Doc Brown's Chemistry Clinic

Revision Notes Answers to the ATMOSPHERIC and GEOLOGICAL CHANGES on EARTH worksheet

for KS4 Science GCSE-IGCSE Chemistry

based on a GCSE TASK SHEET * Earth Science Homepage

5 multi-word fill GCSE worksheets + answers * EMAIL query?comment

GCSE Earth Science: Foundation-easier m/c Quiz and Higher-harder level m/c Quiz

ANSWERS 1. The Evolution of the Earth's atmosphere and Carbon Cycle ... 2. The Rock Cycle and types of rock (details 'evolve' through sections 3. to 9.) ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Structure of the Earth ... 8. Plates and their movement ... 9. Plate Tectonics ... 10. The Moon and Planets

1. The Evolution of the Earth's atmosphere breathe easy!

1(a)(i) Today's atmosphere consists of the elements: 78% nitrogen (about ⁴/₅ths), 21% oxygen (about ¹/₅th), 1% argon (¹/₁₀₀th), traces of other Group 0 Noble Gases (He, Ne, Kr, Xe), plus the compounds 0.036% carbon dioxide (360 ppm or parts per million) and variable amounts of water vapour (depends on humidity) and traces of many other gases from natural or man-made pollution sources (e.g. sulphur dioxide, nitrogen dioxide and carbon monoxide from fossil fuel combustion (see Oil Products notes) and methane (greenhouse gas) from cows and decomposing plant material). The composition of our atmosphere is thought to be relatively unchanged for about 200 million years due to the Carbon-Cycle balance.

1(a)(ii) One method to determine the % of oxygen in air is to use two 100cm³ glass gas syringes are connected either side of a piece of silica tubing containing copper powder or fine granules. One syringe is empty and the other filled with 100cm³ of air. The silica tube/copper is strongly heated and the gas syringes moved to and fro to pass the air over the hot copper. The oxygen in the air reacts with the copper to form copper(II) oxide. 2Cu_(s) + O_2(g) ==> 2CuO_(s) This is a black solid of little volume. Eventually the total volume reading reaches a minimum value when all the oxygen in the air has reacted. 100 - final reading gives the % oxygen in air.

1(a)(iii) Removal of carbon dioxide (CO₂): Photosynthesis* in green land plants absorbing carbon dioxide to form biomass (and oxygen), then some plant biomass is converted to animal biomass. Some of the CO₂ will dissolve in the seas/oceans => where it may be further changed in photosynthesising marine organisms to produce biomass, forming soluble carbonates and insoluble minerals e.g. calcium carbonate (sedimentary limestone rock) as the shelly remains of creatures and coral etc., decay (without oxygen) of any organic material from dead plant and animal remains to form the fossil fuels coal, oil and gas over millions of years. * Photosynthesis:

water + carbon dioxide + sunlight energy ==> glucose sugar + oxygen 

6H₂O_(l) + 6CO_2(g) ==> C₆H₁₂O_6(aq) + 6O_2(g)

1(a)(iv) Production of carbon dioxide: Natural burning of biomass like forests, plant and animal respiration**, biological decay of plant and animal material, 'mans' burning/combustion of fossil fuel, volcanic activity e.g. the thermal decomposition of minerals like carbonates in magma/lava. See Oil notes for more on fossil fuels. ** Respiration:

glucose sugar + oxygen ==> water + carbon dioxide + energy (exothermic, energy given out)

C₆H₁₂O_6(aq) + 6O_2(g) ==> 6H₂O_(l) + 6CO_2(g)

1(a)(v) Oxygen balance: The main process of CO₂ removal by photosynthesis also produces oxygen. Respiration and combustion (natural/man) mainly remove the oxygen from our atmosphere. So this means the Carbon-Cycle effectively maintains a constant % of oxygen in the atmosphere as well as controlling the carbon dioxide levels. (Note: as far as we know the Greenhouse Effect will not significantly change the oxygen level in the Earth's atmosphere). There is no evidence to suggest that the increase in the world's population (respiration!)  or the burning of forests (deforestation by combustion) is having any effect on the oxygen level BUT increase in 'man's' industrial and domestic activity by burning fossil fuels is causing the carbon dioxide concentration to rise.

1(a)(vi) Global warming, temperature and CO₂ imbalance: The average temperature of the Earth depends on the net effect of the Sun's input and the Earth's output of energy [mainly by heat/infrared (ir) radiation]. However, the relatively rapid rate of burning massive amounts of fossil fuels over the last few hundred years is threatening this and the CO₂ balance. The CO₂ in the atmosphere absorbs some of the re-radiated ir to keep the Earth warm and a constant CO₂ concentration, also means a steady temperature. The increasing CO₂ levels means more ir is absorbed and the global temperatures are rising - the Greenhouse Effect. This global warming is predicted (maybe happening?) to: affect sea levels by melting glaciers, change in weather patters e.g. more drought in Africa, more rain and storms in other parts of the world, forcing change in agriculture management with weather/temperature changes etc. etc. but its all a bit uncertain!

