The Rock Cycle and Different Types of Rocks

Rocks are made of grains that fit together. Each grain is made from a mineral, which is a chemical compound.

Two types of grains:1. Interlocking Grains: Fit tightly together (looks like a puzzle)

a) Example: Granite2. Rounded Grains: have gaps between the grains that absorb water.

2) Example: Sandstone

Porous rocks are rocks with rounded grains that absorb water. Water can get into the gaps between the grains.

Porous rock grains

This is an example of rocks with interlocking grains are more likely to be hard and non-porous.

This is an example of rocks with rounded grains that are more likely to be crumbly and porous.

Sedimentary Rocks

Sedimentary rocks are made of layers of compressed sediments (caused by weathering).

Sedimentation- sediment (pieces of broken rock) is transported by a river to the ocean, lake or sea. The sediment is deposited and settles to the bottom and this continues over time building up the amount of sediment in the location in layers.

Compaction- the weight and pressure of the sediments on top squashes the sediments at the bottom. The water is squeezed out from between the pieces of rock and crystals of different salts form.

Cementation- the crystals form a sort of glue or cement that makes the rock pieces stick together.

The processes to form a sedimentary rock in order:1. Sedimentation2. Compaction3. Cementation

Sedimentation:

The river transports bits of rock, and deposits them on the bottom of the river bed.

Compaction:

Deposited rocks build up in layers, and the weight of the top layers compresses the bottom layers

Cementation:

The compression squeezes out water, leaving salt crystals that cement the rocks together.

These processes take millions of years, heat and pressure to form a sedimentary rock. Most sedimentary rocks are found under water or were once under water.

Examples of Sedimentary Rock:1) Chalk2) Limestone3) Sandstone4) Shale

The oldest layers are at the bottom and they youngest layers are at the top.

Sedimentary rocks may contain fossils of animals or plants trapped in the sediments as the rock was formed.

Igneous Rocks

Igneous rock is a rock formed by magma after it cools and becomes a solid.

The inside of the earth has a molten liquid that forms when rocks melt. This molten rock is called magma until it reaches the earth’s surface, then it is known as lava.

Igneous rocks contain randomly arranged interlocking crystals. The size of the crystals depends on how quickly the molten magma changed to a solid. The more slowly the magma cools, the bigger the crystals. The faster the magma cools, the smaller the crystals.

Extrusive Igneous rocks form when magma erupts onto the earth’s surface and becomes lava. The magma cools quickly and has smaller crystals. Examples: obsidian and basalt

Intrusive Igneous rocks form when magma stays under ground. The magma cools slowly and has larger crystals. This can only happen if the magma cools deep underground. Examples: granite and gabbro

Igneous rocks do not contain any fossils, as it would have melted when the magma formed.

Rocks that do melt form igneous rocks.

Metamorphic Rock

Metamorphic rocks are formed from other rocks that are changed due to heat or pressure or both. Rocks can be deeply buried under ground or squeezed. The rocks are heated and put under great pressure. They do not melt, but the minerals they contain are changed chemically which causes the formation of metamorphic rock.

Metamorphic rocks are sometimes formed when the rocks are close to some molten magma and get heated up.

Metamorphic rocks may form from rocks heated by magma.

Metamorphic rocks are NOT made from melting rocks. Igneous rocks are rocks that melt.

When metamorphic rock is formed under pressure, its crystals become arranged in layers. Example: Slate

Metamorphic rocks sometimes contain fossils if they were formed from sedimentary rocks, but the fossils are usually squashed out of shape.

Metamorphic rocks can be formed from any other type of rock-sedimentary or igneous.

Examples of Metamorphic rock formed from other types of rock:-Slate formed from shale.-Marble formed from limestone.

Weathering

Weathering is the gradual wearing away of rock.

Three types of weathering:1) physical weathering2) chemical weathering3) biological weathering

Physical weathering is caused by physical changes such as changes in temperature, freezing and thawing, and the effects of wind, rain and waves.

Temperature ChangesWhen a rock gets hit it expands a little, and when a rock gets cold it contracts a little. If a rock is heated and cooled many times, cracks from and pieces of rock fall away.

Wind, rain and wavesThe wind can blow tiny grains of sand against a rock. These wear the rock away and weather it.Rain and waves can also wear away rocks over long periods of time.

Freeze-thawWater expands slightly when it freezes into ice. The formation of ice can also break rocks. If water gets into a crack in a rock and then freezes, it expands and pushes the crack further apart. When the ice melts later, water can get further into the crack. When the rock freezes again, it expands and makes the crack even bigger. This process of freezing and thawing can continue until the crack becomes so big that a piece of rock falls off.

Freeze-thaw process:

Water gets into a crack in a rock,

The water freezes and expands, making the crack bigger,

The water can get further into the crack, making it bigger, and

This process of freezing and thawing can continue until the crack becomes so big that a piece of rock falls off.

Biological Weathering

Biological weathering is when animals and plants wear away rocks. Example: An animal burrowing into a crack in a rock, making it bigger and splitting the rock.

Example: Plant roots can grow in cracks and as they grow bigger, the roots push open the cracks and make them wider and deeper causing pies of rock to fall away.

Example: People can cause biological weathering by walking which causes the breakdown of rocks.

Chemical Weathering

Chemical weathering is the weathering of rocks by chemicals. Rainwater is naturally slightly acidic because carbon dioxide from the air dissolves in it. Mineral in rocks may react with the rainwater, causing the rock to be weathered.

Rocks easily weathered by chemicals: limestone and chalk

Rocks not easily (weathers slowly) weather by chemicals: granite and gabbro

Acid Rain

When fossil fuels (coal, oil and natural gas) are burned, carbon dioxide and sulphur dioxide escape into the air. These dissolve in the water in the clouds and make the rainwater more acidic than normal. When this happens, acid rain is produced.

Acid rain makes chemical weathering happen more quickly.

Erosion and Transport

Weathering is the wearing away of rocks.

Erosion is the movement of the broken pieces away from the site of weathering.

Transport is when pieces of rock move within streams and rivers.

The Rock Cycle

Earth’s rocks do not stay the same forever. They are continually changing because of processes such as weathering and large earth movements. The rocks are gradually recycled over million of years, which is called the rock cycle.

Process of the Rock Cycle:

Sedimentation creates layers or rock particles.

Compaction and cementation presses the layers and sticks the particles together. This creates sedimentary rock.

Rocks underground that get heated and put under pressure are changed into metamorphic rock.

Rocks underground that get heated so much they melt turn into magma. Magma is liquid rock. Magma also comes from deeper inside the Earth, from a region called the mantle.

Pressure can force magma out of the ground. This creates a volcano. When the magma cools it turns into solid rock, called extrusive igneous rock.

Magma that cools underground forms solid rock called intrusive igneous rock.

Areas of rock can move slowly upwards, pushed up by pressure of the rocks forming underneath. This is called uplift.

Weathering breaks down rocks on the surface of the Earth. There are three types of weathering - physical, chemical and biological.

Wind and water move the broken rock particles away. This is called erosion.

Rivers and streams transport rock particles to other places.

Rock particles are deposited in lakes and seas, where they build up to form layers. This starts the process ofsedimentation which will create sedimentary rock.