‘Geology Rocks!’

an

AS GEOLOGY

guide

(2006-2007)

As taught by

Mr. Dr. JR Aggett

and

Ms. H Boocock

SECTION 5.1, Module 2831: Global Tectonics and Geological Structures

• The Earth’s Structure
• Earthquakes
• Tectonics
• Geological Structures

5.1.1: THE EARTH’S STRUCTURE

The Earth has a volume of 1.1 million million cubic kilometres. Its surface is covered 70.8% by water. The land has an average height of 840 metres rising to a maximum, at mount Everest, of 8,846 metres above sea level. The land is mostly continental crust, which makes up approximately 30% of the earth’s surface. Continental crust is from 20-90km thick, on average 35km, and made up mainly of acidic rock, meaning the dominant igneous rock type is Granite (although this crust type is made up of igneous, sedimentary, and metamorphic rock). Most continental crust is from 0-4000 Million years old. The crust is 99% composed of the following elements:

· Oxygen

· Silicon

· Aluminium

· Iron

· Calcium

· Magnesium

· Sodium

· Potassium

· Hydrogen

The oceans have an average depth of 3,798 metres down to a maximum, in the Marianas trench, of 11,033 metres, and are mainly on Oceanic crust. Oceanic crust is much thinner than continental, on average 7km thick, ranging from 5-12km thick in some places. Oceanic crust is made up mainly of basic igneous rock, meaning the dominant rock type is Basalt. Oceanic crust is also denser than continental, which is why it subducts beneath continental crust, and it is up to 200 million years old. It has a characteristic structure:

This structure can be observed in only a few places above sea level, such as Cyprus.

Surface temperatures of the planet range from -89 to +58 degrees centigrade.

The boundary between the crust and mantle is called the Mohorovičić discontinuity, but you can call it the Moho.

THE MANTLE

The mantle cools very slowly, and therefore has large or coarse crystals. Gabbro is a ‘macrocrystalline’ form of basalt. The Mantle is the only place where diamonds can form, and is composed mainly of Ultrabasic Peridote, rich in the mineral Olivine.

HOW CAN WE TELL THE STRUCTURE OF THE EARTH?

We can tell the structure of the Earth from plate boundaries, volcanoes, meteorites, the earth’s magnetic field, the refraction of earthquake waves (one type of earthquake wave, the S Wave, cannot pass through liquid-see below), surface observation, and subterranean observation (mines and drilling).

Seismic wave velocities and shadow zones

A shadow zone is an area in which an S-Wave is not detected due to it not being able to pass through the core of the earth. When an earthquake occurs, seismographs near the epicenter, out to about 90° distance, are able to record both P and S waves, but those at a greater distance no longer detect the S waves. This is due to the fact that shear waves cannot pass through liquids. The earth can be shown to have a liquid outer core because the S waves do not travel through it.

P waves slow down when they pas through the liquid outer core, meaning they also have a shadow zone.

The deeper refracted wave travels faster than the direct wave, even thought it travels farther.

Ophiolites

Ophiolites are pieces of oceanic crust obducted or lifted over continental crust, such as that in Cyprus. They allow observation of the structure of Oceanic Crust.

Xenoliths

Xenoliths are pieces of rock brought to the surface by liquid magma which remain solid during transport from a great depth. These are pieces of the earth’s crust that can be used to observe the structure and properties of the crust below the depth to which man is able to travel.

Volcanoes

Very little volcanic magma comes from the liquid mantle or other parts of the Earth’s interior. Most volcanic magma is continental or oceanic crust, which, although melting has potentially changed the structure or other attributes of the rock, can be used to examine the composition of those rocks.

Surface Features

The average height above sea level of continental crust compared to the depth below sea level of oceanic crust clearly displays the thicknesses of the two: due to the principle of isostacy, which states that tectonic plates “float” at an elevation which depends on their thickness and density, crust must have equal amounts protruding into the mantle as into the atmosphere.

Kimberlite Pipes

Some volcanic activity does bring mantle material to the surface, such as Kimberlite pipes. These are vents that pass right through the crust to the mantle, as shown by the fact they contain diamonds, which can form only at specific temperatures and pressures found only in the mantle. The material brought up by these pipes is Ultrabasic Peridote, rich in the mineral Olivine.

Density and Newtonian equations

The density of the Earth’s crustal rock is between 2.7 and 2.9g/cm³. Newtonian equations taking into account the Earth’s orbit, the mass and the size of the earth, the average density of the Earth of 5.5g/cm³ can be deduced. This means that the Earth must contain areas denser than the crust to make up the average density.

Magnetism

The Earth’s magnetic field implies a large part of the Earth must be composed of magnetic elements Iron, Cobalt, and Nickel. The movement of the field over time implies that this magnetic material must be a liquid.