1(b) The  early Earth atmosphere consisted of mainly carbon dioxide, water vapour and small amounts of ammonia and methane from intense volcanic activity (mainly in the first billion years!). There would be little or no oxygen (rather like on Mars or Venus today). Some texts also refer to small amounts of hydrogen, nitrogen, carbon monoxide and sulphur dioxide. Note: We can't be absolutely sure, but study of the atmospheres of other planets are helping in understanding of the Earth's early atmosphere, and all of these gases exist on one or more of the planets. Much of the hydrogen and helium originally present would have been 'boiled' off as the Earth's gravity would not be strong enough to hold these fast moving molecules at high temperatures!

1(c)(i) Seas and oceans would form from condensed water vapour as the early Earth surface cooled down. Carbon dioxide and ammonia would dissolve in this water.

1(c)(ii) The carbon dioxide could form soluble sodium carbonate, sodium hydrogencarbonate or calcium hydrogencarbonate and insoluble calcium carbonate.

1(c)(iii) The seas and oceans contain large quantities of dissolved salts which were once part of rock formations, now weathered, eroded and washed away in rivers. These salts do NOT evaporate, unlike the water, so the oceans have gradually  become more concentrated in salts. Much later some salts are removed as shells of marine organisms, some chemical reactions produce precipitates which form part of the sea-floor sediments, and crystallisation to form salt deposits e.g. in high concentration warm parts of the world like the Dead Sea and enclosed seas/lakes may completely dry up to give 'rock salt' and 'potash' sedimentary rock formations.

1(d) Primitive bacterial life evolved about 3500 million years ago (3.5 billion y) and the first green algae like plants from about 2000 million years ago (2.0 billion y). The increasingly successful evolution of green photosynthesizing plants colonising both land and water, produced an increasingly oxygen richer atmosphere and in so doing removing most of the carbon dioxide from the original early atmosphere. This 'oxygenated' atmosphere would be 'polluting' and 'toxic' to many microorganisms which could not tolerate oxygen, having evolved in a non-oxygen environment. However, by 1000 million (1 billion years) years ago, there was sufficient oxygen to allow the evolution of respiring animal life.

1(e)(i) Ammonia would be converted to soluble nitrates mainly by nitrifying bacteria or, to a small extent, ammonia would be directly oxidised to nitrogen gas by the newly formed oxygen. The nitrates are absorbed by plants to form proteins or converted to atmospheric nitrogen by denitrifying bacteria.

1(e)(ii) Methane would be oxidised to carbon dioxide and water by the new 'oxygenated' atmosphere.

1(e)(iii) Ozone (O₃) would now be formed and this would absorb and filter out much of the ultraviolet light that is harmful to many organisms. This uv filtering would then allow the much wider evolutionary development of plant and animal organisms.

2. The Rock Cycle The "1st Big Picture View"

Many of the features of the rock cycle are illustrated below in Fig 2.1 The Rock cycle. (see also Fig 8.1 and Fig 9.2)

Fig 2.1

Sections 3. to 9. take you through all the details.

Rocks are classified into groups of IGNEOUS, METAMORPHIC and SEDIMENTARY ROCKS depending on their origin.

• All rocks have crystalline structure based on giant ionic or covalent structures (Chemical Bonding Notes)
• Physically they are relatively insoluble in water, poor heat and electrical conductors and have high melting points. They are generally hard materials but with quite some variation between rock types.
• Generally speaking, igneous rocks like granite and basalt are much harder and 'weather resistant' than sedimentary rocks like limestone, shale and sandstone which erode much more easily. Rock type details are given in 4. to 6. below.
• The three generalised type of rock are discussed in detail in sections 4. to 6.
  • Section 4. Igneous rocks are formed from cooled molten magma or lava from the Earth's mantle below the crust.
  • Section 5. Sedimentary rocks are formed deep in the Earth's crust from highly compressed deposits of weathered rock material or mineral deposits from plants and animals.
  • Section 6. Metamorphic rocks are formed from re-existing sedimentary or igneous rocks by the action of heat and pressure deep underground in the Earth's crust.

3. Weathering of Rocks they all wear away eventually!

(3a) Weathering of Rocks means the process of breaking them up into smaller fragments and it can occur in many different ways.

• (i) physical weathering e.g.
  • Liquid water from rain or melted snow/ice runs into rock cracks. When the temperature falls below 0^oC, the water freezes and there is an expansion in volume in changing from water to ice. The resulting ice pressure cracks the rocks apart and the process can be repeated if the ice melts/thaws and re-freezes etc. So, many mountain sides have a very shattered appearance!
  • The continuous battering of rock surfaces with dust particles carried by the wind,
  • River water carrying rocks and battering into other rocks, which is why down a river the rocks/pebbles tend to become smaller and rounder.
  •  Sea waves crashing on the seashore and against cliffs etc.
  • Layers of rocks flaking off when larger rocks expand and contract with extreme temperature changes.
  • The action of glaciers grinds rock material off the land and sides of valleys with tremendous power.
• (ii) chemical weathering (a sort of 'corrosion' of rocks) e.g.
  • Acid rain water will gradually break up even igneous rocks by slow chemical reactions. Some of the minerals dissolve and free up particles of the insoluble material. Even hard igneous rocks get weathered away eventually over millions of years to form sand grains. Rain is 'naturally' acid from - dissolved carbon dioxide from respiration and forest fires, nitrogen oxides from lightning flashes and sulphur dioxide from volcanoes. Pollution increases the acidity of air with extra nitrogen and sulphur oxides from fossil fuel burning.
  • Water running off from decayed and oxidised plant material is acidic e.g. peat water has a pH of 3.5.
  • Limestone rock (calcium carbonate) chemically dissolves away much quicker than most other rocks, even with just carbon dioxide in the water,
    • e.g. calcium carbonate + carbon dioxide + water ==> calcium hydrogencarbonate
    • CaCO_3(s) + CO_2(aq) + H₂O_(l) ==> Ca(HCO₃)_2(aq)
    • and the process is much faster in polluted acid rain, hence the rather worn appearance of medieval buildings in industrialised Europe made from the 'softer' sedimentary rocks limestone or sandstone.
• (iii) biological weathering e.g. the action and pressure of growing plant roots expanding in the cracks of rocks.

