Your question on writing a thesis statement for an essay on volcanoes is dependent on many items.  For example, is your essay about the kinds of volcanoes, the history of volcanoes, a comparative essay about the eruptions of Vesuvius in ancient times and Mt. St. Helens in modern times, or an argumentative essay about whether scientists should be sounding the alarms when small eruptions may happen? Once you have decided what focus your essay will take and how long it must be, then look at the main points you plan to cover in the body of the essay.  When you are ready to write and are finished with the rough planning, then write your thesis.  A thesis should be your statement about volcanoes that you intend to prove.  For example, if you are doing a comparative essay, a good thesis statement might be that "whether volcanoes erupt in ancient or modern times, many similarities (and differences) exist between the two volcanic eruptions even though the eruptions were centuries apart."  I hope this helps you with your thesis statement which will control what you can write about for the whole of your essay.