Question

Which of the following rocks can float on water?
A. Shale
B. Obsidian
C. Limestone
D. Pumice

Answer 1

Hint: There are three types of rocks. Igneous rocks are formed when magma or lava cools down and hardens. Sedimentary rocks are formed due to accumulation or deposition of organic particles or minerals followed by cementation. Metamorphic rocks are formed when existing rocks are converted by heat, pressure, or active liquids, such as hot mineral carrying water.

Complete answer: Option A: Shale is a fine-grained sedimentary rock composed of clay minerals and mud mixed with other minerals, especially quartz and calcite. It is used to make brick, pottery, tiles, etc. Shale cannot float on water.
Option B: Obsidian is a naturally occurring igneous glass formed by the rapid cooling of lava from volcanoes. It is black or dark greenish in color and is extremely rich in silica. Obsidian does not float on water.
Option C: Limestone is a hard sedimentary rock which is composed mostly of calcium carbonate or dolomite and is used to make building material and for making cement. Limestone does not float on water.
Option D: Pumice is a light colored igneous rock that is formed during volcanic eruptions. Many pieces have a high enough porosity which makes them float on water until they gradually become soaked.

Therefore the correct answer is Option D: Pumice.

Note: A very similar rock to pumice is scoria. They must not be confused with each other. Scoria is always dark in color while pumice is always light grey in color. The major difference is that scoria does not float in water, while pumice does.