The Geosphere

The Geosphere - SQA Key definitions: 

• Bauxite - An ore rich in aluminium oxide, which is found in extractable quantities in areas associated with subduction zones. 

• Chemical weathering - The destruction of rock caused by chemical interaction with carbonic acid in rainwater. 

• Constructive plate boundary - Occurs where convection currents in the upper mantle are diverging, causing the tectonic plates to move apart, with magma rising to fill the gap, creating new crust. 

• Convection - The process of heat transfer in a fluid (liquid or gas)  due to differences in density, in which warmer low-density material rises and cooler high-density material sinks.

• Convection currents - A circulation pattern of movement of a fluid, caused by convection. 

• Convergence - To move towards each other. 

• Core - The innermost layer of the Earth's structure, made of a liquid outer core and a solid inner core. Both the inner and outer cores are a mixture of nickel and iron.

• Crust - The outermost layer of the Earth's structure, which is made of solid rock.

• Destructive plate boundary - Occurs where convection currents in the upper mantle are converging, forcing the plates above to move towards each other. One plate will slide under the other into the mantle and undergo melting. 

• District heating scheme - A centralised energy network that uses geothermal heat extracted from the Earth to provide space heating and hot water to an entire community or city.

• Divergence - To move apart

• Earthquake magnitude - A measure of the total energy released at the source (hypocenter) of an earthquake, measured on a scale of 1-10. 

• Extrusive rock - Igneous rock that formed on the surface during volcanic action, cooling rapidly. 

• Fault - A fracture in the rock at the boundary of a plate.

• Fold mountains - Mountains formed by the convergence of two plates, causing the crust to crumple, buckle, and fold upwards.

• Frictional heat - Heat energy released when two materials slide past each other. One of the 3 sources of the Earth's internal heat. 

• Geothermal energy - Heat energy found below the surface of the Earth. 

• Geothermal gradient - The temperature difference between different depths at a location, due to the heating effect of the magma below. 

• Gravitational contraction - In the early stages of formation, the Earth contracted into a smaller volume, resulting in gravitational potential energy converting to heat energy.

• Gravitational potential energy - The energy stored within an object that has been raised in a gravitational field. 

• Greenhouse gas - A gas which traps radiated heat from the Earth's surface within the atmosphere, raising the average temperature of the Earth.

• Ground source heat pump - A heating and cooling system that uses the stable temperature of the ground near the surface as its heat source and heat sink.

• Hydrothermal vent - A fissure on the seafloor from which geothermally heated, nutrient-rich water discharges.

• Intrusive rock - Igneous rock which formed below the surface, cooling slowly. 

• Kinetic energy - The energy associated with movement. 

• Laterite - A soil that is rich in iron and aluminium, typically found in hot, wet tropical and subtropical regions.

• Lava - Molten rock on or above the Earth's surface.

• Leaching - The process by which soluble substances (like mineral salts or plant nutrients) are dissolved and carried downward through the soil by percolating water.

• Magma - Molten rock below the Earth's surface. 

• Mantle - The layer of the Earth's structure below the crust but above the core, consisting of hot, dense, iron and magnesium-rich molten rock known as magma. 

• Metallic mineral - An element or chemical compound formed as a result of rock cycle processes, containing a metal. 

• Ocean trench - A deep point on the ocean floor caused by the subduction of an oceanic plate below a continental plate. 

• Open-cast mining - A form of mining, the surface material (rock, vegetation, soil, etc.) is removed to expose the layers of ore below. 

• Ore - A rock that contains enough of a valuable metal or mineral that it can be extracted profitably. 

• Ore deposit - A location where an ore has accumulated. 

• Overburden -  The surface material and non-ore material that was removed to expose the underlying ore in open-cast mining. 

• Particulates -  Extremely small solid particles and liquid droplets that are suspended in a fluid (liquid or air). 

• Percolation - The process of a fluid moving (usually downwards) through a porous material.

• Plate boundary - The edge where two or more of the Earth's plates meet and interact. 

• Plate tectonics - The crust of the Earth is split into seven large rigid plates and several smaller plates. These plates are all in motion, caused by convection currents occurring in the underlying mantle.

• Radioactive decay - The disintegration of natural radioactive elements into smaller daughter elements, releasing heat energy. One of the 3 sources of the Earth's internal heat. 

• Rift Valley - Formed when two plates move apart. As the plates move away from each other, the stress on the rock increases until cracks start to form. These cracks spread, and the land between drops down, forming a rift valley. 

