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Rock Classification

Want to learn how to identify that rock in your back yard?  Rocks can be identified using the following characteristics: 

1. Class

Rocks are ‘divided’ into three principal categories: IGNEOUS, SEDIMENTARY, and METAMORPHIC. The relationships between these categories are seen quite well in the following diagram, illustrating the "Rock Cycle".

 

2. Grain Size

There are added wrinkles to all of this, as you might expect. For example, as you can see above, igneous rocks form from the cooling of magma (or molten rock). Crystals actually grow from this liquid over a long period of time, depending upon their environment. For example, if a magma is cooling at depth, the surrounding host rock (like an insulating blanket) and the pressure of all the rock above generally results in slow cooling. The crystals can grow relativly large. On the other hand, if magma is tapped by fractures or faults, which provide channelways to the surface, then the magma or lava cools very quickly. The mineral crystals have little time to grow, so they are typically fine to very-fine-grained. Simple recognition of mineral grain sizes is a common field tool for distinguishing between plutonic igneous rocks (intrusive) (medium to coarse-grained) and volcanic (extrusive) igneous rocks (fine-grained).

3. Texture

But wait. How do we know for sure whether a rock is igneous or sedimentary or metamorphic? Well, that can be a tricky subject. A good tool to use in making a preliminary decision is texture. Because an igneous rock ‘grows’ from a liquid, the various minerals grow in a variety of directions. The resulting rock has an igneous texture as shown in the diagram at the right.

On the other hand, many sedimentary rocks, or the sediments from which they are made, are composed of mineral grains that have been eroded or weathered off original rocks somewhere else and carried along by agents such as water, wind, and ice. As a result, these grains have been ‘tumbled’ in a natural mill, and the sharp edges rounded off. When compressed and cemented into rocks, the grains still retain their sedimentary or clastic texture:

Metamorphic textures show more variety, but a quick guide is to look for colour banding of dark and light-coloured minerals, with a pronounced foliation or layering present. If one of the most common minerals present is a variety of mica, you may well be looking at a metamorphic rock:

4. Name

Many sedimentary rocks are named from the predominant particle or chemical present. If the predominant particle is sand, then the resulting rock is called sandstone. Similarly, if the predominant chemical is lime (CaO), then the resulting rock will be limestone. Metamorphic rocks are commonly named according to their texture or amount of foliation (e.g. gneiss, schist), and then by predominant mineral content (e.g. biotite schist). Igneous rocks are also named according to mineral content, but the scheme is much more complex. The key indicators for igneous rock names are:

1. Quartz content – commonly more or less than 5%
2. Feldspar ratios – mainly the relative amounts of Ca, Na, and K present
3. Colour index – the sum total of the dark-coloured (mafic) minerals present. The terms, mafic and felsic, are useful ones for quickly distinguishing rocks on the basis of colour, specific gravity, and to some extent, hardness. Mafic minerals/rocks have a higher percentage of Fe and Mg, and are, therefore, darker in colour and heavier. Felsic minerals/rocks are lighter in colour and lighter in weight. Because they also are ‘richer’ in silica, they have greater viscosity and tend to be harder.
4. Grain size – as briefly summarised above
5. Texture – in addition to what’s mentioned above, one looks for such signs as porphyritic texture, or volcanic textures such as vesicular or amygdaloidal

5. Age

Still another consideration must be weighed when classifying rocks – and that is age. The earth is approximately 4.6 billion years old (4.6 X 109). That huge span of time has been divided into smaller intervals; epochs, eras, and periods. One can get a rough appreciation of how much time is involved by inspecting the ‘wheel of time’ shown below:

Notice that most life forms are only significant in the fossil record from about 545 million years (5.45 X 108) ago to the present day. That number represents the transition into the Cambrian time period, named after the Roman name for Wales (Cambria) where much of the early investigative work was done. At the time, it was thought that any rocks pre-dating the Cambrian were devoid of fossils, and such rocks are termed PreCambrian.

One may be confused into thinking that since a certain sandstone, for instance, is Ordovician in age, then all sandstones are Ordovician in age. This is simply not true. The conditions for forming certain rock-types have repeated throughout geological time, so that it is quite common to find sandstones or most other rock types represented in any geological time period.

Students are often given a mnemonic device to help them learn the order of the geological time divisions starting with the Cambrian. Although these mnemonics vary, the following one may be of use to you: