This is the force of attraction between objects. There is a relationship between gravity and mass so measurements of gravitational force can be used to find the mass of, e.g. the earth or other planets.

The acceleration due to gravity can be measured with a gravimeter. There is a theoretical value for gravity on the surface of the earth but where there is an excess or deficit of material the value may be higher or lower. These are gravity anomalies, excess gives a positive anomaly and deficit gives a negative anomaly. Anomalies are stated in milligals (mgal). Anomaly maps can be prepared by joining up points of equal anomaly - isogals.

The lithosphere can be regarded as floating on the more fluid asthenosphere. The normal laws of flotation apply.

• If the landmass is floating so that there is no tendency to rise or sink it is in isostatic equilibrium and there is no gravity anomaly.
• If it is floating too high it has a positive anomaly. Denser material now occupies the under part of the mass and there is a positive gravity anomaly.
• If it is too low it has a negative anomaly. It now displaces the denser material (less of it) and there is a negative gravity anomaly.

Concept suggested by Pratt (1855) when gravity pull of the Himalayas (plumb-line myth) was less than expected. Various theories - finally Airy/Dutton.

Mountains - negative anomaly - low density (SiAl) material

Ocean trenches - negative anomaly - less mass

Oceanic ridges - slight negative anomaly - material less dense because hot

Ocean floor - positive anomaly - high density (SiMa) material

East Indies - belt of positive anomalies parallel to Sumatra and Java, bordered by positive anomalies - isostatic inequilibrium. Crust buckled so that light SiAl forced up (negative anomalies).

• Weight of ice on land. There are thick ice sheets on Antarctica (4 km thick) and Greenland (3 km thick) and the land is depressed. Scandinavia had thick ice sheets in the last glaciation. When these melted (10 000 years ago) it left a negative gravity anomaly. The land is now rising at a rate of about 1 cm yr^-1. Uplift of 100 m so far and 200 m to go to reach isostatic equilibrium. Same in UK - east coast rising - Tynemouth Priory built on old beach level.
• Erosion of mountains can potentially produce a negative anomaly but usually the mountains rise isostatically to keep pace as in the Himalayas. As they rose to maintain equilibrium, Indus and Ganges cut gorges. Also Colorado plateau and Grand Canyon.
• Deposition of sediment can potentially produce a positive anomaly but usually the land sinks to maintain isostatic equilibrium, as in the Mississippi delta.
• In the oceans, volcanic islands can produce a positive gravity anomaly but usually they subside to maintain equilibrium.
• Continental shelf of Antarctica low - weight of ice depressed land.
• Hoover dam (Colorado River) /Lake Mead - crust has subsided

• The axis of the Earth's magnetic field in about 11° to the axis of rotation. The north magnetic pole is located in Northern Canada. The angle between geographic (true) north and magnetic north varies with location and is changing.
• Throughout geological time it has reversed polarity many times.
• Measurements can be made of:

• the strength of the magnetic field on the surface and the points of equal strength joined up to form a magnetic contour map;
• the angle between geographic north and magnetic north (declination) and the points of equal declination joined up to form a declination contour map;
• the angle to the horizontal (inclination) and the points of equal inclination joined up to form an inclination contour map.

The strength and position of the magnetic field changes with time. The field drifts west - it has moved from 64°W to 70°W in the last 140 years.

Electric currents are flowing inside the Earth, probably circular currents flowing west under the equator. These are produced by a dynamo effect in the outer liquid core due to the rotation of the Earth. Convection currents may also be involved and be responsible for the westward drift.

Ancient magnetism (palaeomagnetism)

Iron-containing minerals orient like magnets. They orient along the magnet field and retain this orientation provided the rock is not heated. The inclination and declination can be measured in rocks of known age and this can be used to calculate latitude, as well as the position of the pole at the time. The alignment of the minerals can take place:

• as the rock solidifies after an eruption
• when particles are eroded and deposited as wet slurry to form sandstone

• when the rocks undergo metamorphism and folding

Pole positions shown in many ancient rocks do not coincide with present day. The position of the pole for different ages of rock can be calculated in a continent, e.g. Africa and repeated in a different continent, e.g. S America. A comparison shows poles in different positions. As there cannot be two poles, the position must be reconciled by moving the continents to their original position - evidence for continental drift. NB The continents move, not the pole.

Apparent polar wandering curves have been plotted for the past 2700 Ma.

Data on palaeomagnetism and palaeoclimates agree.

In some rocks the pole positions were reversed. Field reversals take place fairly regularly over about 20 000 years and last 100 × 10³ to 50 × 10⁶ years. A polarity time-scale can be constructed from lava flows from cracks in the crust - looks like a bar code. Has been plotted over the past 4.5 Ma - before this the rocks cannot be dated accurately. Field reversals coincide with many extinctions, possibly due to:

• climate changes induced by reduction in strength of the field;
• increased radiation from the sun (field no longer mops up radiation);
• decrease in fertility.

