Some types of rock have interlocking grains that fit tightly together. Granite is a rock with interlocking grains. Other types of rock have rounded grains. Sandstone is a rock with rounded grains.
Rounded grains - Sandstone:
Interlocking grains - Granite:
Porous rocks
Rocks with rounded grains are more likely to absorb water than rocks with interlocking grains. THIS IS BECAUSE THE WATER CAN GET INTO THE GAPS BETWEEN THE GRAINS. Rocks that absorb water are called POROUS.
Rocks with rounded grains are usually softer and easier to crumble because they lack the structure of rocks with interlocking grains. Plus, if the porous rock gets more water into it, that alone will make it softer.
Rounded grains with water effect:
Interlocking grains with water effect:
So from this, we can gather that a rock with interlocking grains will be more structured and durable enough to resist damage either via water or compaction.
Sedimentary rocks:
There are 3 main types of rock; sedimentary, igneous and metamorphic.
The formation of sedimentary rocks:
A river carries/transports many things besides water and one of them are broken pieces of rock as it flows in its given direction. All rivers flow to the sea and when the river meets the sea there is a great difference in depth, so anything that has a bit of weight will sink to the bottom of the sea. We refer to this as the rocks being DEPOSITED. These DEPOSITED ROCKS build up in layers from the seabed, called SEDIMENTS. This process is called SEDIMENTATION.
Simply put, layer upon layer stack on top of each other. As this happens the layers that are at the bottom will be compacted and crushed due to the weight pressure of the layers above. With this happening in the sea, water is squeezed out from between the pieces of rock and crystals of different salts are formed.
IT'S PROBABLY A GOOD IDEA TO MENTION HERE THAT ALL ROCKS ARE MADE OF MINERALS.
The crystals that form act as a cement because they cause the pieces of rock to stick together. This is known as CEMENTATION.
These processes make the type of rock known as SEDIMENTARY ROCK. This process may take MILLIONS of years for the SEDIMENTARY rocks to form but due to the nature of the CYCLE, SEDIMENTARY rocks are constantly moving on to the next phase.
This is the established order of the SEDIMENTARY rock formation:
SEDIMENTATION - COMPACTION - CEMENTATION
IGNEOUS ROCKS:
Igneous rocks are a product of the inner Earth - a very hot place that has the ability to melt rock. MOLTEN (LIQUID) ROCK is the product of melted rocks and is known as MAGMA. MAGMA does eventually undergo the law of cooling and when cool enough, IGNEOUS ROCKS are formed.
You may be wondering to yourself - isn't a rock a rock? True, but you can tell a tremendous amount of information from a rock and it's composition by taking a closer (much closer) look.
Igneous rocks are comprised of randomly arranged INTERLOCKING crystals. Now, the size of the crystals depends upon how quickly the MAGMA cooled and solidified. The longer the cooling process, the bigger the crystals. That is, they have more TIME to stick together and arrange themselves, meaning a larger crystal.
Metamorphic rocks!
The 3rd type of rock in the rock cycle. The key word to pick out here is MORPH - that is, metamorphic rocks are formed from OTHER rocks by a change in HEAT or PRESSURE. They are MORPHED from other rocks.
As we know, the Earth is constantly moving, whether it be spinning on its axis, orbiting the Sun or even the movement of its tectonic plates.
Earths movements cause rocks to be either crushed or buried. All this compaction causes heat via the pressure that the is applied on the rock. The heat doesn't cause any melting but instead causes a CHEMICAL REACTION. It allows the chemical composition of the rock to be change. The MINERALS that make up the rock are altered, giving us a NEW ROCK. The heat allows the reaction to take place as HEAT IS ENERGY or rather HEAT GENERATES ENERGY.
On occasion, METAMORPHIC rocks are formed when rocks close to some MOLTEN MAGMA and are subsequently HEATED UP.
A KEY POINT TO NOTE HERE IS: ROCKS THAT DO MELT FORM IGNEOUS ROCKS - NOT METAMORPHIC.
When a METAMORPHIC rock is formed under pressure, its crystals become arranged in layers. Slate, which is formed from shale, is like this. The slates on your roof may spring to mind here. Slate can be split into flat sheets - hence why it's so appropriate for roofing.
MARBLE is another fine example of a well used metamorphic rock and is formed from limestone. Metamorphic rocks can contain fossils if they were formed from sedimentary rock (the fossils would be compacted) but the fossils are usually squashed due to the nature of the sedimentary rock process.
METAMORPHIC rocks can be formed from either IGNEOUS or SEDIMENTARY rocks.
Weathering is also a key part of the rock cycle. Rocks are gradually worn down by the weather. There are 3 types of weathering that can occur:
Physical
Chemical
Biological
Physical weathering is caused by actual physical changes in the surrounding weather, changes such as temperature (increase/decrease), freezing, thawing, wind, rain or waves.
Temperature:
When something gets hot it expands - a rock is no different. It's also the same scenario if something gets cooled it contracts - a rock is no different. This constant expansion and contraction causes the rock to crack and crumble. In climates such as the DESERT, the temperature goes from one extreme to the other so this type of weathering is very common.
Wind has the ability to blow tiny grains of sand against a rock. These wear the rock away and weather it. Rain and waves too have the ability to wear away rock over long periods of time.
When water freezes, it expands, this is how many things are damaged IE water pipes and rocks are no different. Water will find get into anything it can, it can and getting into the crack of a rock is easy. Once the water freezes, it could either widen the crack or split the rock. It could take years for a very large rock to split as the crack could be very minor to begin with and be gradually expanded.
Biological weathering:
Animals and plants too can wear away rocks. Animals can set up home within a rock and create or widen cracks that split it.
We all have weeds in our garden and plenty in between the flags/stones of our paths, where there is soil and the ability to receive water, you will find a plant.
This plant may start small and stay small or it could grow to be very big. It's roots would cause the cracks to expand (it's important to note that the cracks may be minuscule but are still cracks). Eventually, pieces of rock would break away.
Me and you are examples of biological weathering, we tread on rocks often and the constant pounding will eventually cause wear and tear of the rocks.
Chemical weathering:
Rainwater is naturally slightly acidic because CO2 from the air dissolves in it. Minerals in rocks may react with the rainwater, causing the rock to be weathered.
Some types of rock are more reactive than others. Limestone is made of a mineral known as CALCIUM CARBONATE (CaCO3). When acidic rainwater hits the limestone a reaction takes place that results in the formation of new soluble substances. These are washed away and the rock is weathered.
The more ACIDIC the rain is, the greater the chemical weathering. Rain that is more acidic contains fossil fuels that have been burned IE oil.
Difference between WEATHERING and EROSION:
Weathering is the wear and tear of the rocks.
Erosion is the transportation of the broken pieces of rock from the weathering.
Examples of the types of each rock:
SEDIMENTARY: Limestone, sandstone.
IGNEOUS: Basalt and granite.
METAMORPHIC: Slate and marble.
Many thanks, Adam. :-) X
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