Meteorites

Meteorites are ‘shooting stars’ that hit the earth’s surface (not to be confused with meteors, which burn up on entry, comets, or asteroids). Meteorites come in three forms, stony (86% of meteorites are (stony) chondrites, containing glass and material from which the earth formed, 8% are achondrites, made up of igneous rock similar to the moon and mars) , iron (composed of iron and 5-15% nickel, or alloys thereof, representing the cores of destroyed planets), and stony-iron. 92.8% of meteorites are stony, 5.7% are iron, and 1.5% are stony-iron. Iron Meteorites ar thought to be similar in composition to the earth’s core, and stony iron meteorites similar to parts of the outer core; the stony meteorites represent crustal composition.

Interior of the Earth-properties

Solid(rigid)

RHEID. 1-10%liquid layer (the asthenosphere)
			Gutenberg Discontinuity

(see ‘LAYERS OF THE EARTH WITH SELECTED PROPERTIES’)

5.1.2: EARTHQUAKES

An Earthquake is a Release of stored energy. Earthquakes can be measured either in the Mercalli Scale, which measures the damage caused by the quake, or in the Richter scale, which measures the amount of energy released. The Mercalli scale can be used to produce a map with isoseismal lines, where values of the Mercalli scale are surrounded by lines:

Isoseismal Lines (X marks the epicentre)

Seismic waves

Energy from an earthquake is transferred in the form of waves. There are two forms of wave:

Body Waves: travel through the earth and across the earth’s surface.

Surface Waves: Travel only across the surface.

Body Waves

Body waves come in only two significant forms to be learnt about at AS, the P (Pressure or Primary) wave and S (shear or secondary) waves. P waves are fast, longitudinal waves (like compressing links on a slinky toy) which can travel through solids, gases, or liquids. P waves are the least destructive Seismic Waves. S (shear or secondary) waves are slower, transverse waves, which can travel only through solids, as liquids do not support shear stresses: they travel through the crust and the solid upper mantle but not the asthenosphere. They have a larger amplitude than P waves.

because P waves pass through the core but S waves don’t, there is what is known as a ‘shadow zone’ where no S waves are felt.

Surface waves

Surface waves come in the form of L (or Love) waves as well as some ones less important to AS level. Love waves are the most destructive waves. L waves have the greatest amplitude, and are slower than P or S waves.

Earthquake waves arrive in the following order: first P WAVES, then S WAVES, followed by L WAVES.

Why do earthquakes occur where they do?

Earthquakes occur on Plate Boundaries and Fault Lines where the meeting of plates allows energy to build up and be released due to friction and elasticity.

Deep and Shallow Earthquakes

Shallow Earthquakes occur in Solid Rock, and deep earthquakes occur in molten rock. Deep earthquakes ‘shouldn’t happen’, as molten rock cannot store elastic energy from friction. They are probably caused by the decomposition of subducted plates at a depth of around 160km, releasing energy. Ruptures in these plates happen very slowly and so deep earthquakes consist of closely spaced ‘sub-events’. Shallow earthquakes occur along all plate boundaries, where solid rock rubs against other solid rock, but deep earthquakes only occur along subduction zones where plates are subducted, in the benioff zone, a zone where earthquake foci occur concentrated sloping from an ocean trench.

Earthquake terms

Foci: The focus of the earthquake is the point at which it originates at the surface or below the ground.

Epicentre: the epicentre of the earthquake is the point on the surface directly above the Focus, the centre of the destruction where the first effects are felt or recorded.

Epicentre

focus

Isoseismal lines (not necessarily circular)

fault

Determining the Epicentre

Use seismograms of the P and S waves at three different stations. Measure the time difference between the arrival times of the two different waves.

Using a graph like the one in fig. TDSG we can convert the time difference to a distance:

Fig. TDSG

Measure the time dilation on the graph for the S-P curve, and read off the distance associated with it. This gives the distance away from the epicentre for each station.

Then using a compass, mark on a map the distances from the stations. Where the three circles cross is the epicentre.

Reference and activities: http://www.sciencecourseware.org/VirtualEarthquake/

Earthquakes may occur on transform faults by mid-ocean ridges, deep sea trenches, deep below fold mountain ranges, but seldom at continental shield areas or basins.

The area beneath an oceanic trench where earthquakes occur is called the Benioff zone.

Properties of Rocks

Rocks can deform under pressure. This deformation can be categorized as either Brittle or Ductile Behaviour. Brittle behaviour is where a material breaks, whereas ductile behaviour is where it bends.

Deformation can occur as a result of forces in three main ways:

Stress results in deformation called strain. Under strain circles become elipses and lines remain straight. This applies to fossils.

Earthquake Measurement

Earthquakes are measured using seismographs. On seismographs a pen is suspended in such a way that it will not move if the base is shaken, but the rotating drum on which it writes will move. Seismographs come in threes: one for measuring vertical movement, one for measuring horizontal movement east and west, and a third for measuring horizontal movement south and north. Electric seismographs are similar but use suspended electromagnets and coils to create a current which varies with the magnet’s movement.

