How are Rocks Classified?
|Every rock has characteristics that reflect its process of formation; these are used to determine whether the rock is igneous, sedimentary or metamorphic. For example, a rock with rounded grains cemented together is sedimentary, while one with a strong banding and orientation of minerals is metamorphic. Identification of rock types within the three divisions depends on the nature of the mineral grains, including their composition and size and relationship to surrounding grains; the way the rock occurs; how uniform it is; its hardness; how it reacts with acid; its colour; and the way in which it breaks. Before trying to identify a rock, break a piece off with a rock hammer. This will expose a clean, fresh surface. (WARNING - do not use a carpenter's hammer, they are made from hardened steel and will chip, possibly causing injury.)
When a rock is dirty or covered with lichen or moss, many of its characteristics are obscured preventing you from accurately identifying the rock. Identification of beach pebbles that are smoothed and rounded is not considered here. Many pebbles are relatively easy to identify without breaking them, but many are extremely difficult, and breaking them usually destroys their interest and charm. As a rule a pebble catches the eye and is picked up because it is an object which shows some special effect of colour or pattern, often due to layering, veining or alteration. Such pebbles may be rare and not the common rock types considered here.
To identify a rock which is of interest, first, take a fresh surface and examine it carefully with the naked eye. Decide whether the grains which make up the rock can be distinguished or not. Click on the appropriate answer. Where only some of the grains can be seen, for example, a rock in which there are a number of large grains in a very fine matrix, click on "grains not distinguished". From here on use a hand lens and, where it is required, a knife or dilute hydrochloric acid (5%). Keep answering the questions and clicking on the answer which most closely fits your specimen. Continue until you come to a rock name and description. If the features and the way the rock occurs fit your sample - BRAVO!! Otherwise go back through the key (use the back button on your browser) and check for alternative choices where you may have been unsure of your observations; rocks vary so don't be surprised or discouraged if this happens.
Key to Classifying Common Rock Types
- 1. Inspect the rock carefully with the naked eye. Are some or all the grains distinguishable or not?
- if some or all grains are distinguishable - go to 2
- if the grains are not easily distinguishable - go to 13
- 2. Look at the grains in the rock - their shapes and relationship to one another.
- if the grains are rounded or angular - go to 3
- if the rock is made up interlocking crystalline grains - go to 6
- 3. This is probably a sedimentary rock. Are the grains rounded or angular?
- if the grains are mostly rounded - go to 4
- if the grains are mostly angular - go to 5
- 4. Look at the size of the grains. How large are they?
- if the grains are mostly sand sized (less than 2 millimetres) - the rock is a sandstone
- if the grains are mostly gravel sized or larger (larger than 2 millimetres), or a mix of coarse and fine grains - the rock is a conglomerate
- 5. Look at the grains and the matrix between them.
- if the grains are mostly coarse (larger than 2 millimetres) in a fine matrix - the rock is a breccia
- if the grains are volcanic fragments and possibly include glassy grains - the rock is a volcanic breccia
- 6. This is probably an igneous or metamorphic rock. Does the rock react with dilute hydrochloric acid?
- if the rock fizzes - the rock is a marble
- if it does not fizz - go to 7.
- 7. Does the rock show layering of minerals?
- if the rock does not show any layering - go to 8
- if the rock is noticably layered - go to 12
- 8. Look at the size of the crystals.
- if they are very large (usually larger than 1 to 2 centimetres) - the rock is a pegmatite
- if the crystals are roughly equal in size - go to 9
- if there are large crystals in a fine matrix - the rock is a porphyry
- 9. Estimate the relative proportions of light and dark coloured minerals.
- if the rock is mostly light coloured minerals with two types of feldspar - go to 10
- if the rock has more light minerals than dark, and mostly one type of feldspar - the rock is a granodiorite
- if the rock has equal quantities of light and dark minerals (intermediate colour) - go to 11
- if the rock is mostly dark minerals - the rock is a gabbro
- if the rock is all dark coloured minerals - the rock is an ultramafic rock
- 10. How much quartz is present in the rock?
- if the rock is mostly feldspar with little or no quartz - the rock is a syenite
- if the rock has both quartz (often more than 10% of the rock) and feldspar - the rock is a granite
- 11. Is there any quartz present in the rock?
- if there is some visible quartz (usually less than 5%) - the rock is quartz diorite
- if there is no visible quartz - the rock is diorite
- 12. Look at the minerals present and how easily the rock splits along layering.
- if the rock is made up of granular quartz crystals that break across the grains - the rock is quartzite
- if the rock micaceous and splits easily along the layers - the rock is schist
- if the rock has coarse light and dark layers which do not split very easily - the rock is a gneiss
- 13.Is the rock glassy or not?
- if it is glassy - go to 14
- if it is not glassy - go to 15
- 14.What is the overall colour of the rock?
- if the specimen is light coloured - it is quartz
- if the rock is dark coloured - the rock is obsidian
- 15.How hard is the rock? Try to see if it will scratch a knife blade.
- if the rock is hard and scratches a knifeblade - go to 16
- if the rock is soft and will not scratch a knife blade - go to 19
- 16.Look at the rock closely with a hand lens.
- if the rock is smooth with no visible grains - the rock is chert
- if the rock is granular - go to 17
- 17.Is the rock layered?
- if the rock is layered - the rock is tuff
- if the rock is not layered - go to 18
- 18.What is the overall colour of the specimen?
- if the rock is light in colour - the rock is felsite
- if the rock is intermediate in colour - the rock is andesite
- if the rock is dark in colour - the rock is basalt
- 19.Does the rock react with dilute hydrochloric acid?
- if it fizzes - go to 20
- if it does not show any reaction - go to 21
- 20.How vigorous is the reaction with acid?
- if the reaction is vigorous - the rock is limestone
- if the reaction is slow, but better with rock powder - the rock is dolomite
- 21.Does the specimen show any layering?
- if it is layered - go to 22
- if it is not layered - go to 23
- 22.Test how easily the rock splits and how it feels.
- if the rock is rough to touch and light in weight - the rock is tuff
- if the rock does not split easily - the rock is shale
- if the rock splits easily into smooth sheets - the rock is slate
- 23.What is the colour of the rock? How does it feel?
- if the rock is green to black with a slippery feel - the rock is serpentinite
- if the rock is light coloured and feels greasy - the rock is talc
Remember that rocks vary, so don't be surprised or discouraged if you fail to identify your specimen first time. Review the description of the features and the way the rock occurs. If they do not seem to fit your sample - go back through the key (use the back button on your browser) and check for alternative choices, particularly where you may have been unsure of your observations.