Another reversal may be starting, the field has been weakening for the last 150 years.

Continental drift is the theory that originally there was a single large landmass from which the continents split and drifted apart.

• The concept of continental drift was first suggested by Roger Bacon in the16^th century and seriously debated in the 19^th century but was not accepted until a feasible mechanism was proposed from the 1960s on.
• Late 19^th century the close fit of Africa, South America, India and Australia was demonstrated (Edward Suess) - called Gondwanaland.
• In 1912 the close fit of Eurasia and North America was demonstrated (Alfred Wegener) - called Laurasia.
• Then suggested (Alfred Wegener) that Gondwanaland and Laurasia formed a single continent called Pangea.
• From Precambrian onwards this has been breaking up, moving apart and reforming.

The present continents fit closely together - best fit about halfway down the continental shelf. Some overlap occurs but can be accounted for by delta formation.

• demonstrates that continents have moved and gives a time-scale

Rock types in different continents can be shown to be the same - i.e. they were once continuous.

• Sub -surface folds have a good match across the line of fracture.
• Charnockite (a granite) - distinctive - found in India, Africa, W. Australia and Antarctica.
• Anorthosites (Precambrian) extend across India, Madagascar, Africa, Australia and South America.
• Diorites (Jurassic) in South Africa match with those in Tasmania and Antarctica. This means that Gondwanaland may not have split until Jurassic.
• Caledonian Mountains can be traced Norway , Greenland, Scotland and North America.

The effects of glaciers can be traced across lines of fracture.

• Tillites (fossilised boulder clays) in S Africa to equator - correspond with those in S America.
• Large, erratic blocks of quartzite, dolomite and chart found in Brazil and match those in S W Africa.
• In S W Africa glacial erosion but no deposition - centre of the glacier dumping to the E and W.

• Permo-Carboniferous glaciation in Gondwanaland at the same time as coal seams were being laid down in N America and Europe which must have been tropical. Continental drift cause continents to change position relative to poles and equator.
• Wind direction - sandstone in Cullercoats laid down in different prevailing wind direction (E wind) - Britain rotated through 35°.

A wide ocean is a barrier to migration to shallow water dwellers. The same types are now often found in widely separated continents.

• Mesosaurs are found in S Africa and S America and nowhere else.
• Glassopteris (a Pteridosperm) found in S Africa, India, S America, Australia and Antarctica.
• Lystrosaurus only in S Africa and Antarctica.
• Fossil elephants in all continents except Australia and Antarctica.
• Tapir now in S and central America was in all of Eurasia in the Tertiary.
• Australia - marsupials and monotremes - separated before mammals evolved.
• Evolution in the Holocene produced many different groups - continents must have been separated.

• The direction of magnetised particles in rocks show that in the past they were orientated in a different direction. This assumes a constant relationship between rock magnetism and the poles and axis of rotation.
• Palaeolatitudes of the same rocks in S America and Africa are different in the Permian-Carboniferous than in the Upper Cretaceous.

• Production of new material and recycling of crust
• Height of ridge - no aerial erosion but rarely reaches surface
• Density gradient and heat flow - cooling and contraction
• Ocean depth - deeper furthest from ridge (cooling and contraction)
• Age of floor - oldest furthest from ridge
• Magnetic anomalies - stripes
• Sediments - thickness and age. All laid down from Cretaceous onwards - split of Africa from S America began during Cretaceous. This corresponds with calculated values of actual sea floor spreading in the Red Sea. Sea floor spreading is due to the production of material at the oceanic ridges, which then spreads outwards. The oldest sea floor is less than 200 Ma old. Thickness varies from ridge to edge.
• Direct measurements in Iceland - ground-based laser and GPS satellites

A long chain of submarine mountains circles the world.

• 60 000 km long and 1000 to 1500 km wide
• peaks reach 1000 to 4000 m above the abyssal plain, some reach the surface as islands, e.g. Iceland
• topography rugged as there is no aerial erosion
• most have a deep cleft (a rift valley) along the axis - Median Valley or Axial Rift Zone
• symmetrical on either side of the median valley

• central mountains composed of basalt with little sedimentation
• intermediate zone (plain) covered with sediment
• outer rugged foothills, highest peaks not covered by sediment -guyots, submarine volcanoes

• magnetic anomalies on either side
• crossed by Transform Faults

Atlantic islands formed at the Mid-Atlantic ridge, moving outwards with the plates, the nearer the ridge the younger, i.e. Azores younger than Cape Verde.

Atlantic spreading at a rate of 1 - 2 cm yr^-1.

Plates (about 7 major and 8 minor) cover the entire surface of the Earth. They are in continual motion relative to each other and most volcanic and seismic activity is localised at the margins. A plate may contain:

• oceanic crust;
• continental crust;
• both oceanic and continental crust.

