Rocks and Minerals
The earth's crust ( outer layer of the earth) is made of rocks and minerals.
Mineral- This can be defined as a naturally occuring, inorganic( non-living), solid substance.
There are thousands of minerals such as Quartz,
Diamond, Feldspar and Iron. Minerals are very difficult to tell apart because many minerals look the same.
There are certain properties of minerals that we can use to identify the mineral.
1)Lustre- This refers to how shiny the mineral is. There are three types of lustre. i) Glassy- If the mineral reflects light like a piece of glass. ii) Metallic- If the mineral reflects light like the polished surface of a metal. iii) Dull- If the mineral does not reflect light well.
Dull Glassy Metallic
2) Streak- If a mineral is rubbed against a porcelain plate, it leaves a powdered mark. The colour of the mark can help identify the mineral. A mineral can be different colors but it's streak is always the same color.
3) Hardness- This refers to how difficult it is to scratch a mineral. The hardness of minerals are determined by using the Mohs Hardness Scale.
(You DO NOT have to memorize this scale.)
4) Cleavage and Fracture.
Some minerals may have cleavage which means the mineral breaks along smooth flat surfaces called planes. A good example of this is a mineral called Mica. ( remember the lab) Some minerals break along jagged edges in no particular pattern. These minerals have fracture. For example Quartz.
Colour is a property that is unreliable for two reasons.
One, a mineral may be many different colours. For example, quartz may be rose colored, white, colorless.
Two, many minerals can be the same colours. For example, both pyrite and gold have a golden colour.
Some other properties that are important to identify minerals are: heft( how heavy it is), magnetism, odour (smell), and texture ( feel).
A rock is a mixture of two or more minerals. Rocks are formed in different ways. Rocks can be grouped into three different types or families based on how they form.
1) Igneous Rocks
This type of rock forms when melted rock cools and hardens.
Liquid rock that is found below the earth's surface is called Magma. If this magma reaches the Earth's surface through a volcano, it is called Lava.
Igneous rocks can be grouped into two types;
A) Intrusive Rock. If the magma cools and hardens below the earth's surface, the rocks formed are intrusive( inside the earth) rocks. These rocks cool slowly and there is time for large crystals to form. An example of a intrusive rock is Granite.
B) Extrusive Rock. If the magma reaches the earth's surface( lava) it cools and hardens quickly. There is no time for crystals to form and the rock looks homogeneous. For example: Basalt. Sometimes the lava hardens so quickly that it forms volcanic glass which is called Obsidian. Another type of extrusive rock forms from frothy ( foamy) lava and is called Pumice.
2) Sedimentary Rocks. These rocks are formed from tiny pieces of other rock ( sediments)that form layers and then harden into rocks. This usually takes place on the ocean floor. The processes that form sedimentary rocks are compaction and cementation.The layers in sedimentary rocks are called Beds.
Sedimentary rocksare named based on the size of their particles.
Shale-This type of rock has very fine grains. It forms from mud or clay. .
This type of rock forms from sand sized particles.
Conglomerate- This type of rock is made of different sized particles, some big some small.
Limestone- This type of rock is formed from the shells and skeletons of marine organisms.
3) Metamorphic Rocks : Metamorphic means changed. This is a type of rock that was once a sedimentary, igneous, or different metamorphic rock but has been changed over a long period of time due to high temperatures, pressure or the presence of hot fluids. The original rock is called the parent rock. Metamorphic rocks often have bands because the crystals in the parent rock have been squeezed into lines... Some examples are:
Parent Rock Metamorphic Rock
Rocks are always changing. One type of rock may turn into another. An example is sedimentary rocks may get pushed underground and get melted into igneous rocks, or igneous rock may get put under high pressure and change to Metamorphic rock. This process of rocks continuously changing is called " The Rock Cycle". Another way of saying this is that rocks are continuously recycled into different forms.
We can see the processes that cause this recycling of the rocks in the diagram above. Weathering and erosion can turn igneous and metamorphic rocks into sedimentary rocks. Heat and pressure can change sedimentary and igneous rocks into metamorphic rocks. Melting and cooling changes metamorphic and sedimentary rocks into igneous rocks.