(3b) Erosion is the wearing away of the rock as a result of the weathering processes described above. Examples of erosion are the wearing away of mountains and the creation of river valleys and gorges.

• Transportation is the process by which the eroded weathered rock fragments are moved away from the erosion area.
  • This happens mainly due to 'falling under gravity' and then the rocks or fragments carried away by stream and river water as well as sand by wind.
  • There are powerful currents in the sea which transport huge masses of eroded material.
  • You find in rivers that as you go from high mountains to an estuary, the 'rocks' become smaller and more rounded the further they have travelled due to the constant collisions in the water chipping of the edges.
  • Glaciers also carry considerable eroded material away, particularly in the 'ice ages'
• Deposition will eventually occur giving rise to sediments or sand dunes in river delta's sea etc. The smaller (lighter) the particles, and the faster the current, the further they are carried. This means most deposition will occur in a slow moving but distant location e.g. fine silt deposits in estuaries.

4. Igneous Rocks form the fastest, and last the longest!

4(a) Igneous rocks are formed from hotter less dense (than surrounding rock) molten rock called magma, welling up and pouring out from the mantle and sometimes from re-melted crust (see 6. and 7.)  The rising 'plumes' of magma break through the crust from volcanoes and mid-ocean ridges (see Fig 6.1 and Fig 8.1) and cooling to solidify to form igneous rocks. Sometimes the magma does not break through the surface and cools within the crust (see igneous intrusion below). Most igneous rocks consist of interlocking crystals from cooled magma and are physically hard and relatively dense and do not erode easily.

Note (i): There is quite a variety of mode of formation though e.g. some volcanic rocks are very hard and 'glassy', others form from ash deposits from volcanic eruptions. They sometimes occur as intrusions into other pre-existing rocks (see below) and the crystal size and type of igneous rock also depends on the rate of cooling.

Note (ii): You often see the lava bubbling as dissolved gasses under pressure in the mantle are released into the atmosphere - sometimes with explosive force!

4(b) The igneous rock granite is formed by the slow cooling of magma in the crust or perhaps inside a volcano after it stops erupting and the top becomes plugged. It is called an intrusive rock because it is formed 'inside' the crust and not on the crust surface. The crystals are relatively large due to slow cooling and 'speckled' as different minerals of different colours crystallise out within the rock structure. Granite tends to be lighter in colour than basalt (see 4(d) below). Granite type rocks are sometimes called course-grained rocks because of the mixture of interlocking larger crystals.

Fig 4.1 An igneous Intrusion

4(c)(i) An igneous intrusion is where a mass of very hot 'plastic' magma from the mantle rises and 'bulges' up into the crust and cools to form igneous rock. This is often granite because it will cool very slowly as the surrounding rocks act as an insulator. The intrusion may 'push' up through many layers of previously formed sedimentary rock (see section 5. and section 6.).

4(c)(ii) If these sedimentary rocks are then weathered away, the harder wearing granite remains as a hill or mountain.

4(c)(iii) The igneous intrusion rock must be younger than the surrounding sedimentary rock because it is formed by the magma cooling in the previously existing rock layers.

4(d) The igneous rock basalt is formed much more quickly than granite and in several locations e.g.

• molten lava from undersea volcanoes and mid-ocean ridges is rapidly quenched by the cold water.
• molten lava pouring out of volcanoes onto land and cooled by air (or pouring out from land into water).

Basalt is described as an extrusive rock because it 'extrudes' out into air or water to cool and form the solidified rock. It is formed by the fast cooling of magma and the crystals are relatively small because of the fast cooling. It consists of interlocked microscopic crystals which are darker in appearance compared to granite. This situation is found when lava/magma cools rapidly when flowing out into air or water. Basalt rocks are sometimes called fine-grained rocks because of the mixture of interlocking tiny crystals.

5. Sedimentary Rocks slow to form, and weather the fastest!

5(a) A sedimentary rock bed is formed from plant/animal remains or weathered and eroded particles from pre-existing rocks. These may be transported, usually by water (and wind in the case of sand) and deposited to form sediments. These become buried under later forming sediments and water or by major tectonic activity, and then become subjected to compression as enormous pressures are created deep in the crust. For those from eroded pre-existing rocks, water is squeezed out and the particles cement together with the help of dissolved salts and silica crystallising out.  Other changes come about depending on the type of material from which the sedimentary rock is formed.

