Basalt is a very common volcanic rock with low silica content.
Rhyolite is a volcanic rock with high silica content.
Most volcanic rocks share a number of common minerals.
Differentiation of volcanic rocks tends to increase the silica (Si O) content mainly by fractional crystallization.
On land, they are very common at plate boundaries and in flood basalt provinces.
Volcanic rocks are among the most common rock types on Earth's surface, particularly in the oceans.
Volcanic rocks are usually fine-grained or aphanitic to glass in texture.
They often contain clasts of other rocks and phenocrysts.
Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks.
For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct.