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Plate Tectonics and Igneous Rocks.

There are two ideas about igneous rocks that are geologically important. The first idea is that igneous rocks evolve they change from one kind of rock to another. The second idea is that rocks are not randomly distributed across the earth. Specific kinds of rocks are always found in specific places for specific reasons, all tied into plate tectonic processes. These ideas of igneous rock evolution and distribution are related, and eventually we will explore their relationship. Igneous rocks, evolution does not just happen anywhere. It takes special plate tectonic conditions.In terms of earth processes, fractionation occurs at two main location divergent plate boundaries and convergent plate boundaries. Divergent plate boundaries are mostly under water so we can not see them easily. But magma rises up from deep of the earth and oozes out into the ocean floor to form new oceanic lithosphere. In the process, the parent rock of the earth’s interior fractionates to form mafic igneous rocks, for example basalts and, at depth, gabbro.

At convergent plate boundaries, part of the ocean lithosphere descends into the earth again, where it heats up and fractionally melts. This generates intermediate rocks at first, such as diorite, but may eventually create felsic rocks such as granite. A final outcome of all this is that different igneous rocks are found in different places on the earth and all these different distributions are related to plate tectonic process, and to the history of the earth. At its simplest, continents are made of felsic igneous rocks, ocean basins made of mafic igneous rocks and volcanic arcs of intermediate igneous rocks.

Characteristics of Igneous Rocks:

  1. Rounded particles are asymmetrically arranged in igneous rocks.
  2. No fossils are found in igneous rocks.
  3. They are compact or massive.
  4. No layers but joints.
  5. They are hard and are less eroded by weathering.
  6. They are crystalline.
  7. The rocks are non porous.

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