Final answer:
Pumice and obsidian are both created by explosive volcanic eruptions, but they differ greatly; pumice is a light, frothy rock that can float on water, while obsidian is a dark, smooth volcanic glass with sharp edges.
Step-by-step explanation:
Pumice and obsidian are two types of rocks that are formed from explosive volcanic eruptions. Both are classified as extrusive igneous rocks, which means they solidify from lava that erupts onto the surface of the Earth. However, pumice and obsidian have very different textures and physical properties due to the conditions under which they form.
Pumice
Pumice is characterized by its frothy texture, which is a result of the numerous gas bubbles that were trapped in the lava as it solidified. This texture gives pumice a very low density, and it's often light enough to float on water. Pumice is typically light in color, varying from white to light gray or light brown, and due to its porosity, it commonly features a rough, abrasive surface. Composed of volcanic glass, pumice does not have a true mineral structure.
Obsidian
In contrast, obsidian is a volcanic glass that is usually dark in color, commonly black, and has a smooth, glassy appearance. It forms from the rapid cooling of high-viscosity lava, which inhibits the formation of crystals. Because of its conchoidal fracture, obsidian can break with a curved surface, and freshly broken pieces can have incredibly sharp edges - sharper than any man-made material.