Social and Economic Effects of Earthquakes

Ground movement causes building damage. Liquefaction can occur when unconsolidated (loose) material is saturated with water and turns into a liquid when shaking causes grains to move so they are no longer resting on one another, meaning they can no longer support weight. In rock formations if a layer of sand becomes liquefied large bits of land can move freely across the lubricated surface, leading to landslides. Landslides are also caused by the waves causing loose rock to fall. Earthquakes can cause fire when the waves knock over naked flames or electric equipment, especially in the presence of wooden structures or broken gas mains. (Fires may also be set by humans to remove damaged buildings or in order to claim insurance when not insured against earthquake damage). Tsunamis are generated when the sea floor abruptly deforms and vertically displaces overlying water. All these things can lead to deaths and injuries.

Law and order can break down after an earthquake, leading to higher crime rates. Places depending on tourism for income can suffer economically after an earthquake which can discourage visitors from going, either from lack of facilities or fear. This loss of income makes rebuilding difficult and greatly reduces the area’s income. The state may need to take out World Bank loans to pay for repairs, causing greater debts, and insurance against earthquakes can rise greatly. In developing countries many children may not be able to afford to continue education, leading to widespread illiteracy and a lack of skilled workers in future. Businesses are unlikely to be attracted to the area.

Earthquake Prediction

· Earthquakes can be predicted by a number of methods.

· Measuring movement

· Seismic history of location (patterns, or time elapsed between each quake and time since last one-Seismic Gap Theory)

· Elastic Rebound Theory

· Animal Behaviour

· Water levels (The water table may drop as water leaks into a fault)

· Chemical Changes-i.e. water acidity

· Gas release

· Ground level changes

· Magnetism

· Pre-shocks

· Stress in Rocks

Seismic Gap Theory

A seismic gap is a segment of an active geologic fault or subduction zone that has not slipped in an unusually long time; they are often considered susceptible to future strong earthquakes.

Often earthquakes occur at predictable intervals. Frequent earthquakes do not have long to build up significant elastic charge. The longer the period between earthquakes, the more energy is built up, and the the larger the eventual earthquake is.

Hazard Maps

Hazard maps ma be produced to show where the most dangerous places are and so increase education of risk and reduce risk of injury or damage, as precautions can be taken.

Other Measures

Specially designed buildings, warning systems (alarms etc.), and constant measurement of factors that can be used as warnings ( detailed measurement of gas, magnetism, water and ground levels, etc.) can be use to reduce the impact of earthquakes.

5.1.3: Global Tectonics

Evidence for Continental Drift

1: Matching Structural Geology and Fossil deposits/Rock Sequences

Large scale geological structures like cratons and Fold Mountains are seen to match well if continents are reconstructed. Palaeozoic Sedimentary rock strata match across southern hemisphere continents and India, and record similar climate changes over these continents. These Rocks are called the Gondwana Succession. The Mesozoic rocks of Africa and South America are very similar, recording the opening of the Atlantic Ocean (see below).

Fossil species (such as Mesosaurus, found in south America and Africa, and the fern glossopteris, found in all southern continents) occur across one or more continents despite them now being currently separated by oceans. Opposing hypotheses are that the animals floated across the oceans on driftwood and so on, that they travelled across land bridges or Isthmian Links which were up thrust and removed periodically by volcanic activity or changing sea levels, or that the animals used island chains as ‘stepping stones’, swimming the short distance between each island as opposed to travelling the whole distance from one continent to another at once.

2: Continental Fit

There is a jigsaw like fit between the coastlines of Africa/south America and northern Europe/north America. The match is best if made at the continental slope (around the 1000m submarine contour). There are places where the continents would overlap, but these can be accounted for by the growth caused by river deltas and other areas that appeared since the separation. Iceland for instance is a recent volcanic uprising and is left out of drawings. ( see fig 5.1.3.1.2)

3: Rocks and Climate

Rock types formed on some continents are inconsistent with their current climate, e.g coal (formed in tropical swamps) is found in Antarctica and the UK and there is evidence of glaciation in the Sahara desert. All the continents in the southern hemisphere and India show evidence of extensive glaciation during the Carboniferous.

fig 5.1.3.1.2

4:Paleomagnetism

As igneous rocks form from magma, tiny iron crystals align with the earth’s magnetic field and record this direction in the rock formed. The inclination of the magnetic field changes with latitude, so the rocks contain a record of the latitude at which the rock formed. The magnetic inclination data for rocks of many continents do not match their current latitude. Rocks of different ages record the north-south movement of the continents over time. IT is customary on maps to keep the continent in its current position and plot the apparent position of the North Pole as a ‘polar wander curve’. Such curves can be used to track the joining and break up of continents.

These alignments also record the periodic reversal of the earth’s magnetic field, and with it the position of the poles.

