the extreme pressures are very difficult to model or attempt to recreate so while we may know the chemicals down there we don’t know much about how they fit together. In the upper mantle the most common mineral is olivine, which is magnesium and iron silicate. It has a major influence on plate tectonics and the way the crust and continents move.

In the crust, the most common mineral is feldspar. They are also silicates, but with aluminium and potassium, sodium or calcium. They are very common in igneous rocks and are the white parts you can see. Feldspar is extensively used in glassmaking and ceramics.


Photo courtesy of Rob Lavinsky.

In continents, so around you, the most common mineral is quartz. It is the simple silicate silicon dioxide – this is why oxygen and silicon are so common on earth! It is the clear part of stones like granite. Quartz is an important source of silicon which is used throughout the electronics industry and has many industrial uses.

5.     The photo is a natural garnet crystal. Garnets have been used as jewellery for thousands of years, partly because they form large, almost perfect natural crystals. Like other minerals there are several forms of garnet, this one is a pyrope. Pyrope is important in geology because it only forms under high pressure, so it is a marker of metamorphic rocks. Most of it is formed in the mantle but some comes from ultra-high-pressure metamorphosis.


A quick video of some people knapping tools.

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{ 1 comment… read it below or add one }

Stephanie September 30, 2011 at 8:38 am

I love this. My daughter’s class just finished a unit on rocks, so we’ve been going over similar information together lately.
Stephanie´s latest amazing offering ..What Tools Do You Need to Encourage Your Child’s Interest in Science?My Profile


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