• Runoff -  The flow of water that occurs when rainwater, meltwater, or other sources flow over the Earth's surface, instead of percolating into the ground.

• Smelting - The process of using heat and chemical reactions to extract a raw, desired metal from its ore.

• Subduction zone -  A location where one tectonic plate sinks beneath another and into the Earth's mantle. It is a type of convergent plate boundary. 

• Superheating - A liquid remaining as a liquid beyond its boiling point due to intense pressure.

• Tailings - The fine, sludge-like residues from ore mining, which are stored in artificial lagoons.

The Geosphere

The geosphere consists of all of the Rocks and Minerals that make up the Earth. This incudes the material below the surface all the way down to the Earth's core, as well as surface rocks, mountains and beaches. The Geosphere also includes the abiotic parts of soil as well as the fossilised remains of organisms that died millions of years ago. 

There are several processes that form part of the Geosphere, with the processes collectively making up the 'Rock Cycle'.  

The Formation of the Earth

The Solar System started forming ~4.5 billion years ago from a huge cloud of dust and gas. The cloud collapsed under gravity until the temperature and pressure at the core were large enough for nuclear fusion, forming a star. Gravity causes the leftover dust and gas to collapse together to form the planets, including the Earth. Small rocks and dust were then drawn together by gravity to form moons around the planets. 

The image below is an artist's impression of the early Earth, a ball of molten rock, continuously bombarded by other small (and not so small!) objects : 

The Structure of the Earth

The Earth is an oblate spheroid (a sphere that is wider than it is tall) with a radius (distance from centre to surface) of approximately 6400 km. It is due to the Earth's rotation that it is not fully spherical in shape.

The Earth rotates once every 24 hours (approximately), giving the Earth its day and night cycle. The Earth orbits around the Sun once every 365.25 days, giving the length of one year:- 

As can be seen from the above simulation, the earth does not rotate vertically, its 'axis of rotation' is at an angle to the vertical, at 23.5 degrees to the vertical. It is this angle to the vertical that causes the Earth to have 'seasons':-

Below the Surface

The Earth's structure is split into three main sections:- 

The Crust - The outermost layer of the Earth's structure, which is made of solid rock, consisting mostly of basalt (the oceanic crust) and granite (the continental crust). The crust is the thinnest layer of the Earth's structure, with the continental crust about 40km thick and the oceanic crust only about 10km thick. 

The Mantle - The mantle is the largest layer of the Earth's structure (~2900 km thick), consisting of hot, dense, iron and magnesium-rich molten rock known as magma. The mantle acts like a viscous fluid, moving over geological time.  The movement of the mantle has caused the crust above to be broken into 'plates' which move over time in a process known as 'Plate Tectonics'.

The Core - The core is the innermost layer of the Earth's structure and is made of a liquid outer core and a solid inner core. Both the inner and outer core are mostly a mixture of nickel and iron, and are responsible for generating the Earth's magnetic field.

Scientists cannot directly sample the inner structure of the Earth. The deepest hole ever drilled only reached a depth of 12,262 meters, less than half of the thickness of the continental crust. 

Scientists understand the structure of the Earth by analysing seismic waves generated by earthquakes. As the seismic waves pass through the Earth, they are refracted or blocked by the changing density of material. By analysing the changes in the waves detected across the Earth, the structure of the Earth can be modelled:-

Plate Tectonics

Plate tectonics is the theory explaining the motion of the Earth's crust over geological time. Due to forces provided by gravity and the movement of the mantle below, the crust of the Earth is continually in motion, with plates moving apart, together or past each other. 

The diagram below shows the major plates that make up the Earth's crust, as well as their relative motion:-

This motion is caused by the movement of the molten mantle below the Earth's crust. The Mantle of the earth remains molten due to the internal heat of the Earth. This internal heat comes from three key sources : 

• Extraterrestrial impacts : During Earth’s formation the kinetic energy contained in colliding extra-terrestrial bodies was converted to heat energy upon impact.

• Gravitational contraction : In the early stages of formation, the Earth contracted into a smaller volume, resulting in gravitational potential energy converting to heat energy. At the same time, frictional heat was generated by denser iron and nickel-rich material sinking to the core.

• The decay of radioactive elements : Disintegration of natural radioactive elements in the mantle and crust generates heat energy. 