THE MECHANISM FOR CONTINENTAL DRIFT

The mechanism for continental drift is Sea Floor Spreading, which occurs at constructive plate boundaries. Magma is forced to the surface at the mid-ocean ridge, making new rock and forcing the two plates apart.

Evidence for the Mechanism

Apart from the fact of the existence of continental drift, it can be shown in several ways that the sea floor spreads at the mid ocean ridge. Magnetic surveys show that parts of the ocean floor have magnetic anomalies lying roughly parallel to the mid ocean ridges. These magnetic anomalies are caused by a similar mechanism to the one described above: as the igneous rock is formed at the edge of the plate by rising magma, tiny iron crystals align with the earth’s magnetic field and record this direction in the rock formed. As the magnetic poles switch places repeatedly, this forms the ‘stripes’ of rock whose crystals are aligned in the same direction as the present magnetic field, and the rock whose crystals oppose the present field. This shows that the ‘stripes’ are being continually formed. It also means that the stripes on either side are roughly symmetrical.

By comparing the pattern of ridge anomalies with the approximate dates of magnetic pole reversal (derived from study of continental lavas) it is possible to ascertain the rate of spread by the usual d/t=s equation.

On top of this, it has been shown that the oceanic crust grows older the further away from the midocean ridge it is measured, although the oldest part of oceanic crust yet found is only 150Ma old.

The shape of the midocean ridge suggests uplift, and its heat and gravity profiles suggest increased heat and density at the middle of the ridge, indicating rising magma.

There are more and thicker and older layers of sediment the further you get from the ridge as well. This is because the older crust has been there for longer so more sediment has settle on it. Island arcs and hotspots, typical from the rising magma of the ridge) are too far away from the volcanic activity to have formed there, or can be shown to have moved over time. The shallow earthquakes on the transform faults indicate movement along them, at right angles to the ridge.

Plate Margins

CONSTRUCTIVE ( DIVERGENT ) PLATE MARGIN

Mid ocean ridges; mechanism of continental drift. Shallow earthquakes along ridge & part of transform fault. Features are ridge, rift valley, and shield volcanoes.

HEAT AND GRAVITY PROFILE

H/G

Heat and gravity increase over the ridge itself.

Destructive ( CONVERGENT ) boundary

1. oceanic/oceanic boundary

Collision between two plates of oceanic crust causes one plate to subduct beneath the other. As the plate descends it is heated and melts to form andesite (intermediate lava). This magma rises to the surface to form volcanoes in an island Arc (such as Aleutian Islands, Indonesia).

Features: Trench, island arc, volcanoes along trench, shallow/deep earthquakes, burial metamorphism/contact metamorphism.

HEAT AND GRAVITY PROFILE

2. oceanic/continental boundary

Complex volcanic activity, acidic lava (more violent eruptions), mixture of oceanic and continental lava, partial melting of subducted oceanic crust, (intermediate magma (andesite) and of continental crust (granite).

Volcanoes of intermediate/acidic nature-steep cones, shallow/deep earthquakes (benioff zone).

HEAT AND GRAVITY PROFILE

3. continental/continental boundary

Fold mountains, no volcanic activity, some intrusion of granitic magma occurs as a result of melting at base.

HEAT AND GRAVITY PROFILE

Shallow to intermediate earthquakes, regional metamorphism (the alps and Himalayas).

CONSERVATIVE (large transform) BOUNDARY

Crust is neither created nor destroyed.

Types: Transform faults, fault plains, minor volcanic activity (no subduction, minor frictional heating). Shallow earthquakes and dynamic metamorphism (grinding)

HEAT AND GRAVITY PROFILE

POSSIBLE MECHANISMS FOR PLATE MOVEMENT

Convection currents:

heating and cooing magma in the earth’s interior causes convection currents. Where these bring magma to the surface there are constructive boundaries, where they suck magma down are destructive ones. The mantle is layered so presumably there are layers of such currents.

Another possibility is that the movement is driven by the pulling of spreading magma ridges, although this is unlikely since rift valleys indicate tension. Alternatively plates may be pulled apart by their density compared to surrounding rock or other bits of crust, causing them to sink into the magma, or even the plates sliding down the 1 in 3000 slope into the mantle.

There is a fundamental balance in that as much crust is subducted as is created at any one time, preventing the earth’s crust from ‘growing’.

HOTSPOTS

Hotspots are stationary areas of seismic activity caused by mantle plumes. They form island chains in oceanic crust by their rising magma coming to the surface and erupting, forming volcanic islands, while the crust moves over them: the direction of plate movement can be ascertained by finding the direction from the youngest of two islands to the oldest, and from that to the even older one.

They are the mechanism behind the breaking up of continents. When hotspots form under continental crust, a build up of heat occurs because of the crust’s insulating effect. This causes the crust to be lifted in a dome, until it cracks into a triple junction of rift valleys. One of the rifts always ‘fails’ and a spreading ridge forms in the two left.

