Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The color distinction comes from the differences in iron and magnesium content. Igneous rock.
Igneous rocks are classified on the basis of their composition and their texture. As we just learned, there are two main types of igneous rocks: How Are Igneous Rocks Classified? The compositions of typical mafic , intermediate, and felsic magmas are shown in Figure 3.
Examples includeobsidian and pumice. A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture.
A number of processes that take place within a magma chamber can affect the types of rocks produced in the end. Discuss the role of melting and subsequent cooling in the rock cycle. Volcanic rocks break down into two more categories: Cooled lava forms basalt with no visible crystals.
For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle. These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock.
Scoria has more holes and may be black or red in color. As magma cools slowly the minerals have time to grow and form large crystals. First, the rule does not work for glassy igneous rocks.
Magma is created when rocks are at the correct temperature and pressure to cause melting also know as passing the Liquidus aka melting point with respect to pressure and temp … erature. Different cooling rate and gas content resulted in these different textures.
Once you have determined the texture and composition of an igneous rock, you can name it and you can also say something important about how it formed. Figure 5.
Share to: Share This Book. Would you like to merge this question into it? As magma cools, elements combine to form minerals.