How Rocks & Minerals are Formed
1. THE EARTH'S CRUST
The whole earth is made of rocks & minerals. Inside the earth there is a liquid core of molten rock and on the outside there is a hard crust. If you compare the earth to an egg, the shell on an egg is like the crust on the earth. The crust is made up of rocks and minerals. Much of the crust is covered by water, sand, soil and ice. If you dig deep enough, you will always hit rocks.Below the loose layer of soil, sand & crumbled rocks found on Earth is bedrock, which is a solid rock.
- The CRUST makes up less than 1% of the Earth’s mass (0.4%)
- There are 8 elements that make up 98% of the Earth’s crust. They are: Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium and Magnessium.
- The continents are about 35 km thick and the ocean floors are about 7 lm thick.
- The MANTLE is the solid casing of the Earth and is about 2900 km thick. It makes up about 70% of the Earth’s mass (68.1%). It is made up of silicon, oxygen, aluminum and iron.
- The CORE is mainly made of iron and nickel and makes up about 30% of the Earth’s mass (31.5%).
- The OUTER CORE is 2200 km thick and is liquid and the INNER CORE is 1270 km thick and is solid.
2. ROCKS, MINERALS, CRYSTALS, GEMS, SOIL, SAND & DIRT
All rocks are made of minerals. The rocks you see around you - the mountains, canyons & riverbeds, are all made of minerals. A rock is made up of 2 or more minerals.
Think of a chocolate chip cookie as a rock. The cookie is made of flour, butter, sugar & chocolate. The cookie is like a rock and the flour, butter, sugar & chocolate are like minerals.
A mineral is composed of the same substance throughout. If you were to cut a mineral sample, it would look the same throughout. There are about 3000 different minerals in the world.
Minerals are made of chemicals - either a single chemical or a combination of chemicals. There are 103 known chemical elements. Minerals are sorted into 8 groups.
- Native Elements ~ copper, silver, gold, nickel-iron, graphite, diamond
- Sulfides ~ sphalerite, chalcopyrite, galena, pyrite
- Halides ~ halite, fluorite
- Oxides & Hydroxides ~ corundum, hematite
- Nitrates, Carbonates, Borates ~ calcite, dolomite, malachite
- Sulfates, Chromates, Molybdates, Tungstates ~ celestite, barite, gypsum
- Phosphates, Arsenates, Vanadates ~ apatite, turquoise
- Silicates ~ quartz, almandine garnet, topaz, jadeite, talc, biotite mica
- Crystals are minerals that have had the chance to grow in the shape that they were meant to be.
- Just like your DNA determines the colour of your eyes, how tall you will get to be and the shape of your bones, the chemicals that a mineral is made of determines what shape it gets to be.
- We can tell different minerals apart by what crystal shape they are.
- Sometimes minerals form in spaces where there is not a lot of room, so they don't have a crystal shape.
- When there is just a big hunk of a mineral, it is called a massive mineral.
- If there is a definite shape with easy to see flat sides, it is called a mineral crystal.
- Most of the earth's crystals were formed millions of years ago.
- Crystals form when the liquid rock from inside the earth cool and harden.
- Sometimes crystals form when liquids underground find their way into cracks and slowly deposit minerals.
- Most mineral crystals take thousands of years to "grow" but some like salt (halite) can form so quickly that you can watch them grow at home!
- Some people think of crystals as clear pretty rocks that are used for jewelry. Amethyst is a very common quartz crystal.
- Crystals do not have to be clear, but those are the kinds you will usually see in the stores.
- Gemstones are often what people mean when they talk about "crystals".
- There are many gemstones and most are used for jewelry or decoration. These are minerals that are usually transparent and have been cut and polished.
- Some gemstones look similar to what the mineral looks like when found in nature and others are very different.
- Few minerals found in nature are suitable to use unaltered in jewelry. One exception is the "Herkimer Diamond" which is a doubly terminated quartz crystal which forms in vugs of gray rock and are found near Herkimer, New York. These are not real diamonds - they are quartz crystals that look like they have been cut & polished like a diamond.
SOIL, SAND & DIRT
- When rocks break down into smaller & smaller pieces, they turn into sand.
- If you look at the sand under a microscope, you will see that sand is made up of the same minerals as the rocks that the sand came from.
- When plants start to sprout up in sand, it is turning from being just small bits of rock to being soil.
- Soil is very important to life on earth. It supports plant life. We could not live without plants.
- Soil is made up of sand and decomposing plants and animals.
- Soil has many names including: clay, silt, mud, dirt, topsoil, dust, potting soil and humus
- For a good description of the importance of sand and How Sand Is Made, visit http://www.madehow.com/Volume-3/Sand.html
Rocks are constantly being formed, worn down and then formed again. This is known as the ROCK CYCLE. It is like the water cycle but it takes a lot longer. It takes thousands and millions of years for rocks to change. For more information on IGNEOUS, SEDIMENTARY and METAMORPHIC ROCKS click here.
EROSION is a key part of the Rock Cycle. It is responsible for forming much of the interesting landscape that is around us. To learn more how WATER, TEMPERATURE and WIND break up rocks, click here.
Click here for a list of some of the more common words and phrases used when describing rocks and minerals. If a word is not on our list, then look for it on an online geological dictionary.