- A theoretical model of the formation of magnetic striping. New oceanic crust forming continuously at the crest of the mid-ocean ridge cools and becomes increasingly older as it moves away from the ridge crest with seafloor spreading.
- Early in the 20th century, paleomagnetic those who study the Earth’s ancient magnetic field such as Bernard Brunhes in France in 1906 and Motonari Matuyama in Japan in the 1920s recognised that rocks generally belong to two groups according to their magnetic properties.
- One group has so-called normal polarity, characterized by the magnetic minerals in the rock having the same polarity as that of the Earth’s present magnetic field. This would result in the north end of the rock’s “compass needle” pointing toward magnetic north.
- The other group, however, has reversed polarity, indicated by a polarity alignment opposite to that of the Earth’s present magnetic field. In this case, the north end of the rock’s compass needle would point south.
- This answer lies in the magnetite in volcanic rock. Grains of magnetite behaving like little magnets can align themselves with the orientation of the Earth’s magnetic field.
- When magma molten rock containing minerals and gases cools to form solid volcanic rock, the alignment of the magnetite grains is “locked in,” recording the Earth’s magnetic orientation or polarity normal or reversed at the time of cooling.
- In 1961, scientists began to theorise that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest.
- New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust.
- There are two types of plate: continental and oceanic. Continental is made up of sial which is older, lighter rock of granite type.
- It is dominated by mineral rich in Silica (Si) and aluminium (Al). Oceanic crust is made up of sima, magnesium and silica, and is younger and denser rock of basaltic composition.
- Continental (Sial)
- Oceanic (Sima)
- Thickness
- 30-70km
- 6-10km
- Age
- Very old over 150million years
- Very young 200 million years
- Weight/density
- 6
- 0
- Composition
- Granite, silica, aluminium
- Basalt, silica, magnesium
- The movement of the earth’s crustal plates is believed to be due to convection currents which occur in the semi-molten mantle.
- These convection currents are created by heat from within the earth much of which is generated by radioactive decay in the core.
- As semi-molten rock in the mantle is heated it becomes less dense than its surroundings and rises. As it reaches the crust above, it spreads out carrying the plates above with it. As the semi-molten rock then cools, it gradually sinks back down to be re-heated.
- Destructive plate margins are most commonly found around the edge of the Pacific Ocean.
- The pacific plate and Philippines plates move North West to collide with the Eurasian plate, while the Nazca, Cocos and Juan de Fuca plates travel eastwards towards south, central and North America.