 5(b) Types of sedimentary rock

• Shale and Mudstone is formed from relatively fine grained weathered rock material transported into seas and lakes before settling out as clay or  mud sediment. It then becomes compressed under the weight of water and other sediments and the water is squeezed out and the particles cement together. These rocks are clearly layered and crumble easily. Shale can contain significant amounts of oil-like organic material.
• Limestone is formed from the deposition of hard mineral remains of sea creatures and chemically is mainly calcium carbonate CaCO₃. It contains fossils and sand grains. The 'shelly' remains, including coral, get buried and compressed and cemented together by the weight of water and other sediments. Limestone tends to form beneath warm shallow seas rich in plant and animal life.
• Chalk is formed from the mineral remains of tiny marine organisms in the sea and is chemically relatively pure calcium carbonate and it contains microscopic fossils readily seen under a microscope.
• Sandstone is formed from weathered particles of igneous rock and these particles mainly consist of colourless silica (silicon dioxide, SiO₂). The rock particles are laid down in lakes, estuaries or seas from water transportation or wind blown to form sand dunes. The layers of sand get buried and compressed and the particles get cemented together by other minerals including iron oxides which give sandstone its distinctive orange or red colour.
• Coal is formed from the decayed (without oxygen) remains of plant materials e.g. giant ferns and trees from hot swampy forests. The organic materials are buried, compressed and form clear sedimentary layers often showing well preserved fossils of leaves or tree trunks. The deeper and older the layer, the more carbonised is the coal (anthracite is almost completely the element carbon).
• Salt deposits : These are formed from the evaporation of ancient seas or lakes and become buried and compressed underground e.g.
  • Rock salt is mainly sodium chloride. It can be mined as a solid or extracted as a concentrated solution. It is used for food preparation, de-icing  roads or to make chlorine etc. via the process of electrolysis.
  • Potash contains potassium chloride, sodium chloride and magnesium sulphate and used in fertiliser manufacture.

5(c) Since limestone is mainly calcium carbonate CaCO₃, and a simple test is to add acid - should giving fizzing of a colourless gas that turns limewater 'milky' i.e. carbon dioxide CO₂ is formed. Heating limestone to a high temperature in a limekiln produces calcium oxide (quicklime, a strong alkali). Lime is used in agriculture to treat fields which are too acidic for healthy crop growth. Limestone is used as building stone and in the manufacture of glass and concrete.

5(d) Any rocks which are not eroded away, are eventually returned to the mantle when plates descend in tectonic activity - see later. 

5(e) A potted history of fossils ...

• Fossils are formed by plants and animals becoming trapped in deposits or sediments. In most cases the original organic material is replaced by other minerals but this leaves the trace and structure of the original plant or animal.
• In undisturbed sedimentary layers the lower the layer the older the layer, so the geological sequence of formation can be worked out.
• Fossils allow us to date the age of the rocks from the species present and also the sort of 'environment' present at the time of fossil formation e.g. the climate and the nature of the land. The older the fossil, the older the rock! Note: Fossil dating is NOT absolute and accurate dating can only be obtained from radioisotope studies.
• The fossil record provides powerful evidence for species evolution as the development of individual species can be followed and their divergence into other later species.
• Fossils 'emerge' when the sedimentary rocks in which they lie in are eroded away. The original harder parts of the organism tend to be better preserved e.g. shell, bone, coral or bark etc. They then require careful extraction from the surrounding rock or mud material.

5(f) 

• You would not expect fossils in igneous rocks because they are formed from molten mixed up magma. Even if a sedimentary rock had fossils in it, they would be destroyed if the rock was re-melted e.g. in a subduction zone - see plate tectonics later.
• Fossils are rare in metamorphic rock but their trace can sometimes be preserved in e.g. slate, despite the effects of heat and pressure involved in their formation (see 6.). It is not impossible for the 'traces' of fossils in sedimentary rock to be preserved through the re-crystallisation process. However the fossils are likely to be distorted or destroyed by the heat and pressure factors involved in metamorphic rock formation.

5(g) At the surface of the Earth younger sedimentary rocks usually lie on the top of older rocks. All sorts of features found in sedimentary rock formations allow scientists to work out their origin and what has happened to them over long time periods of time e.g.

1. order of layers - the deeper the layer, the earlier the sedimentary rock was formed
2. discontinuous deposition where different layers of different rocks are successively laid down at different times. (see Fig 9.1)
3. a more recent (younger) rock layer might cut across an older layer.
4. ripple marks can show the layer was formed from a sea-bed or river bank from waves or currents.
5. tilting of rock formations can show very large scale movement and the angle can be followed over a large distance to show the relationship between distant rock formations.
6. folding shows the compression of layers due to plate movement, a curve down is called a syncline, a curve in an arc upwards is called an anticline. (see Fig 9.1)
7. fractures and fault lines provide evidence of earthquake activity. (see Fig 9.1)
8. inverted layers (turned upside down!) provide evidence of massive plate movement and give geologists much food for thought on deducing the 'event sequence'!
9. rock layers can be buried by these massive upheavals as well as burial by subsequent sedimentary rock formation.
10. Points 5. to 9. are evidence for the crust being unstable and subjected to tremendous forces.

