<span>The Roman empire was physically united by good roads and bridges and secure sea lanes.</span>
Intrusive igneous rocks include gabbro, granite, and diorite. Each rock differs in that it was created by a separate silica-containing magma.
Despite having different silica concentrations, granite, diorite, and gabbro are all products of magmas. Gabbro is a typical rock that contains 45% to 55% silica. With a silica content ranging from 55% to 65%, diorite is a middle-ground rock. With a silica concentration of more than 65%, granite is an acidic rock.
The most common elements in the Earth's crust, silicon and oxygen, are combined to form silica, also known as silicon dioxide (SiO 2). Silica, which makes up the majority of more than 95 percent of all known rocks, makes up 59 percent of the Earth's crust. Quartz, which is by far the most prevalent crystalline variety of silica, tridymite, and cristobalite are the other two.
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