How many different rock types are in this photo?
A beach or river bed is a good place to see a lot of different rock types since the rocks there represent the entire drainage system. How could you tell how many different rock types were in the photo? What characteristics would you look for?
What Are Rocks?
We all know what rocks are right? The are lying on the ground everywhere! They are the mountains, and canyons that you can’t help but notice. We have all thrown them, sat on them, and dug them out of our gardens. However, to pin down an exact definition… that is something not all of us could do.
A rock is a naturally formed, non-living Earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass (Figure below).
The different colors and textures seen in this rock are caused by the presence of different minerals.
A rock is:
- A solid
- Naturally occurring
- Is made up of minerals or mineral like matter.
Some rocks are composed of just one mineral. Pyrite and quartz are two common rocks that fit this category. Most rocks are a solid mixture of several minerals like granite.
In each group, distinctions are made for texture or grain size and chemical or mineral content.
That is pretty straightforward isn’t it? That is the textbook definition. However, there are some gray areas.
Exceptions to the What Is A Rock definition
What about mercury? It is not a solid at normal temperatures but if it gets cold enough it becomes solid.
Then there is coquina. Coquina is a sedimentary rock made of seashells. The shells are made of minerals but they are not minerals.
Coal is considered a rock but it is not made of minerals it comes from organic matter plants.
These last two are called biogenic rocks.
Petrology is the study of rocks.
What Is A Rock? Perhaps the more important question is “why study rocks?”
Rocks are the pages in the history book of the earth. They tell stories of times long past. From rocks, we have learned of dinosaurs, trilobites and thousands of other life forms that existed on earth once but are now no more.
We have learned that the earth is constantly moving, shifting, and changing. Rock is continually being created. It is heated, squeezed, and weathered into new rocks. Finally, it is remelted to start the process anew. By studying rocks, we learn to read the pages of earth’s history.
How is a rock different from a mineral? Rocks are made of minerals. The mineral grains in a rock may be so tiny that you can only see them with a microscope, or they may be as big as your fingernail or even your finger (Figure below).
A pegmatite from South Dakota with crystals of lepidolite, tourmaline, and quartz (1 cm scale on the upper left).
Rocks are identified primarily by the minerals they contain and by their texture. Each type of rock has a distinctive set of minerals. A rock may be made of grains of all one mineral type, such as quartzite. Much more commonly, rocks are made of a mixture of different minerals. Texture is a description of the size, shape, and arrangement of mineral grains. Are the two samples in Figure below the same rock type? Do they have the same minerals? The same texture?
|Sample 1||plagioclase, hornblende, pyroxene||Crystals, visible to naked eye||Magma cooled slowly||Diorite|
|Sample 2||plagioclase, hornblende, pyroxene||One type of crystal visible, rest microscopic||Magma erupted and cooled quickly||Andesite|
As seen in Table above, these two rocks have the same chemical composition and contain mostly the same minerals, but they do not have the same texture. Sample 1 has visible mineral grains, but Sample 2 has some visible grains in a fine matrix. The two different textures indicate different histories. Sample 1 is a diorite, a rock that cooled slowly from magma (molten rock) underground. Sample 2 is an andesite, a rock that cooled rapidly from a very similar magma that erupted onto Earth’s surface.
A few rocks are not made of minerals because the material they are made of does not fit the definition of a mineral. Coal, for example, is made of organic material, which is not a mineral. Can you think of other rocks that are not made of minerals?
Who is Mohs and what is his hardness scale all about?
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science.
The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another mineral. The samples of matter used by Mohs are all different minerals. Minerals are pure substances found in nature. Rocks are made up of one or more minerals. As the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5.
The Mohs scale is a purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as corundum. The table below shows comparison with absolute hardness measured by a sclerometer, with pictorial examples.
|Mohs hardness||Mineral||Chemical formula||Absolute hardness||Image|
On the Mohs scale, graphite (a principal constituent of pencil “lead”) has a hardness of 1.5; a fingernail, 2.2–2.5; a copper penny, 3.2–3.5; a pocketknife 5.1; a knife blade, 5.5[clarification needed]; window glass plate, 5.5; and a steel file, 6.5. A streak plate (unglazedporcelain) has a hardness of 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.
The table below incorporates additional substances that may fall between levels:
- Nearly all rocks are made of minerals. A few are made of materials that do not fit the definition of minerals.
- Rocks are typically identified by the minerals they contain and their textures.
- The texture of a rock describes the size, shape, and arrangement of mineral grains and is a reflection of how the rock formed.
Use this resource to answer the questions that follow.
1. What is a rock?
2. What type of rock is this?
3. What mineral produces the pink pieces?
4. What mineral produces the white pieces?
5. What mineral produces the black pieces?
6. What is a mineral?
1. Name a rock type that is not made of minerals and state how a rock could not be made of minerals.
2. Can a rock be made of only one type of mineral, or do rocks need to be made of at least two minerals?
3. Why is texture so important in classifying rock types?
Rock – a naturally formed, non-living Earth material
Texture – a description of the size, shape, and arrangement of mineral grains
Moh’s Hardness Scale – characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material