5.1.4 Geological Structures

UNCONFORMITIES

An unconformity is a buried erosion surface. A bed or beds of rock were eroded and then covered with sediment. This causes a break in the sedimentary record, with younger rocks above the unconformity than below, unless the sequence has been overturned.

An Angular Unconformity, the only type of unconformity you are required to know about according to the specification, is the laying of horizontal beds across the top of tilted or vertical beds. This sequence is still angular unconformity if overturned or tilted.

Formation of Unconformities:

Competent and incompetent beds

Competent rocks are hard and brittle and break under stress.

Incompetent rocks are ductile and may bend under stress.

Joints and Faults

A Joint is a fracture in a rock where there is no relative movement

A Fault is a fracture in a rock where there is relative movement

Competent Rocks under stress may break along Joints. The three main types of Joint are the Tectonic, cooling and unloading joints.

Tectonic Joints are formed by folding (like those in the competent beds above)

Cooling Joints are formed by steadily cooling igneous rock forming columnar joints such as the giant’s causeway.

Unloading joints are formed by the reduction of weight on top of some rock, the reduction of pressure and the expansion of the rock by tension.

Slaty Cleavage:

Mud is made up of clay minerals with plate-like structures. When mud is deposited the plates are randomly oriented, but when compressed the particles align perpendicular to the direction of the force. In folds slaty cleavage is often parallel to the axial plane.

Beds are more ductile and incompetent at higher temperatures.

Faults

a tear fault is most often known as a strike slip fault.

The fault dip is the angle of the fault from the horizontal. Not to be confused with the dip of the bed, as so often happens.

NORMAL faults are formed by tension

REVERSE faults are formed by compression

STEP faults are always a series of normal faults formed by lots of compression

THRUST faults are shallow Reverse faults

slickensides is formed by the friction of the hanging wall against the footwall, which also breaks off grains and pieces of each wall, fault breccias and the surrounding grains.

Folding

Fairly self explanatory: with incompetent rock, when it bends rather than faulting under compression and tension you get folds. Folds need to be described as well:

Dome: antiform shaped like an upturned bowl: dips in all directions

Basin: synform in bowl shape, dipping from all directions into centre

Geological Maps show the rock type at the surface.

A geological map show the surface geology of an area. For most purposes the topography is considered as flat. Dipping strata show series of horizontal stripes (they always dip from old to young). Folded strata appear to have repeating beds:

GEOLOGICAL MAPS: FOLDS

Asymmetrical folds show the steeper limb as having a narrower outcrop. Plunging folds will not be any different in section view, but in map view can be seen the closure of the beds, with dip arrows pointing outwards from the centre/middle of the fold for an anticline. The map view allows you to see whether a fold is an anticline or syncline from whether the oldest beds are in the middle or not. For a syncline the dip arrows point towards the centre of the fold.

Faults seen on geological maps should be assumed to be vertical. Therefore normal and reverse faults cannot be distinguished from one another. The upthrown and downthrown side of the fault can be determined from the map because the older bed visible will be the upthrown side. Where beds dip this can have the effect of shifting the pattern of the beds:

Normal fault

Sinistral Strike slip fault

(a sinistral strike slip fault is one in which the two sides are going to each others’ left, dextral is where they go to each others’ right)

unconformities will appear as abrupt changes in the beds.

Igneous features such as intrusions and dykes cut across strata, thus proving they are younger than them. Assume the sides of the intrusion are vertical.

THE ROCK CYCLE (MODULE 2832)

Approx temp. ( C)	processes	Approx depth (km)	product
20	weathering	0	sediment
100	Diogenesis (compaction/cementation)	<5	Sedimentary rock
200	recrystallisation	10-30	Metamorphic rock
250-1000	melting	50-100	magma

temperature

lithification involves compaction and cementation

Surface Processes

WEATHERING

The more a sediment is weathered and the longer therefore it is transported the smaller, rounder, more spherical and more prismoidal the grains get.

CHEMICAL WEATHERING-take place in solution

Hydrolysis-breakdown of silicate based minerals by water

Oxidation-Combination with oxygen to make softer rock

Carbonation-reaction with carbonic acid (CO2 plus H2O) i.e limestone

fHydration-water causes rock to swell and alters volume and structure

Chellation- removal of mineral ions from organic compounds

PHYSICAL WEATHERING

Exfoliation- expansion or repeated heating and cooling of rock causes flakes of rock to fall off

Frost shattering – water collects in cracks and freezes, expanding and breaking apart the rock

BIOLOGICAL WEATHERING

Animals burrow through rock

Treeroots grow through rock

Transport

Gravity: bits of rock fall off and fall to the ground

Grain size: no effect: coarse or fine

Shape: usually angular but depends on prior transport or erosion

Sorting: poor

Wind: sand blows around the desert: only the finest grains are carried by wind. Moisture prevents wind transport.

