I lava you.
Ha, ha, right? My 7 year old used to tell that joke often, laughing every time. He is a volcano guy from way back. Here are some books that other young vulcanologists might appreciate, too.
Why Do Volcanoes Blow Their Tops? by Melvin Berger. Picture book, lots of facts, Q. & A. format. Includes directions for making a grand baking soda/vinegar/dish-soap explosive concoction using an empty soda bottle. Because of this book, "magma" has been a part of my vocabulary for the last four years.
Hill of Fire, by Thomas P. Lewis. Illustrated beginning reader about the farmer who stumbled across a volcano (the beginnings of one) while plowing. About the 1943 eruption of Mexico's Paricutin. A Reading Rainbow selection.
Volcanoes, by Stephanie Turnbull. From the Usborne Beginners series, a nice introduction to the subject, short bits of text, index, glossary, recommended web sites—all in 32 pages.
The Magic School Bus Blows Its Top: A Book About Volcanoes. You can't go wrong with Ms. Frizzle, the extraordinary science teacher, and her class.
Volcanoes! Mountains of Fire, by Eric Arnold. Advanced beginning reader about the eruption of Mt. St. Helens.
Volcanoes! by Jeremy Caplan. Another advanced beginning reader, with photos.
Volcano, by Nicholas Harris. Pompeii from the Ice Age into the present, with tabbed pages. Picture book for older readers. Part of a series called Fast Forward. Vesuvius included, of course.