Final answer:
Felsic magmas, with higher silica content, are more viscous than mafic magmas, which have lower silica levels and are less viscous. Mafic lavas, like basalt, flow more easily due to this lower viscosity, whereas felsic lava flows much slower.
Step-by-step explanation:
Felsic magmas are more viscous than mafic magmas. This increased viscosity is due to the higher silica content in felsic magmas, which include rocks such as granite and rhyolite. Mafic magmas, rich in magnesium and iron with rocks like basalt and gabbro, have lower silica levels and are consequently less viscous.
The color of an igneous rock is indicative of its composition. Mafic rocks tend to be dark due to their abundant ferromagnesian minerals. In contrast, felsic rocks are generally lighter in color since they contain more nonferromagnesian minerals, such as feldspar and silica-rich quartz.
When assessing whether a rock is extrusive or intrusive, texture plays a key role. Extrusive rocks, like rhyolite, usually have an aphanitic texture, meaning the crystals formed in the rock are small due to rapid cooling on the Earth's surface. However, intrusive rocks, such as granite, have a phaneritic texture, where the crystals are larger because of the slower cooling within the Earth's crust.
In terms of lava flows, mafic lavas such as those from basaltic eruptions can flow quickly and cover great distances. This is because their lower viscosity allows them to move more easily. Meanwhile, the high viscosity of rhyolite lava causes it to flow much slower, which can result in a texture that appears to have flowed like oozing toothpaste.