6. Metamorphic Rocks formed through the action of heat and pressure!

6(a) A metamorphic rock is one that is formed directly from a pre-existing rock.  Heat and pressure are the 'driving forces' for metamorphic rock formation in which the grains of pre-existing rocks are re-crystallised. The pre-existing rocks involved are usually deep in the crust where they are subjected to great pressure. The high temperatures often needed, are due to rocks being near the hot mantle, or when an igneous intrusion rises, or volcanic rock heats other surrounding rock and when continental plate meets oceanic plate (see (3) in Fig 8.1).

6(b) The link between metamorphic rocks and igneous intrusions is shown on the left and in diagram Fig 4.1. The rising magma heats up the surrounding sedimentary (or igneous) rocks producing metamorphic rocks such as marble, slate, gneiss or schist. Note: There are high-low grades of metamorphism depending on the high-low temperatures and pressures particular pre-existing rocks are subjected to. For example, the rocks become 'less metamorphic' the further you go from the igneous intrusion as you go to a lower temperature.

6(c) Slate is formed from sedimentary rocks such as shale, mudstone or clay deposits and the re-crystallised layers are easily split - hence its use in roofing. Sometimes, but rarely, fossil traces are preserved in the layers through the crystallisation process.

6(d) Marble is a hard rock formed from the action of heat and pressure on the sedimentary rock limestone. It will still give carbon dioxide with acid but is much harder physically than limestone or chalk.

6(e) Gneiss, quartzite  and schist are metamorphic rocks formed by the action of heat and pressure on pre-existing igneous or sedimentary rocks. They can form from igneous rocks* like granite or basalt, from metamorphic rocks* like slate or from sedimentary rocks like shale, mudstone or sandstone, and chemically they are mainly 'silica' SiO₂. * Note, the original pre-existing rock does not have to be sedimentary!

• NOTE: The terms ....
• Regional Metamorphism refers to large scale metamorphic rock regions associated with mountain building from tectonic activity. (see Fig 8.1)
• Contact Metamorphism refers to localised metamorphic rock formation around an igneous intrusion. (see Figs 4.1, 6.1, 6.2)

6(f) Metamorphic rock has the same chemical composition as the original rock it was formed from (in terms of % elements). This is because no minerals are added or lost in the recrystallisation process. For example, the Ca:C:O ratio is the same in the sedimentary limestone rock as it is in the resulting metamorphic rock marble, because chemically they are both mainly calcium carbonate CaCO₃.

7. The Structure of the Earth A sort of egg?

7(a-b) The three layered structure of the Earth.

X is the crust: is the relatively thin and cool outer layer of the Earth. The thickness ranges from 6 to 40km. It is much cooler, harder, brittle and less dense than the other layers. The crust is divided into sections or 'plates' which 'float' and move on the mantle. 2/3rds of the surface is water.

Y is the mantle: is very hot rock material, it is almost solid but the 'plastic' rock can move very slowly as huge convection currents driven by the heat from radioactive decay in the core. It is these convection currents which move the 'plates'. The mantle's 'thickness' is 3000 km and its temperature is usually over 1000^oC. It consists mainly of non-metallic silicates with some metal ions. Magma is heated molten rock, from the more 'runny' mantle material and comes up to the surface in volcanic activity or igneous intrusions. The mantle has a higher density and a different chemical composition compared to the crust. It is relatively cold and rigid just below the crust, but lower down it is much hotter and non-rigid and so is able to flow.

Z is the core: is composed mainly of iron, nickel and other metals. Its diameter is about half that of the Earth (3500 km radius) and its is very hot and dense. The core consists of an outer liquid layer and a solid inner layer. The heat is generated by radioactive decay of longer lived isotopes and is transferred into the mantle. It is this heat that drives the convection currents in the mantle, which ultimately moves the tectonic plates of the crust. The mainly iron core generates a magnetic field through and around the Earth.

Some general points:

The overall density of the Earth is much greater than the average density of the rock of the crust. This is evidence that the inner layers of the Earth are made of different more denser materials from that of the crust e.g. the metallic core.

The lithosphere is the rigid, relatively cool crust, and the outer or upper part of the mantle. It is split into sections called plates.

7(c) The age when rocks where formed in or on the crust can be estimated in various ways ..

• Fossils: As plants and animals evolve, species die out and new ones emerge. The sequence and type of fossils can be worked out and the timescale estimated. Therefore the fossils present in a layer can be used to estimate the age of the sedimentary rocks. This dating method is not absolute like radioisotope studies of igneous rocks but its the most useful for sedimentary rocks.
• Radioactive isotope dating: This is a more accurate method for dating very ancient igneous rocks. As certain isotopes, with VERY long half-lives, decay to form more stable atoms, there is a change in the isotope ratio of less stable / more stable. This ratio gets smaller, and by knowing the rate of change from the half-life of the more unstable atom, the age at which the magma cooled to give igneous rock can be estimated.
  • For example: potassium-40 decays to Argon-40 with a half-life of 1300 million years (1.3 x 10⁹y). This ratio can be measured in an analytical instrument called a mass spectrometer. The ratio of potassium-40 / Argon-40 is measured. If 50% of the potassium-40 remains, the rock is 1.3 x 10⁹y old; if 25% is left the age is 2.6 x 10⁹ y old;  if 12.5% is left the age is 3.9 x 10⁹ years etc.
  • Age of the Earth: Using this method it is estimated to be 4.5 x 10⁹ years.
  • The radioisotope carbon-14, ¹⁴C, is of new use for dating rocks. Its half-life is too small at only 5700 years and is not very long in terms of geological time.