Intense geological activity occurs at plate boundaries (the edge where two or more of the Earth's plates meet and interact), where plates move away from each other, past one another, or towards each other.

The motion of the Plates is caused by rising convection currents within the Mantle pushing upwards on the crust in some places, and providing lateral (sideways) forces in other places : 

The video below shows how the movement of the Plates has affected the landforms of the Earth over the last 1.5 billion years:-

Constructive Plate Boundaries

Constructive plate boundaries occur where convection currents in the upper mantle are diverging, causing the tectonic plates to move apart, with magma rising to fill the gap, creating new crust.

What type of crust forms depends on the location of the plate boundary : 

• Continental Crust - Rift Valley formation 

• Oceanic Crust - Oceanic Ridge formation

Constructive Plate Boundaries : Continental Crust

Rift valleys form when two plates move apart underneath continental crust. As the plates move away from each other, the stress on the rock increases until cracks start to form. These cracks spread, and the land between drops down, forming a rift valley : 

Rift valleys form when two plates diverge (move apart) underneath continental crust. As the plates move away from each other, the stress on the rock increases until cracks start to form. These cracks spread, and the land between drops down, forming a rift valley. 

An example of a rift valley can be seen in East Africa, where the Nubian and Somalian sections of the African Plate are moving apart, creating the 'Great Rift Valley' of East Africa : 

Constructive Plate Boundaries : Oceanic Crust

Oceanic ridges form when two plates diverge (move apart) underneath oceanic crust. As the plates move apart, the underlying mantle melts, forming magma. The magma rises upwards, erupting through the crust and filling the gap between the plates, creating new oceanic crust. Due to the slope of the ridge, the older material gets dragged downhill due to gravity, pushing the surrounding crust away; this is known as 'Ridge Push'. 

Due to the rising magma and the gaps in the crust of the ridge, a chain of underwater volcanoes is created along the spreading plate margins. 

An example of this is the 'Mid-Atlantic Ridge' between North America and Europe. This oceanic ridge runs from the Arctic Ocean (87° North) to just off Bouvet Island in the Southern Atlantic Ocean (54° South).

Most volcanoes on the Mid-Atlantic Ridge are underwater, due to the depth of the water. These continually release molten rock, creating new seafloor and increasing the separation of North America and Europe by approximately 2.5 cm per year. 

Due to the volcanic activity around the Mid-Atlantic Ridge, Hydrothermal vents also form. Deep faults form when the oceanic plates move apart. Cold seawater percolates (to move through a porous material) down through the faults and is superheated (water that is still liquid at temperatures above 100°C due to the intense pressure) through contact with the magma, before returning to the seafloor via hydrothermal vents. The superheated fluids often contain dissolved metallic minerals (compounds containing metals). As the fluids make contact with the seawater and cool, the dissolved minerals are deposited on the sea floor. The minerals often accumulate in substantial volumes, which can potentially be exploited. 

The exact minerals that are deposited depend on the 'type' of hydrothermal vent - 'Black Smokers' and 'White Smokers'.  The table below shows the difference between the two types : 

The map below shows the sites of Hydrothermal vents along the Mid-Atlantic Ridge :

Hydrothermal Vents are also support unique ecosystems. These ecosystems are not based on Photosynthesis, as light cannot reach these depths, but instead are based on Chemosynthesis. Autotrophs (Bacteria in this case) use the chemicals (mainly hydrogen sulphide) within the superheated fluid to produce their own food, forming the base of the food webs around the hydrothermal vents : 

Destructive Plate Boundaries : Subduction Zones

Subduction zones occur where continental plate and oceanic plates converge (move towards each other). A subduction zone is a location where one tectonic plate sinks beneath another and into the Earth's mantle. The oceanic plate moves below the continental plate because it is denser, also carrying down oceanic sediments and seawater in a process known as 'Slab Pull'.

Destructive Plate Boundaries : Ocean Trenches

As the oceanic plate moves below the continental plate, a deep ocean trench forms at the junction. These trenches form the deepest parts of the oceans, with the deepest point on Earth found within the Mariana Trench, off Guam in the Pacific Ocean. 

The deepest point of the Mariana Trench is known as the 'Challenger Deep', with a depth of 10,935 meters. 

Despite the crushing pressure and total darkness at this depth, life still exists. Organisms have been found in the lowest depths of the Challenger Deep, including giant, single-celled amoebas, small, shrimp-like crustaceans and sea cucumbers.

