Lab 2 5. intrusive rocks crystallize below the earth's surface, while extrusive rocks erupt and crystallize at the surface. why do the grain sizes (texture) vary in these two types of igneous rocks? 6. examine a sample of porphyritic igneous rock. how do you explain the existence of two different grain sizes in one rock? 7. if pumice is a rock, then how does it float?

Intrusive rocks are the igneous rocks that are formed due to the crystallization of magma below the earth's surface at a very slow rate. They are not exposed to the surface. For example, granite.  

Whereas, the extrusive rocks are igneous rocks that are formed due to the crystallization of magma on the earth's surface. The rate of cooling in this rocks is significantly very high. For example, basalt.

The size of the crystals formed in this two type of rocks are directly dependent on the rate of cooling of the crystals. When the rate of cooling is slow, then it forms coarse-grained crystals, whereas, when the rate of cooling is fast then it forms fine-grained crystals. Due to this reason, there occurs variation between this two type of rocks.

The porphyritic igneous rocks are the rocks which contains the porphyritic texture where the larger crystals are found to be embedded in between the smaller crystals or ground-mass. This larger crystals are called phenocryst and they are formed earlier than the smaller crystals. This is how two types of grain size occurs in the same rock.

Pumice rocks are light colored, less weight, and porous rock that are comprised of vesicles. These rocks are formed during volcanic eruptions and they have the ability to float on water. If the pores are filled fully with water then it eventually sinks.