Click image for larger version. 
Igneous rock evolution does not just happen anywhere. It takes special plate tectonic conditions. A basic understanding of plate tectonics is necessary here, and if you are unfamiliar with the ideas go to this link for a quick, simple introduction. In terms of earth processes, fractionation occurs at two main locations, divergent plate boundaries, and convergent plate boundaries. Divergent plate boundaries (drawing or definition) are mostly under water so we do not easily see them working. But here magma rises up from deep in the earth and oozes out onto the ocean floor to form new oceanic lithosphere (simple drawing; or a more detailed drawing). In the process the parent rock of the earth's interior fractionates to form mafic igneous rocks, e.g. basalt and, at depth, gabbro.
At convergent plate boundaries (drawing or definition) part of the ocean lithosphere (created at divergent plate boundaries) descends into the earth again, where it heats up and fractionally melts (drawing). This generates intermediate rocks at first, such as diorite, but may eventually create felsic rocks such as granite.
On an earth time scale, igneous fractionation is responsible for the formation of all the world's volcanic arcs and continents, the implication being, the earth began without continents, and the total size of the continents has grown with geologic time. Contemplate the earth with no continents. It is easy to appreciate the importance of igneous fractionation to just about everything about the earth.
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 processes, and to the history of the earth. At its simplest, continents are made of felsic igneous rocks (such as granite), ocean basins made of mafic igneous rocks (such as basalt and gabbro), and volcanic arcs of intermediate igneous rocks (such as diorite and andesite.)
0 komentar:
Posting Komentar