Destructive Plate Boundaries : Fold Mountains & Volcanic Action

As the oceanic crust moves lower, the heat of the mantle causes the rock to melt, forming magma. The magma generated from subduction zones has high levels of gas present and is very explosive. 

This molten material forces its way up through faults to the surface of the continental plate, erupting as a volcano. 

Molten material reaching the surface is known as lava and cools to form extrusive rock. Magma that cools and solidifies before reaching the surface forms intrusive rock. The different types form when the rock cools at different speeds. The faster the rock cools, the smaller the 'grains' within the rock : 

The magma at a destructive plate boundary carries concentrated metallic minerals into the upper crust. Significant metallic mineral deposits are often found near ancient plate boundaries. For example, Copper, Gold and Silver deposits are overwhelmingly found in volcanic regions above subducting plates, e.g., the massive Silver and Gold deposits of the Andes mountains of South America. 

The largest silver mine in the Andes, historically, is Cerro Rico in Potosí, Bolivia. Discovered in 1545, it was the world's largest single source of silver at its peak and financed the Spanish Empire. While now also mined for zinc and tin, the mountain is still a source of silver, and the region remains one of the world's largest silver deposit systems. Over its nearly 500 years of mining, this single mine is estimated to have produced 60,000 metric tons of Silver, a volume that would be worth approximately $101 billion USD today. 

 Subduction also causes crumpling along the edge of the continental plate, forming fold mountain chains. It is this process that has raised the highest mountain range on Earth - the Himalayas. The Himalayas are located in Asia between the Indian subcontinent and the Tibetan Plateau. 

They formed due to the collision of two continental tectonic plates: the Indian Plate and the Eurasian Plate. Over geological time, the Indian Plate moved North, colliding with the Eurasian Plate, a collision that is still continuing today :

As both plates consist of continental crust, neither could easily be subducted beneath the other. Instead, the immense compressive forces caused the Earth's crust in the collision zone to crumple, buckle, and fold upwards, creating the the Himalayan mountains. 

The image below shows some of the major landmarks of the Everest massif : 

Destructive Plate Boundaries : Earthquakes

The forces acting between plates at a destructive boundary can cause major earthquakes. Earthquakes at this type of boundary can be of high magnitude due to the release of frictional pressure that builds up between the two plates as they move relative to each other. As the plates converge, strain builds up in the fault (a fracture in the rock at the boundary of the plate). When the stress exceeds the fault's strength, it ruptures, causing the overriding plate to thrust upward and seaward suddenly :

This sudden movement sends seismic waves through the surrounding crust - an earthquake. The upward movement can also trigger tsunami waves in the ocean due to the mass movement of water caused by the seabed shift. 

Some of the most powerful earthquakes ever recorded have occurred along destructive plate boundaries. Earthquake strength is measured using a magnitude scale, a measure of the total energy released at the source (hypocenter) of an earthquake. 

Earthquake Magnitude is a 'logarithmic' scale - each number increase on the scale is a 10 x increase in amplitude of the seismic waves.  This corresponds to a ~32x increase in energy, and therefore, potential damage caused. 

Aluminium : Bauxite Ore Formation

The rocks that make up the Earth's crust contain many valuable metals and minerals that can be used to create a wide range of materials. However, for these to be used, they first must be separated from the rest of the rock itself. 

If a rock contains enough of a valuable metal or mineral that it can be extracted to make a profit, then that rock is known as an 'Ore'. 

Some examples of ores are shown below:-

Bauxite is an ore rich in aluminium oxide and is found in extractable quantities in areas associated with subduction zones. 

Bauxite forms in tropical areas around the equator, where high temperatures and abundant rainfall result in extreme chemical weathering of rock. As water percolates through the soil in the hot, humid climate, nutrients dissolve and leach (carried in solution)  downwards. 

The soil that remains, known as laterite, can be rich in aluminium oxides and form rich ore deposits near the surface. 

The diagram below shows the layered structure of laterite soil : 

Aluminium : Bauxite Ore Mining

The majority of the world's bauxite production is from surface mines, extracted by open-cast mining. In 'open-cast' mining, the surface material (rock, vegetation, soil, etc.) is removed by giant excavators to expose the layers of ore below. The ore is then extracted by blasting (explosives used to break up the rock) and then removed and transported to a processing site. 

Once the mining is completed, the surface material and non-ore material (known as 'overburden') are returned to the site and landscaped to repair as much of the site as possible. 

