What does it mean for a good to be elastic ?

When a product is elastic, a change in price quickly results in a change in the quantity demanded. When a good is inelastic, there is little change in the quantity of demand even with the change of the good's price. ... If the market price goes up, firms are likely to increase the number of goods they are willing to sell.

Magma is outside the volcano  and lava is inside the volcano.