Pumice is a lightweight rock that sometimes forms when lava is very rich in volcanic glass.
Step-by-step explanation:
Pumice is a lightweight rock formed after cooling of lava erupted from the volcano. These rocks are composed of minerals which are present in deep underground of volcano which after eruption in high temperature and pressure comes out as lava.
As time goes the lava will get cooled down and form rocks which can be classified as igneous rocks. One of the rocks formed from lava are termed as Pumice.