Open-cast mining has several associated environmental issues. These include :

Environmental Impact of Mining : Particulates

The dust that can be released during the mining process is classed as a form of 'particulate' pollution. Particulates are extremely small solid particles and liquid droplets that are suspended in a fluid (liquid or air). These particulates are hazardous to human health, and the control of emissions of these is legislated in mining in Scotland. Particulates are classified in two forms, based on their size : 

• PM10 (Coarse dust) - Particles that are between 10 and 2.5 micrometres in size. These particulates irritate the eyes, nose, and throat, worsening conditions like asthma.

• PM2.5 (Fine dust) - Particles that are smaller than 2.5 micrometres in size. This particulate matter is so small can penetrate deep into the alveoli of the lungs and enter the bloodstream. These particulates can cause the short-term health effects of the bigger particulates, such as eye, nose, throat and lung irritation, but also affect long-term lung function and worsen medical conditions such as asthma and cardiovascular disease.

Mines can reduce the impact of dust be employing methods to limit its production, or its spread, mostly through wetting the materials to prevent dust from becoming airborne : 

Aluminium : Bauxite Ore Processing

Once the Bauxite has been mined and transported to a processing facility, the aluminium can then be extracted from the ore. In order to extract the aluminium from its ore, it undergoes a two-stage process : 

1. Refined into 'Alumina' - a highly purified form of aluminium oxide.

2. Electroysis to produce Aluminium - Separating the oxygen from the Alumina to produce aluminium. 

Aluminium so reactive that even heating with a reduction agent such as carbon cannot extract it from the Alumina. In order to extract aluminium from the Alumina, electricty must be used. This process is known as Electrolysis :

The carbon electrodes apply an electrical current through the molten alumina & cryolite mixture. Cryolite is added to reduce the melting point of the alumina, making the process more energy efficient. 

The Aluminium is attracted to the negative eletrode and sinks to the bottom of the chamber, whilst the oxygen is attracted to the positive electrodes and rises to the surface, where it is extracted through the waste gas pipe. 

The molten aluminium is then tapped off from the base and taken for further processing into a range of products. 

Bauxite processing into pure Aluminium provides the raw materials for a huge range of applications, including : 

Bauxite ore processing has several associated environmental issues. These include :

Geothermal Energy

Geothermal power uses the heat Energy contained within hot rocks deep underground to generate Electricity. 

The diagram below shows a simplified diagram of a Geothermal power station : 

A Geothermal power station uses heat from deep underground to boil water into steam. Energy in the thermal store of the rocks can be obtained by drilling shafts into the Earth's surface to expose the heated rocks. Water is then injected into the shaft, which is heated by the rocks. The water is returned via another shaft as steam. This steam is then used to drive a turbine and generator in the same way that a fossil fuel power station does. 

The energy changes in a Geothermal power station are:-

1. Geothermal Rocks -  Heat Energy.

2. Steam driven Turbine - Heat Energy converted to Kinetic Energy.

3. Generator - Kinetic Energy converted to Electrical Energy.

Geothermal energy : Geothermal Gradient

Geothermal energy is usually limited to geologically active areas (such as Iceland) as they require a steep 'Geothermal Gradient' in order to be cost-effective. The geothermal gradient is the temperature difference between different depths at a location, due to the heating effect of the magma below. 

In places where the magma is deep below the surface (such as away from plate boundaries), the geothermal gradient is shallow, changing by only 10-20°C per km. 

But in geologically active locations, the geothermal gradient will be much steeper, changing by 50-100°C per km. In Iceland, as the magma lies very close to the surface, the geothermal gradient is very steep, with the steepest gradient globally, found on Iceland's Reykjanes Peninsula, at approximately 250°C per km. 

Geothermal energy can also be used to provide heating through 'District Heating' schemes. A district heating scheme is a centralised energy network that uses geothermal heat extracted from the Earth to provide space heating and hot water to an entire community or city. Instead of each building having its own furnace or boiler, the heat is generated centrally and distributed efficiently through a network of highly insulated underground pipes.

In a addition to district heating schemes, ground source heat pumps can also use to heat homes using heat from the Earth. 

Unlike the surface, where temperature changes widely over the course of a year, below the ground the temperature remains at a constant stable value of about 12°C. 

Through the use of a heat pump, water pumped down into underground pipes can access this heat during the winter, and then the warmed water can be used to heat the home : 

In the summer, the cooler underground temperatures can be used to cool water, reducing the heat in the home :