how do igneous rocks for
Sedimentary rocks often form in deltas and rivers because the constant flow of the river drags sediments from the river bed and deposits them in a confined area. Igneous rocks are formed when molten rocks cool. Magma and lava spewing from volcanoes are the two most common source for igneous rocks. Granite, pumice, obsidian and scoria are all examples of igneous rocks. Metamorphic rocks begin as either sedimentary or igneous rocks.