Final answer:
Volcanic rocks are classified by silica content, ascending from basalt through andesite and dacite to rhyolite, with obsidian as a glassy form and pumice as a vesicular variety.
Step-by-step explanation:
Volcanic rocks can be classified by their silica content, which affects their properties and the way they are formed. The types of volcanic rocks from the lowest to highest silica content are basalt, andesite, dacite, and rhyolite.
Basalt is a mafic rock with a low silica content and a high content of iron and magnesium. Andesite has a moderate silica content and may contain minerals such as plagioclase and pyroxene. Dacite has a higher silica content with a physical appearance between andesite and rhyolite. Finally, rhyolite has the highest silica content among these volcanic rocks and often has a light color and a high-viscosity lava that cools quickly.
Volcanic glasses such as obsidian are formed from rhyolitic composition lava with rapid cooling, resulting in a glassy structure without crystals. Vesicular volcanic rocks, like pumice, contain gas bubbles that give them a frothy texture. It's important to understand these characteristics to correctly identify the various types of volcanic rocks.