Please enable JavaScript.
Coggle requires JavaScript to display documents.
8.1 The Earth and it's rocks (The rock cycle (Igneous rocks form from…
8.1 The Earth and it's rocks
Structure of the Earth
Scientists cannot see what is inside the earth but information form earthquakes, meteors and volcanic eruptions provide data
Geology is the study o rocks, and the scientists who study them are geologists
The crust is the outermost solid layer of the Earth, the crust varies in thickness from about 7 kilometres under the oceans to about 30-50 kilometres under the continents
Beneath the crust is a layer called the mantle, the mantle is much hotter than the crust and is chemically different for it
Within the uppermost portion of the mantle is a layer called the asthenosphere, the rocks here behave plastically
The core of the Earth is very dense, and scientists think it is composed mostly of iron and nickel, the inner core s solid and the outer core is liquid
The rock cycle
Igneous rocks form from red hot melted rock from inside the Earth
Sedimentary rocks form when small pieces worn off other rocks(sediments) become stuck together
The rock cycle is a model used by scientists to explain the endless cycling of materials that make up rocks by processes within Earth and on its surface
Metamorphic rocks have been changed by heat and pressure within the Earth
The rock cycle shows how each of the three types of rock can be changed into the other types by a variety of process
Weathering-rock is broken down into smaller pieces
Erosion-the broken rock material is transported from one place to another
Deposition-the broken rock material settles, forming sediment
Compaction and cementation-the sediment becomes packed together, and then chemically bound
Melting-the solid rock is heated, forming magma
Crystallisation-the magma becomes solid
Metamorphosis-the rock is changed by intense heat and/or pressure
Weathering
In geology, the term weathering covers all physical, chemical and biological that break rocks down into smaller pieces
Physical weathering
Wind
Temperature change
Physical weathering can happen because of the action of:
Water and ice
Crystallisation of salts
Wind
Fine particles of rock carried by wind can also blast away at a rock surface wearing pieces away
Anyone who has been on a beach on a windy day will know the feeling of sand blasting against your skin
Temperature change
If there is a hot day and cold night the temperature change can cause the rock to expand and contract to quickly and crack
Solids expand (get larger) when they are heated, and contract (get smaller) when they are cooled
The action of water and ice
As glaciers move, the ice wears and knocks pieces off the surrounding rocks, forming valleys that are U-shaped, rather than the V-shape carved by a river
Water can enter cracks in a rock, if the water freezes and becomes ice, the ice expands and makes the crack wider
Glaciers are like frozen rivers that very slowly carve the rocks they pass over
Water running or washing over rock can also gradually wear away the rock, the rater of weathering depends on the type of rock the water flows over, the speed and the power of water, and what materials the water is carrying
Crystallisation of salts
Salts such as sodium chloride and gypsum form crystals as the water evaporates
Water in the soil often contains dissolved chemicals called salts
If the crystals from inside a rock, then their growth can exert tremendous force on the rock, causing pieces to break off
Chemical Weathering
Air contains oxygen and carbon dioxide, when these gases are dissolved in water they can react with minerals in the rock changing them
You often see red colours wherever dolerite rock weathers, this is because the iron reacts to oxygen to produce iron oxide
Water dissolving chemicals from the soil and air will react with rock, changing it or breaking it down, this is known as chemical weathering
Biological weathering
Plant roots can grow through fine cracks, or even create their own cracks, as the root grows and thickens, it splits the rock
Living things and their products can also break down rock, this is known biological weathering
Tree roots can be strong enough to seriously damage homes, paving and drivewas
Erosion
The small rock particles are moved elsewhere by water, wind, gravity or ice, known as agents of erosion
The power of erosion is strongest near the source of weathering, as small rocks move further away from the source, the power of erosion weakens(the force of erosive power decreases with distance
The small rock particles formed by weathering rarely stay near the rock they came from, instead they are moved elsewhere, this is called erosion
A river near its source can carry large and small pieces of rock, but once it reaches coastal plains it will carry only very small pieces such as sand and mud
Deposition
Deposition occurs where moving water, wind or ice that is carrying broken rock material slows down, in rivers this slowing down occurs at bends, and where rivers enter plains
The slower low of the river on the inside of the curve means that more sediment is deposited there, over time this causes a river to bend more
The settling of broken rock material is called deposition
When a river meets the sea, the water slows down, causing the deposition of most of its materials
Compaction and cementation
Minerals in the remaining groundwater cement the compacted sediment together to form sedimentary rock, this is how mudstone, sandstone and limestone are made
Layer upon layer of sediment builds up over time, and most of the water held between the small rocks and sand is squeezed out by weight from above, the water is progressively squeezed out and sediment is compacted
Melting and crystallisation
The rock is completely melted and recrystallised, then an igneous rock is made, its chemical composition often resembles that of the original source rock
If the rock is only partially melted and recrystallised then metamorphic rock is made, the rock often retains the same chemical composition of the original rock but also new minerals may form and these rocks often retain some traces of the original rock
Deep in the Earth, where intense hear and pressure occur, solid rock can melt, when it cools again, new crystals form, the rock's chemical composition often remains the same