Surrounding Rock Are Called: The minerals become concentrated by magmatic segregation. Examples of deposits formed by this process are the chromite [5] deposits of South Africa, the famous iron ore deposits of Kiruna, Sweden, and the nickel sulphide of Sudbury, Ontario. Deposits formed at the same time as the surrounding rock are called syngenetic. When a mineral deposit is formed later than the surrounding rock it is known as epigenetic.
If the direction from which the wind comes varies, the pebble will acquire several flattened surfaces giving it a pyramidal shape. Such a pebble is called a ventifact or a drei-kanter. The sand also erodes the solid rock over which it is blown, etching out any softer or weaker parts and leaving the harder rock standing up in ridges called yardangs, or in "rock mushrooms" called zeugens [6].See Also Molten Rock:The movement of the earth's crust may carry the rock as much as 700km (454 miles) below the surface. Here the temperature and pressure will be even higher and the rock will begin to melt. Molten rock is lighter than solid rock and it will begin to rise up through the overlying rock towards the surface. If it reaches the surface as a lava flow it will immediately be ready for weathering and erosion and the start of a new cycle. More often the molten rock solidifies underground and then all the rock above it must be eroded away before it can begin the cycle again.
The earth's upper mantle, under the crust, is nearly molten. A slight drop in pressure caused, say, by crustal plates drifting apart, completes the melting process. The molten rock (magma), being lighter than the surrounding rocks, then rises slowly to the surface, often along faults. A small increase in heat, will also melt the rock and it is believed that pockets of radioactive elements generate enough heat to form magma.
On The Other Hand See Metamorphic Rock:Metamorphic recrystallization may be A thorough and accompanied by marked change chemical compositions, especially if it takes p at depths where high temperatures and pressi prevail. The distinction between igneous metamorphic rocks then becomes vague. Not naturally there are many geological controver concerning the origin and mode of formatior these mixed igneous-metamorphic rocks, or r matites. Granite was formerly regarded a typical igneous rock, but it is now thought some granites may have crystallized from n mas, while others may have been formed by thorough recrystallization of pre-existing ro Distribution of Rocks.
A third, rare type of metamorphism is dislocation metamorphism, caused by large areas of rock moving past one another. The pressure shatters the rock and the friction is so great that the rock is partially melted, proŽducing a rock called mylonite. It occurs only in narrow strips, of which the Lizard, in CornŽwall, England, is an example. Unlike sedimentary rocks, metamorphic rocks are of no great use to mankind, containing no oil and few useful minerals, and are of little use in building, apart from slate for roofing and marble used in decorative work.
|
