M - R

Magnetic stripes Linear magnetic stripes, resembling a bar code, run parallel to mid ocean ridges. The stripes reflect repeated magnetic reversal of the Earth’s magnetic field. The magnetic anomalies are also symmetrical either side of the mid ocean ridges. The discovery of magnetic anomalies was crucial evidence for sea floor spreading.

Mantle The zone lying between the Earth’s crust and core. It includes the lower part of the lithosphere and all of the asthenosphere.

Mantle Decompression The process by which basaltic magma is generated at divergent (constructive) plate margins. As the plates move apart, pressure on the underlying mantle is reduced and it partially melts. Also known as decompression melting.

Mantle Plume Hot buoyant rock rising through the mantle from the core-mantle boundary. Thought by some geologists to rise beneath hot spots causing doming up of the crust. Cylindrical in shape, seem to be fixed in position and have a radius of around 150 km.

Matthews Drummond Matthews was the PhD supervisor of Fred Vine at Cambridge University. Together, they were the first to interpret correctly the pattern of normal and reversed magnetic anomalies in ocean floor basalts (eg in the Pacific Ocean floor next to California) as the result of sea floor (ocean-floor) spreading.

McKenzie Dan McKenzie is a Cambridge geophysicist who is known for his work on the structure and thermal properties of the mantle in relation to plate tectonics, and magma generation at mid ocean ridges and hot spots.

Metamorphism A ‘change in form’ as a result of the effects of mainly heat and/or pressure to produce a rock with a different texture and/or mineralogy.

Metamorphism, regional Large scale metamorphism (see above) involving heat and pressure. For example, at convergent (destructive) plate margins, and around large intrusions.

Mid Ocean Ridge The junction between two oceanic plates along a divergent (constructive) plate margin. The ridge comprises a submarine mountain chain of basaltic volcanoes and is up to 1.5 km higher than the adjacent abyssal plain. The central part of the ridge has a rift valley running through it and the ridge is offset sideways by transform faults eg the Mid Atlantic Ridge.

Mohorovičić discontinuity (Moho) The boundary between the crust and the mantle. The Moho exists at an average depth of 7 kms beneath the ocean basin and at an average depth of about 35 kms beneath the continents. It was discovered in 1909.

Mountain Building The process by which fold mountain belts are formed. These occur at convergent (destructive) plate margins. Another term for a fold mountain building period is an orogeny and the subsequent product is an Orogenic Belt. Evidence of past mountain building can be seen in the UK with the Caledonian and Variscan fold mountain belts.

Obduction The process whereby ocean crust (and even upper mantle) are scraped off the descending ocean plate at a convergent plate boundary and thrust onto the adjoining plate. Often associated with an accretionary wedge (prism).

Oceanic Crust The crust that forms the ocean basins. It is basaltic in composition, and comprises an upper layer of pillow lavas, a middle zone of vertical dolerite dykes and a lower layer of gabbro. The average thickness is 7 km and it is formed at mid-ocean ridges and subducted at ocean trenches. The oldest oceanic crust is less than 200 million years old.

Ocean Trench An elongate depression of the ocean floor which runs parallel to a volcanic island arc or mountain belt. Oceanic trenches are the deepest part of the oceans and can be up to 11 km. They are locations where oceanic lithosphere is being subducted back into the asthenosphere. An example is the Peru-Chile Trench which runs parallel to the west coast of South America.

Ophiolite A fragment of oceanic crust and mantle thrust onto a continental margin during ocean closure and at a convergent (destructive) plate margin. Examples in the UK include the Lizard Ophiolite in Cornwall (Variscan Orogeny) and the Ballantrae Ophiolite in Scotland (Caledonian Orogeny).

Orogenic belt A fold mountain belt such as the Andes or Himalayas (see mountain building).

Pangea The name given by Wegener to the supercontinent that he proposed to explain the ice distribution at 300 Ma. It came into being during the Permian and existed for around 40 million years. It was a single land mass comprising most of the world’s land areas joined as a single unit. It subsequently broke up into Laurasia and Gondwana.

Passive Margin An ocean-continent boundary that is not an active plate margin, i.e. there are no earthquakes at the margin. The east and west sides of the Atlantic are good examples at present. In the future, as the Atlantic continues to widen due to sea floor spreading in the centre, the ocean-continent boundaries will eventually become subduction zones with new convergent (destructive) margins being formed.

Palaeomagnetism The study of the fossil magnetism locked in rocks which record the inclination and direction of the Earth’s magnetic field at the time of their formation. Such data is used to determine the past arrangement of the continents over time and support the theories of continental drift and sea-floor spreading.

Peridotite A coarse grained, dark coloured igneous rock consisting mainly of olivine and pyroxene, which is believed to be the major constituent of the mantle.

Partial Melting Incomplete melting of rock to produce magma of a different composition from the original rock. Partial melting of the mantle produces basaltic magma. When partial melting occurs, the minerals with the lowest melting points will melt first, these tend to be the most silica rich minerals in the rock.

Plume see mantle plume

Plate see lithospheric plate

Pluton A medium sized igneous intrusion of up to 100 km² with a generally oval or circular shape in plan and steep or near vertical sides. The granite intrusions in South West England such as Bodmin Moor, Carnmanellis and Dartmoor are good examples. Most are associated with convergent (destructive) plate margins.

Polarity The Earth's magnetic field is known to reverse periodically. When it reverses, what was the north magnetic pole becomes the south magnetic pole, and vice versa. The current period of normal polarity has lasted for 780,000 years. A polarity reversal gives rise to a distinctive magnetic pattern in ocean floor basalts, with the reversely magnetized ocean-floor usually portrayed as white/pale stripes in illustrations, and normal polarity in black. Reversals take place in less than 10,000 years. The length of a polarity interval, either normal polarity or reverse polarity, is highly variable, but is roughly 1 million years.

Radiometric Dating A technique for determining mineral ages from measuring the amount of a radioactive isotope that has remained in a mineral (such as feldspar) since it was first created. As the mineral decays, its decay products accumulate and they can also be measured. These measurements, together with knowledge of the rate of decay, enable its age to be estimated. The most widely used methods of dating are Argon-Argon and Uranium-Lead. Age-dating of basalts of the ocean floor revealed they increased in age away from the mid-ocean ridges, in both directions.

Regional metamorphism See metamorphism, regional.

Rheic Ocean The ocean that opened between the continents of Gondwana and a collection of micro-plates including “Avalonia” during the Variscan Orogeny.

Rhyolite A fine grained igneous rock with the same mineral composition as granite. It is commonly associated with volcanoes at convergent (destructive) oceanic-continental plate margins. It is rich in silica, viscous, felsic and is characterised by infrequent but violent eruptions.

Ridge Push Mid-ocean ridges are sites where oceanic lithosphere is created. They are hotter and therefore stand higher than older, cooler lithosphere. As a result, the base of the lithosphere slopes away from the ridges. Under the influence of gravity, the lithosphere tries to move down this slope, creating a “ridge-push” force on the plate it is part of. The force is much smaller - perhaps only a tenth - of the force due to “slab pull”.

Rift Valley A central block downthrown between two normal faults that face one another. Continental rift valleys represent areas where continents are being stretched (extended), as in the case of the East African Rift Valley. They may eventually extend to the point where new ocean-floor is created, as in the Red Sea. Slowly spreading mid-ocean ridges, such as the Mid-Atlantic Ridge, may also have rift valleys running down their centres, but fast spreading ridges such as the East Pacific Rise, do not.