|Our planet, we call earth, is about 4.6 billion years old. It formed from a swirling cloud of gas and dust orbiting a newly formed star which we call ‘The Sun’. Gravity drew the gas and dust together to form planets around our star.
Fortunately for us, the earth formed in what is called the ‘Goldilocks Zone’, a zone close enough to our star to be warm and far enough away for water to remain in it’s liquid state.
Our planet was not alway like it is today.
As it grew, it was bombarded by comets and
asteroids which kept the newly forming planet in a molten state.
Another fortunate event in our planet’s history was in a collision with another early planet that was orbiting the sun on a collision course.
The impact with this other planet, known as ‘Theia’, splashed dust and rocks into space which
coalesced to form our moon.
The collision may also have caused the earth to tilt on it’s axis at approximately 23.5º giving us the season as our planets travels on it’s 365.25 day journey around the sun.
The Earth is 93 million miles (150 million Kilometres) from the sun and the moon is 240,000 miles away.
Although the moon formed much closer to the earth it has
gradually drifted away at the rate of 1.48 inches or 3.78 centimetres a year.
As our planet cooled water formed on the surface. In the early days the earth spun at a much faster rate of 6 hours per rotation. The gravity of the moon slowed the earth’s spin to what it is today.
The moon being much closer to the earth at that time created much higher tides. This was due to the force of the moon’s gravity on the earth. The tides constantly rose over the molten magma of the earths surface cooling it down to allow oceans to form. 71% of the earth’s surface is covered by water. Rocks fractured, lakes formed and soon there was sand due to the erosion of rocks.
The flow of water dissolved minerals from the rocks which washed into the oceans. There was no life on earth at this time but with the minerals dissolved in water soon the conditions were right for life to form.
Initially there was only about 250 different species of minerals on the planet. As the land cooled it is thought that some of these minerals had dissolved in the water and started
chemical reactions leading to the formation of amino acids which are the building blocks of life.
These reactions eventually led to the formation of more complex molecules leading to the first microbes some 3.7 billion years ago.
Evidence of the earliest life has been found in hydrothermal vent precipitates.
There are three events specific to the formation of new minerals starting with the birth of the sun. The formation of the planets and asteroids increased the number of minerals to about 250.
The reworking of Earth’s crust through partial melting, plate tectonics and volcanism increased the total to about 1,500 species.
The largest increase in mineral species is due entirely to the development of life
amounting to more that 65% of total known species.
It was as a result of chemical changes made by living organisms that led to the increased number of species
during the Great Oxidisation Event.
Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O2). The atmosphere on early earth was low in oxygen but with the development of life, particularly cyanobacteria, oxygen levels rose.
These microbes conduct photosynthesis, the use of sunshine, water and carbon dioxide to produce carbohydrates and oxygen which was expelled as a waste product. The higher levels of O2 in the water led to the oxidation of minerals in the water, notably IRON, through ‘mass rusting’ which led to the deposition of iron oxide (FeO2) on the ocean floor leaving bands of iron deposits, some of the largest of which are in Western australia.
Stromatolites, are rock type structures that usually form in shallow water. They are formed by cyanobacteria. The mucus secreted by these bacteria collect grains of sediment which are cemented together by calcium carbonate.
Bacteria is only
active on the surface and this causes the build up of the rock structure.
Stromatolites are the evidence of the earliest forms of life on earth dating back to 3,795 Million years ago. Living Stromatolites may be seen at Shark Bay in Western Australia.
Other theories on life on earth suggest that life
arrived on earth aboard a rock from space.
Whatever the case,
rocks or minerals
have played an important role
in the creation of life on Earth.
And life on Earth has
played an important role
in the creation of new mineral species.
Now you have an understanding of how minerals occurred we will add some
Geological information that will enable you to understand the rock types and where
mineral species may occur.
There are three rock types: Igneous, which are formed from the solidification of molten rocks; Sedimentary Rocks, which are formed as a result of weathering of other rock types and Metamorphic which are altered rocks.
There are two types of Igneous Rocks,
Intrusive and Extrusive.
Intrusive rocks form below the earth’s surface and may result in extremely large crystals being formed due to the slow cooling. Examples of these are granite,
diorite, pegmatite and peridotite.
Extrusive rocks are formed from ejecta and cool very quickly not allowing large crystals to form. Some of these rock types are basalt, obsidian, pumice, rhyolite, scoria, dacite, andesite, and tuff.
Some cool so quickly that they form and amorphous glass like material called obsidian.
Mineralisation can occur in some of these extrusive rocks where air bubbles
(vesicles) have formed in the cooling process and may later be filled with minerals.
Typical minerals that form in these vesicles are Agate, Calcite, Prehnite
Sedimentary Rocks are rocks which have formed from the
deposition of small
particles of mineral or organic origin that build up on the bottom of oceans or other water bodies.
The mineral matter was formed as a result of erosion of rocks through weathering and erosion. The organic matter is from the dead remains of living things. Living things become fossil remains.
They range in size from micro organisms to huge dinosaurs. Common fossils are shells, like those of the ammonite, nautilus or molluscs whilst the remains of trees form petrified wood under the right conditions.
Sedimentation may also occur from the minerals dissolved in the water.
Examples of Sedimentary rocks are: sandstone and limestone but sedimentary rocks can be made up of any of the rock types, Igneous, Metamorphic and
Sedimentary which have been eroded.
All the forces of nature are responsible for erosion of rocks. Even plants can cause rocks to erode. A root system can cause rocks to crack and open up. In extreme climates the differing temperatures between day and night can erode rocks.
Metamorphic Rocks are rocks which have undergone changes due heat or pressure or both.
There are three types of metamorphism:
3. Dynamic Contact
Contact Metamorphism occurs when magma comes in contact with an already existing body of rock that may have formed earlier.
For example, if magma comes in contact with coral (composed of calcium carbonate) marble may be formed.
Examples of Contact Metamorphic rocks are quartzite, marble, slate, granulite, gneiss and schist.
Regional Metamorphism occurs when rocks are buried under vast amounts of
sediments to a depth of over 10 Kilometres.
This usually is associated with the
formation of mountain ranges and takes place within the continental crust.
Examples of rocks formed by Regional Metamorphism are slate, schist and gneiss.
Dynamic Metamorphism are rocks that are restricted to narrow zones and
adjacent to faults. Heat caused by high pressure and strains.
Ground water may play a role in the formation of these rocks.
Examples of rocks formed by Dynamic Metamorphism are breccias and conglomerates.
Some rocks found in the Pilbara of Western Australia contain 4.1 billion year old crystals of zircon which can only form in the presence of water leading to
speculation that life on earth may have formed much earlier than originally believed.
THE STORY OF ROCKS
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