There several different kinds of volcanologists that study different aspects of volcanoes. Geologists study the deposits of volcanoes and derive information on what the volcano has done in the past and what it might do in the future from these deposits. Geochemists study the gasses emitted from volcanoes and derive information about the magma and possibilities of an eruption. Geodisists investigate the swelling and shrinking of volcanoes and can determine whether the magma chamber is growing. Petrologists can look at the minerals and geochemical composition of the rocks from volcanoes and determine the conditions, such as pressure and temperature, of the magma chamber that produced the rocks. Remote sensors use satellite images and data to determine various characteristics about volcanoes. Of course many of us actually do work that incorporates aspects of several of these disciplines.