8. Plates and their movement unstoppable tectonics!

(introducing the basic ideas and evidence - see also section 9.)

The 'compact' diagram Fig 8.1 Plate Tectonics above gives the "2nd Big Picture View" view of plate tectonics and the situations at (1) to (4) will be referred to throughout the answer notes to 8. and 9.

"The Earth's lithosphere (the crust and the upper part of the mantle) is cracked into a number of large pieces (tectonic plates) which are constantly moving at relative speeds of a few centimetres per year as a result of convection currents within the Earth's mantle driven by heat released by natural radioactive processes. Earthquakes and/or volcanic eruptions occur at the boundaries between tectonic plates."

8(a) The Earth's lithosphere is the crust and the upper part of the mantle. The Earth’s lithosphere is divided into plates meaning they are divided into sections that meet at plate boundaries (situations (1) to (4) all represent plate boundary regions). The plates effectively float on the more dense mantle material and move at speeds of 1-4 cm/year. The crust is the lightest rock of the three layers of the Earth. The crust plate material under continents tends to be thicker and made of lighter 'granites' but oceanic crust is a thinner but more denser 'basalt' type rock.

• (i) In the core heat is generated by radioactive decay of longer lived isotopes and is transferred by conduction into the mantle. This heat causes huge 'plumes' or currents of hot 'plastic' magma to rise and these convection currents in the mantle 'drive' the tectonic plates of the crust when they reach the crust.
• (ii) If the crust is thin and weak e.g. on the mid-ocean sea-bed, the hotter less dense and more 'runny' magma can break through and spread out on either side forming new crust when the sea water it cools.

8(c) Where the plates of the Earth meet is called a plate boundary. Some of the evidence which is used to ‘map out’ the plate boundaries ...

• bands of earthquake activity - the place origin of an earthquake can be calculated from the readings of seismographic stations around the world
• bands of volcanoes e.g. the 'Ring of Fire' in the Pacific Ocean
• more recent mountain ranges
• deep ocean trenches near continental plate edges
• mid-ocean ridges which can now be accurately mapped with modern echo sounding techniques.

8(d) At one time it was believed that the major features of the earth's surface were the result of the shrinking of the crust as the Earth cooled down following its formation. Wegener's theory of crustal movement ('continental drift') was not generally accepted until more than 50 years after it was proposed, so why not?

Some of the evidence for crustal movement or  ‘continental drift’ i.e. plate movement on a large scale over millions of years in which land masses, once joined as 'super-continents', move apart by several thousand kilometres is outlined below. The German scientist Wegener (1880-1930) first proposed the theory, with considerable evidence, in 1915 but it was hotly disputed, and generally rejected for several reasons e.g. (i) prejudice, he was German and the 1st World War was going on; (ii) he was a meteorologist, not a geologist; (iii) the mechanism could not be explained or the 'timescale' appreciated. It was only the development of sonar echo-sounding, and other technology, during and after the 2nd World War that the oceans were finally 'mapped out' in the 1950's - 60's and the recognition that deep ocean trenches existed and the mid-Atlantic ridge give evidence of sea floor spreading. This was linked with data from the crucial development of radioisotope dating and magnetic recording techniques.

• Several continent shapes seem to fit into each other e.g. South America and Africa.
• Different continents have similar ancient mountain ranges made of the same rocks formed in the same sequence, and of the same age, but now geographically far apart. Sometimes a mountain band in the same country is 'broken' into two displaced sections by side-ways plate movement e.g. granite hills in the Great Glen of northern Scotland.
• Rock types and fossils, and their sequence and age, are very similar in South America and Africa up to about 200 million years ago and then the sequences diverge as the continents parted.
• Animals on different continents seem to have a common ancestor e.g. llama in South America and the camel in Africa.
• Magnetic Pole Reversal Patterns: Bands of rock on either side of a mid-ocean ridge show the same pattern of ...
  • The N-S poles of the Earth's magnetic field 'flip around' every so often, and this is called magnetic pole reversal.
  • The direction of N-S pole reversal is 'trapped' in new rocks formed as magma from the mid-ocean ridge cools and solidifies. The 'flips' happen over about 1000 years? but millions of years elapse between each magnetic reversals
  • It is the iron-rich minerals in the magma that record the direction of the Earth's magnetic field at the time when the rising magma solidified. When the rock crystals set, the iron atoms in the minerals act as tiny magnets, and they will align themselves in the current direction of the Earth's magnetic field* and remain permanently set in that direction when the solid rock forms (* just like iron filings scattered around a bar magnet line up in particular directions, but think 3D).
  • Matching magnetic reversal patterns in oceanic crust occur in stripes parallel to oceanic ridges and on both sides!
  • These bands match the periodic reversals of the Earth's magnetic field and so support the concept of sea floor spreading.
• Geological studies of glaciated areas in east South America match those in West Africa.
• Certain sedimentary rocks seem to be in the wrong place! Coal from hot swampy forests and coral limestone from warm shallow seas can be found  in Northern countries like Scotland and in the extreme cold of Antarctica near the South Pole!