Grain size: grains

Shape: prismoidal, rounded

Sorting: good: deposited in order of coarseness

Water: Solution: invisible

Traction: heavier particles-rolled in flood

Saltation: bouncing

Suspension: water tension supports particles

Grain size: immaterial

Shape: rounded and prismoidal

Sorting: good: deposited in order of coarseness

Ice:

Grain size: fine to coarse

Shape: angular to prismoidal, rounded to angular

Sorting: poor.

Playa lakes

Playa lakes are temporary lakes with a low inconstant water input and a high evaporation rate. They fill during sporadic rain when groundwater is temporarily higher, and are fed by wadis. The Cheshire basin was once a playa lake.

Ephemeral streams: Playa Lakes are the beds of the lakes once evaporated, and evaporates are what forms as crystals when the water evaporates. The evaporates form in sequence: first Gypsum, then anhydrite, then halite.

Wadis

Wadis are river valleys that may or may not contain a stream. They are often steep sided and narrow, and are found mainly in deserts. They are formed by flash flooding.

Mesa

Mesas are outliers or inliers: pieces of rock eroded into islands

Conglomerate

A rock made of compressed, poorly sorted sediment. Wadi conglomerates mark streambeds.

Alluvial Fan

Sediment left by water

Desert pavement

What’s left after the sand is eroded

Ventifacts

Prism shaped rock with a shallow back slope and steep front slope,

Dunes

Barchan dunes and transverse dunes: shallow back slope and steep front slope. Wind blows grains up shallow slope and they fall down the other side.

Formation of coal

Coal forms in deltaic environments: swamps near the sea. The fallen trees become covered in sediment, and they and the sediment are lithified by compression under heat and pressure into coal and sedimentary rock. Coal first forms peat, then brown coal, then bituminous, then anthracite.

Cyclothems

A cycle of rock types:

The rhythms or cycles consist of marine limestone at the the base followed by marine shale, lagoonal shale and siltstone, deltaic sandstone and coal. This is probably caused by the subsidence of the sedimentary basin subsides at regular intervals. Each subsidence allows the sea to flood in so limestones are deposited. As more sediment is deposited the basin fills up and forests grow on the deltas. Sinking then takes place and the cycle begins again. It could also be caused however by the rhythmic rising of the land or the ocean. LO! SBS4? SECT!

Clastic Material

In sediment rich shallow seas, limestone conglomerate, a mixture of organic shell fragments, settle in large amounts along with sand, and mud, forming limestone, sandstone and mudstone. If the sand and mud are in the limestone they are known as disseminations.

Primary Sedimentary Structures

GRADED BEDDING: variable current drops coarse grains first then the fine grains. FINE GRAINS AT TOP

CROSS BEDDING:

RIPPLE MARKS: marks formed by rippling water or wind.

DESSICATION CRACKS: always mud below, sand above. Cracks formed when mud is dry, filled with another sediment. The cracks are narrower at the bottom.

LITHIFICATION: compaction and cementation of sediments to form rock by removing porosity. Compaction forces grains closer together, cementation binds the grains together by filling remaining pore spaces with other minerals.

IGNEOUS PROCESSES AND PRODUCTS

Igneous Intrusions

Igneous bodies or intrusions are formed when magma cools and rises through the crust as diapers, because it is less dense. The diapers take the simplest possible rout through faults and joints. Cold diapers are igneous bodies. Generally plutonic bodies are course grained, hyperbyssal medium, and volcanic fine grained.

Discordant feature:

An igneous body that runs parallel with beds and bedding planes, such as a sill.

Concordant Feature:

An igneous body such as a dyke that cuts across beds and bedding planes.

Extrusive Feature: an igneous feature that has formed in the open air, which may have been covered since its formation by other strata, i.e a lava flow.

Fine Grains

Rhyolite or andesite

Weathered, containing surface material(animal and plant matter etc.)

Reddened with earth on top

Picks up xenoliths (bits of layer below only)

Vesicles formed from pressure release

Heats rock below (contact metamorphism) but no metamorphism in layer above

Not transgressive

May stop at topographic high points.

Intrusive Feature: an igneous body formed below the surface, such as a sill.

Medium grains

Microgranite

Unexposed and unweathered

Contains bits of layers above and below

Very few vesicles (formed under pressure)

Contact metamorphism in rocks above and below

May be transgressive

Minerals

	Olivine Pyroxene Amphibole	Basic
	Mica/ potassium feldspar quartz	acidic

Rocks are made of minerals with distinct chemical formulae. These minerals form crystals when cooled. Minerals can be identified by their properties, although there are over 4000 of them (only 10-20% are common enough to form rocks).

Bouen’s reaction series: the minerals in igneous rock depend on magma composition and rate of cooling. Magma composition can be changed by magmatic differentiation, which occurs because of partial melting and fractional crystallisation where crystal settling occurs.

