Final answer:
Silica content in volcanic rocks is primarily influenced by the composition of magma. Felsic magmas, high in silica, lead to the formation of viscous rocks like rhyolite, while mafic magmas, lower in silica, produce rocks like basalt.
Step-by-step explanation:
The silica content in volcanic rocks can vary highly due principally to the nature and composition of the magma. Magma is the molten rock stored in the Earth's crust. When magma erupts onto the surface, it is called lava. The silica content of the magma reaches the surface and influences the type of volcanic rock formed during solidification.
Different types of magma have varying silica contents, which determine the viscosity and type of volcanic rock formed. For instance, felsic magmas, like those that form rhyolite, are high in silica and typically result in rocks with higher viscosity and greater explosive potential when erupted. Conversely, mafic magmas, like those that form basalt, have lower silica content and are less viscous, allowing for smoother flowing lava.
Volcanic glasses such as obsidian are formed from high-silica magma due to rapid cooling, resulting in no time for crystals to form. Contrastingly, slow cooling allows for the formation of larger crystals in intrusive rocks like granite.