9. Plate Tectonics (using the basic ideas to explain all the effects)

9(a) When plates move apart: New crust is formed mainly at mid-ocean ridges where magma breaks through a huge fractures in the crust. ((2) in Fig 8.1) This is known as sea floor spreading and is happening along oceanic ridges, including the mid-Atlantic ridge. This causes cracks through which more molten magma material from deep below the lithosphere can push through producing new rock. The magma from theses chains of linked undersea volcanoes (or just long gashes of hundreds of kilometres!)  rapidly cools to form basalt type rocks of the new crust spreading out on either side. (see also evidence for this mechanism) Sometimes a long central rift valley forms (4). All in all, what is described below, is the detail of the ultimate rock recycling machine!

9(b) When plates collide [more in 9(c)]: Crust material is removed from the tectonic plates whenever two plates collide head on because one plate descends into the subduction zone to be melted and combined with the mantle material ((1) oceanic-oceanic plates meeting (e.g. Pacific Ring of Fire) and (3) oceanic-continental plates meeting (e.g. Andes Mountains) in Fig 8.1). One plate descends into a deep ocean trench, and mud and sand  pour into these trenches and at (3) can end up as bands of metamorphic rock in the 'fold' mountains - see 9(c).

9(c) When continental plate meets oceanic plate the thinner more dense oceanic plate subducts below the continental plate, and partly melts under the thicker but less dense granitic plate. Deep ocean off-shore trenches are formed and parallel mountain chains with volcanoes and earthquake activity too. The geology can be complex and the sediments of the continental crust get crunched up into fold mountains. Metamorphic rocks can be formed due to the heat and pressure in the processes (casing recrystallisation without melting), accompanied by considerable faulting, folding, igneous intrusions and volcanoes. Some of the molten rock cools deep below the surface to form course-grained grained rocks like granite. The magma which rises to the surface cools rapidly to form fined grained rocks like basalt lava or volcanic ash.

If continental plates meet (i.e. after all the ocean has been squeezed out!), the massive collision and compression can build up huge mountain ranges like the Himalayas. Even the pre-existing sedimentary rocks, like limestone and sandstone from the seas originally between the plates, can be squashed up and become part of the fold mountain ranges (the top of Mount Everest is limestone!). They can also be heated to give regions of metamorphic rock, more folding and compressional faulting. The whole process goes on for millions of years! and these 'new' mountain ranges replace 'older' ones worn down by weathering and erosion processes.

See below for side-ways passing movement.

• When two plates meet e.g. at (1) or (3) in Fig 8.1 then the rocks are compressed and the tension builds up even if one plate is descending. Eventually a point comes were the strain in the rocks is too much for the structure to maintain and the rock layers move suddenly to relieve the tension. The release of energy is enormous and radiates out as 'shock waves' or seismic waves. These can create fault lines which themselves can be centres of seismic activity. Earthquake have enormous destructive power, not just on land, but undersea they create giant tidal waves called 'tsunami'.
• Earthquake power can be measured on the:
  • Richter Scale based on shock wave acceleration and energy.
    • It is a logarithmic scale, meaning an increase in 1 unit means 10x more powerful.
    • An earthquake of magnitude 7 is 1000x more powerful than one of magnitude 4 on the Richter scale.
  • Mercalli Scale is based on a succession of increasingly 'dramatic' observed events.
    • It was devised before Richter's Scale.
    • What the geologist Richter did was to give Mercalli's scale numerical values based on seismometer vibration measurements. The bigger the vibration amplitude, the more powerful the earthquake.

Richter Scale	Mercalli Scale
< 3.5	only detected by seismometers, very sensitive people
3.5-4.2	feels like a heavy truck passing
4.3-4.8	felt by people walking, most sleepers wakened
4.9-5.4	objects swing and overturn causing damage, trees sway
5.5-6.1	walls crack, general alarm
6.2-6.9	buildings damaged, chimneys fall
7.0-7.3	ground cracks, buildings collapse, pipes break
7.4-8.0	most buildings and bridges  collapsed, major services out; landslides
> 8.1	total destruction, objects thrown in air, ground moves in violently in waves

• When plates move apart where no magma breaks through, land between 'slips' down 'fault' lines and this causes seismic activity, see (4) in Fig 8.1. Also at mid-ocean ridges, the new crust movement can trigger earthquakes, see (2) in Fig 8.1. 
•  
• The plates can pass each other sideways and the 'grinding action' causes tension to build up in the rocks either side of the fault line. Occasionally, and unpredictably the stored tension energy is released causing earthquake activity. An example of this is infamous San Andreas fault in California USA. Note: When plates pass sideways there is no loss or gain of plate material and usually little volcanic activity but there are plenty of minor earthquakes and every so often 'the big one' - ask the people of LA!
  • There is good evidence of side-ways movement in Scotland on the SE to NE 'line' along the Great Glen of northern Scotland, though thankfully, there is no seismic activity to worry about!
•  
• Most earthquakes happen many km below the Earth's surface and it is difficult to monitor and evaluate all the factors that might help to predict when an earthquake might happen e.g. temperature, earth tremors, gas emissions etc. So, unfortunately tragedies continue to happen, even though scientists do their best, despite the uncertainties of the situation, to make accurate predictions.