	ACID	INTERMEDIATE	BASIC
COARSE	GRANITE	DIORITE	GABBRO
MEDIUM	MICROGRANITE	MICRODIORITE	DOLERITE
FINE	RHYOLITE	ANDESITE	BASALT

TYPES OF MAGMA

The three types of magma are basic, intermediate and acidic. Acidic magma occurs in continental crust, intermediate occurs in island arcs, basic occurs in oceanic crust, and Ultrabasic occurs in the mantle.

Igneous is the least porous type of rock and has interlocking crystals with no porosity.

The faster magma cools the finer the crystals produced. Magma cools rapidly beneath the sea, forming fine grained extrusive rocks. Underground magma cools much more slowly forming coarse grained intrusive rocks. A rock cooled slowly but then erupted to the surface or cooler area then large crystals will form first, and become surrounded by fine crystals: this is called porphyritic texture: the large crystals are phenocrysts and the fine crystals are ground rock.

A vesicular texture may be formed if gases are trapped inside the magma when it cools.

VOLCANOES!!

There are four main types of volcanoes, okay?

Submarine, fissure, shield and strato-volcanoes.

Submarine Volcano

Basically just produces pillow lava.

Fissure Volcano

Fissure volcanoes occur when lava runs from long fractures in the earth’s surface. At ocean ridges the ocean floor is moving apart and basalts comes to the sea floor through cracks running parallel to the ridges. On land, fissure eruptions are makrked by a string of small cones, or by flood eruptions, for instance producing flood basalt.

Shield Volcano

Where the basic lava has a very low viscosity, it flows very easily and so forms shallow volcanoes shaped like shields. Explosive activity is usually rare because of the ease with which gas is released from the magma.

Stratovolcanoes

Stratovolcanoes basically are formed by the build up of layers of pyroclasts and lava flows over time, leading to a tall steep cone with a layered structure. They may have secondary vents.

Calderas

Calderas are giant craters formed by the collapse of volcanoes following violent eruptions, as the magma chamber is exhausted and the volcano falls into the space created.

Volcanic Products

Volcanoes produce Lava, Gas, Airborne Material, Nuées Ardentes, Lahars, and Geysers.

Lavas: Most lava is basic, divided into two types based on morphology.

Aa (ar-ar) lava is blocky, and

Pahoehoe lava is ropey

(not to mention submarine pillow lava)

Acidic lava is very viscous and flows slowly, forming steep stratovolcanoes and causing violent eruptions. Basic lava is not viscous at all and causes frequent, less violent eruptions, where lava flows easily causing shallow cones.

Airborne Material Tephra: solid rock (blocks) that are parts of the volcano picked up and thrown by the explosive eruption, or molten lava (bombs). Much material is ejected high into the atmosphere where it may circulate the globe.

Nuées Ardentes (‘burning clouds’) some pyroclastic material may be released as a hot dense cloud which flows down the side of the volcano. It is turbulent, which keeps it moving, and keeps blocks/bombs/glass in suspension. Very destructive and dangerous.

Lahars (mudflows). Many volcanoes have crater lakes full of water at their top, which is displaced by the eruption, sending huge amounts of water down the volcano, picking up mud and ash as it goes. Alternatively the mudflows may be caused by the displacement of water from rivers by ash or lava flows, in which case the mud is usually volcanic ash mixed with water. Lahars are dense and destructive.

Geysers jets of hot water ejected into the sky. The water is pushed to the surface by volcanic gas.

Gas magma typically has a small amount of gas in it. Typically 56% of the gas is water vapour, 28% is CO2, 14% is SO2, and smaller than 1% is H2S, HCl, H2, and CO.

Pros and Cons of Living near a Volcano:

pros	cons
Fertile ground Free pumice stones for feet Geothermal energy source Mineral source Mineral water such as volvic (volvic is worth 1000x more than the same volume of crude oil) Nice scenery Lots of wood and logging from fertile ground Opportunities to study vulcanism Cheap land Tourism More land after eruptions	Death Damage to property Forest fires Loss of animals Loss of infrastructure Loss of mineral, wood and farm production Constant fear Low investment Government costs (evacuation, monitoring volcano, education about it) Breathing problems Ash in atmosphere reduces crops, diverts aircraft, prevents cars working Difficult to get insurance Earthquakes

Predicting Volcanic Activity:

• animal behaviour: the animals detect earthquake tremors and vamoose
• earthquakes-near to volcano, unusual, getting gradually larger
• noises and rumbling
• change in ground level
• change in water table level
• change in water acidity
• smoke and ash released
• pre-eruptions of gas
• increased heat in heat profiles
• volcano’s history and regularity

Risk Analysis:

Lava flow paths, blast damage, ash falls, pyroclastic flows and lahars can be predicted and plotted on a hazard map.

