Hot lava samples provide important information about what's going on in a volcano's magma chambers.
We know from laboratory experiments that the more magnesium there is in magma, the hotter it is. Chemical analysis, therefore, provides the means not only to determine the crystallization history of lava but also to establish the temperature at which it was erupted.
For example, Kilauea's 1997 lavas are chemically different from lavas erupted from 1985 to 1997. Chemical analyses show that magma was supplied by two distinct magma bodies.
Learn more: USGS Volcano Hazards Program