9(e) Volcanoes tend to form where plates meet ((1) (e.g. Pacific Ring of Fire) and (3) (e.g. the east Pacific ocean trench and the Andes Mountains on the South American plate) in Fig 8.1). The crust and mantle are disturbed in the subduction zone and extra heat is generated from compression and friction. Some of the upper mantle becomes much more fluid, 'gassy' and less dense. This results in hot magma working its way upwards to break through as a volcano. The explosive force of volcanoes is usually due to the rapid release of high pressure gas trapped in the magma. This can throw out huge quantities of magma, rocks and volcanic ash to form surrounding deposits which can be studied by volcanologists to research the history of a volcanoes eruptions.

9(f) Fig 9.1 Folds and Faults caused by tectonic activity - plate movement

an anticline near Mizen Head, West Cork, Ireland

Fig 9.1

• Folding shows the compression of layers due to plate tectonic movement as plates meet head on! Along the various layers of rock a curve down is called a syncline, a curve in an upwards is called an anticline.
• Sometimes large sections of rock layers are tilted at extreme angles by the tectonic forces.
• Fault lines are huge 'cracks' down through layers of rocks. They are caused by earthquake activity and for subsequence earthquakes, the rock movement is often along these fault lines.
• In the diagram the sequence might be interpreted as follows from 10 up to 1:
  • layers from 10 up to 4 laid down in that order with 10 first
  • the folding occurs later, since newer layers of sedimentary rock would tend to be laid on top and fill up the fold.
  • the faulting occurred after the folding because all the folds are uniformly displaced
  • the left folds have been displaced downwards with respect to the middle section (or middle folds upwards with respect to left folds)
  • the more right linear sections may have been moved upwards with respect to the middle section or the middle section has slipped down.
  • layers 3, 2 and 1 could be the most recent sedimentary rock layers laid down later on top of the eroded layers 4-6 (by weather or glaciations) and have not been subjected to major tectonic forces since there is no evidence of folding or faulting.
• Folding and faulting can give information on the magnitude and direction of the tectonic forces involved.

9(g) A rift valley is formed on continental crust when two plates move away from each other and the land in between falls as shown in (4). This is exemplified by the Great Rift Valley of Africa but it can also be filled with sea water e.g. the Red Sea between the African Continent and the Arabic states.

9(h) In Fig 8.1 the loss of plate at (1) and (3) is matched by the creation of new crust at (2)!

9(i) In situation (2) new crust is formed but at (1) and (3) crust is being moved. So all new rocks have their start at (1) and eventually end up, in whatever rock form, by returning to the mantle at (1) or (3). Hence all mineral material is eventually recycled in the 'big picture' shown in Fig 2.1 and Fig 8.1. Most of these 'answer notes' are looking at the details of all the primary and secondary processes involved. Note in Fig 8.1 the arrow ==> on the right could match up with the ==> on the left i.e. its a 'balanced' global cycle both internally and externally! Any mountain ranges not subducted still get worn away by weathering and erosion, so everything gets recycled in the end!

Fig 9.2 A simpler approach to the "THE ROCK CYCLE" to show the relationship between the three types of rocks - the "3rd Big Picture View"

Fig 9.2

10. Extension! The Moon and Planets What atmosphere and rocks are out there beyond Earth?

10(a) There would be little or no oxygen like all of the other planets, no photosynthesising life on them, but they have gases such as hydrogen, ammonia, methane and carbon dioxide on planets which you find in the atmosphere of Jupiter, Saturn, Neptune and Uranus.

10(b) On Mars there appears to be eroded, but now dry, river beds and cliffs showing 'weathered' or 'erosion' features.

10(c)

• The moon does NOT have an atmosphere, its mass, and hence its gravity, is too low to hold on to it.

• There are no sedimentary rocks on the moon because there is no atmosphere, so there has been no weather to bring about erosion, transportation and deposition etc.

• There will be metamorphic rocks on the moon because there is evidence of volcanic activity and even igneous rocks when heated can re-crystallise to form a 'new' metamorphic rock.

10(d)(i) The surface on Venus is much hotter than the Earth, not only because it is closer to the Sun, but because it has a dense atmosphere of mainly carbon dioxide. This produces a Super-Greenhouse-Effect!

(d)(ii) The surface on Mars is much colder than Earth, not only because it is further away from the Sun, but because it has very little atmosphere even though its mainly carbon dioxide. This means there is little of the so-called 'Greenhouse-Effect', i.e. little  trapping of re-radiated infrared heat radiation from the surface of Mars.

Geological Earth Science geology types of rocks and minerals plate tectonics earthquakes sedimentary metamorphic igneous rock formation atmospheric evolution

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Geological Earth Science geology types of rocks and minerals plate tectonics earthquakes sedimentary metamorphic igneous rock formation atmospheric evolution

Important concepts to understand, facts to know, information points to learn and revise when studying for exams, lesson preparation, teaching and learning to study for yourself or students pupils parents parental info help with revising topics homework coursework projects and assignments in understanding science chemistry courses ~US grades 8, 9, 10