The black bits are past lava flows.

Metamorphic Processes and Products

Metamorphic rock is metamorphosed under heat, pressure, or a combination of the two, changing its composition and properties (chemically and physically). It may change texturally or mineralogically. The chemicas in the minerals of the rock react when heated, and recrystallise.

KEY FACTORS:

Heat: causes recrystallisation. Crystals become larger and more similar.

Pressure: causes mineral alignment (foliated textures-mm scale- and banding-cm scale)

Time: metamorphism may occur over thousands of years.

Rock type/composition: the more complex the original chemistry the more complex the final mineralogy.

CONTACT METAMORPHISM: results from igneous intrusion heating surrounding rock.

REGIONAL METAMORPHISM: results from burial of rocks and mountain building processes heating and compressing the rocks.

BURIAL METAMORPHISM: results from the burial of rocks (by subduction or other activity) compressing rocks.

Contact Metamorphism:

Intrusion of magma heats surrounding rock. The rock directly next to the intrusion becomes the baked margin, the area of the intrusion directly next to the country rock (surrounding rock into which it is intruded) cools fastest and become the Chilled Margin, with finer crystals than the rest of the intrusion. The full area affected by the metamorphism is the Metamorphic Aureole.

The mineral causing the spots in the spotted rock is most often Garnet. If you see garnet, it’s probably a metamorphic rock.

Metamorphic Aureole: affecting factors

· The Larger the intrusion, the more heat energy, and the larger the aureole.

· The more conductive the rock, the larger the aureole

· The more permeable the rock, the better the conductivity, and the larger the aureole (water convection currents)

Regional Metamorphism

Regional Metamorphism is large scale metamorphism associated with burial under sediment, and mountain building processes, involving both heat and pressure. It causes recrystallisation and reaction forming new minerals (i.e garnet). Pressure causes foliation (layering/alignment of minerals).

As a mudstone undergoes regional metamorphism it passes through a series of stages.

Typical regional metamorphic texture is granoblastic. Folition forces out air and pore spaces, so the resulting rock is dense and non-porous.

Grade: Level of metamorphism.

ECONOMIC AND ENVIRONMENTAL GEOLOGY (module 2833)

Water Supply

Porosity: (volume of pore space/total volume of rock)x 100

Permeability: a measure of the ease with which a liquid can flow through the rock. Rocks must be both porous and have access between the pore spaces for fluids to pass from one to another

Hydrostatic Pressure: the water pressure of at any point in a body of water at rest

Hydraulic gradient: the difference in hydrostatic pressure between two points divided by the distance between them.

Aquifer: a reservoir of groundwater containing economic quantities of water.

Water Table: the top level of the groundwater

SPRINGS

Springs occur where the water table meets the surface or a fault.

springs appear wherever the watertable meets the surface, a fault, an unconformity, or impermeable rock.

(18.00 approx: large amount of work lost because of stupid computer crash-everything from springs to the environmental effects of coalmining) Naturally I am very upset about this. I have worked literally non-stop for two days, stopping only to sleep (I did past papers while I ate) ive been drinking a lot of coffee, the flies won’t stay away from me, and my room is the warmest in the house because im right above the aga in the kitchen (even with the fans on and the window open-which only elevates the fly problem). To lose even a little work, not to mention over ten pages plus diagrams, is so INCREDIBLY annoying that my only two options were to vent my rage by writing about it, or go and smash some old bits of pottery in the garden. I am now finding it difficult to continue: if I don’t get the oil and coalmining stuff back, there seems little point in continuing from where I left off, because there will be a huge gap in the middle. And since the only other two options are to do it ALL again, or not continue with this method of revision, there seems little else to do but complain and resign myself to failure tomorrow.

I am conducting wide internet searches to find a solution to my problem, shouting wild abuse both at the computer and Microsoft, and am desperate to find a way to restore my file and with it my will to revise. This was the straw that broke the camel’s back (and I feel a lot like a camel right now) and I feel on the verge of breakdown. Even at 39 pages, the loss of a fifth of my work is intensely painful. It isn’t even coursework or anything. I feel like someone just died.

I NEED TO FINISH REVISING, so ill do metal deposits, hydrothermals, and engineering?

Concentration factors:

Concentration factor=ore concentration/crustal abundance

Residual deposits: Bauxite: everything else dissolved

Secondary enrichment: chalcopyrite: soluble minerals concentrated at water table

Placer deposits: high density metal minerals may be deposited in concentrated amounts by streams in waterfalls

Road cuttings and embankments

Water seepage

Block sliding

Along bedding plane cut into

Tunnels in hard and unconsolidated material

Waste disposal in quarries

Must be below water table

Gas buildup

Quarry already dug, not expense of digging it

Trigger tectonic activity with extra weight

landslides

Dams and reservoirs:

Arch-v shaped valleys

Buttress

Gravity

Embankment

Landslips

Slumping hazards