What Is The Lightest Rock In The World?

lightest rock, pumice

A piece of pumice. Photo via Flickr.

The lightest rock in the world, similar to the heaviest rock, is an igneous rock.  Do you know what this rock, which is so light it can float on water, is called?

The other day I had a post about the heaviest rock, so now is time to talk about the opposite – the lightest rock.  This came up because my son and I were talking about a piece of pumice that he found.  It turns out that pumice is the lightest rock.  It is so light that it momentarily confuses you when you pick it up the first time since you expect it to be heavier.  Here is more about pumice.

Rocks Are Made Up Of Minerals

The smallest building blocks of rocks are called minerals.  There are over 500 minerals in the world that combine in different ways to make the rocks of the world.

What Is The Lightest Rock or The Least Dense Rock?

The kind of rock with the lowest density, making it the lightest, is pumice.  It has a density of less than one.  If you remember from the post on the heaviest rock, water has a density of 1.  Most rocks have a density of between 2 – 4.  The density of a particular piece of pumice varies but generally is less than one.

lightest rock, pumice

Dark pieces of pumice. Photo via Flickr.

Why Is Pumice So Lightweight?

Pumice is so lightweight because it is porous.  That means that, like a sponge, it is full of holes.

In fact, it is so lightweight that it can float on water.  If you google pumice, you may even see examples of a pumice island.  This is a huge amount of pumice floating on the ocean after an eruption.

lightest rock, volcanic eruption

A volcanic explosion. Photo via Flickr.

How Was the Lightest Rock Formed?

The secret to its lightness lies in how it was formed.  Pumice is an extrusive igneous rock that was formed in an explosive volcanic eruption.

It is full of holes due to the expansion of volcanic gases during its formation.  It is created when a volcano erupts sending out liquid magma that is full of gases.  Then it cools so fast that it retains the spongelike structure, full of holes.  It is like a carbonated drink exploding, which then cools so fast that the foam solidifies into a rock form.


Pumice can come in a variety of colors, depending on the amount of silica in it.  It can be white, yellow, brown, or even black.  The most common pumice is white, but other unusual ones are black.

It can be found in the Rocky Mountains, mainly in Idaho and Wyoming.