Plate margin = edge

Plate boundary = active region on either side of the plate margins

A plate consists of crust and the upper part of the mantle = the lithosphere. This floats on the more fluid asthenosphere (about 5% molten).

Evidence for plate tectonics

• Sea floor spreading - new crust continuously formed at ocean ridges.
• New crust forms part of a rigid plate.
• The Earth has a constant surface area - the radius has not changed by more than 0.05% in the last 200 Ma.
• Oceanic crust is destroyed at the edges. Continental crust is not - too buoyant.

Lubricated by the fluid asthenosphere

Mechanism of movement:

• Convection currents in the asthenosphere - rise at oceanic ridges and sink at destructive margins. In the asthenosphere (50-250 km) temperatures are high enough and pressure low enough to allow partial melting - 5% molten. Viscosity similar to that of glass.
• Slab pull - descending slab cooler and more dense, sinks and pulls the plate towards the edges.
• Ridge push - edge is elevated at the oceanic ridge and 'slide downhill' by gravity.
• Mantle plumes - may have caused the original crack, new material forces plates apart.

The plates rotate around a pole of rotation - this SW of the tip of Greenland. Lines drawn at right angles to transform faults intersect at the pole of rotation.

Constructive margin (divergent)

Material (basalt) is added to the plates moving away from the oceanic ridges, forming new crust. Mid-Atlantic Ridge, African/Red Sea Rift Valley. Volcanic activity may form islands, e.g. Azores, Cape Verde which then move away from the ridge.

Plates slide past one another along transform faults. Material is neither lost nor added. Movement is not smooth but restrained by friction so strain may build up and is released as earthquakes. Can occur in the oceans but best known from the continents where they cause earthquakes. San Andreas Fault, California.

Destructive margin (convergent)

The edge is destroyed as plates slide under or are compressed. Where oceanic trenches form, there is a negative anomaly over the trench and a positive anomaly over the excess material on the landward side. The oceanic trenches may be very deep, e.g. Marianos Trench (11.5 km), Challenger Deep (11.5 km), Puerto Rico Trough (8.4 km).

There are three types of destructive margin:

Oceanic/continental

• Subduction - the oceanic crust dips below the less dense continental crust, initially at an angle of 33/34°, then at 61° (average angle 45°), often oblique due to lateral motion of the plate. An oceanic trench forms. Earthquakes occur along this Benioff Zone - focal depth increases with distance from the margin as the subducted plate gets deeper (shallow 70 km, intermediate 30-70 km, deep 70-700 km). At about 700 km depth the slab is assimilated and the old oceanic crust is destroyed.

• An accretionary wedge can form from sediment scraped off the descending slab and buckled and folded into the mountain range.
• The continental crust overlying the subduction zone forms an active continental margin - a line of volcanoes parallel to the trench. The descending slab heats the rock (friction) and releases fluids at 100 km which lower the melting point so molten basaltic magma rises. Material of different densities rises at different rates, the lighter fractions reaching the surface first. So:

• near surface, mainly Basalt (heavy), e.g. Eycott lavas - fairly rare
• deeper, Andesite, e.g. Borrowdale Volcanics
• very deep, Rhyolites and Trachytes (light, Basalt left behind), e.g. N Wales

NB Deep and intermediate earthquakes are restricted to the Benioff Zone, shallow focus earthquakes can occur at trenches, ocean ridges and transform faults.

• Subduction occurs producing a curved oceanic trench and an accretionary wedge plus volcanoes forming an island arc. Tonga, Phillipines, Japan. Tends to be Basalt, Andesite. Island arcs show a strong positive gravity anomaly as they are composed of heavy basalt. An arc may be a single chain (Aleutions) or double (Japan).

• Many island arcs are the result of compression, metamorphism and isostatic uplift of marine sediments, composed of:

• basalt pillow lava
• andesite lava

• No subduction because the crust is not dense enough. The crust thickens to form a mountain range - higher and deeper by solid state crystalline flow (creep). Himalayas, Alps, Andes, Caledonian Range.

Hot spots may produce volcanoes. As the lithosphere moves over a hot spot a series of volcanic islands may form, e.g. Hawaiian Islands. The islands may erode and sink to form seamounts.

Yellowstone is also over a hot spot.

• May form along both constructive (ocean ridge) and destructive (ocean trench) plate boundaries.
• They slide past one another without gain or loss, i.e. they are themselves conservative.
• Due to stress generated by movement around the pole of rotation.
• The fault plane is vertical.
• The movement is horizontal and parallel to the strike (slip-strike fault).
• The active part is the region between the two ridge segments, induce earthquakes of shallow focus (less than 15 km) - very destructive. Example, San Andreas Fault and Anatolia Fault (Turkey)
• They form as a continent splits to form divergent plates. The pattern matches that of the coast lines.
• They do not change with time. The distance between ridges (active part) remains the same.