Create a Google Form with one short answer question. An example may be, Describe the volcano in one word. Export all of the students' answers into a spreadsheet. Drag down to highlight and copy all of the words. Paste these into a word cloud generator. There are several but my new favorite is ABCya's Generator. A list of word cloud generators is below.
Tagxedo (This is Gail's favorite!)
Another great feature about using the Google Form is that you can click on Form, then choose Show Summary of Responses, and there are great graphs which summarize your data!