The first objectives for your students will be to learn that all mountains are not volcanoes. This is best illustrated by asking questions of the students to find out what their perceptions are about volcanoes. Use the geography nearest you, if possible, so that students can relate to what you're talking about. Explain what makes a mountain different than a volcano and give examples.
The second objective will be to dramatize for your students how volcanoes erupt. This can be the focus of the assignment and really engage your young learners. Do this by creating a volcano model together in class. While building the model, address the other lesson objectives and still keep your students engaged. Before building the model go over lots of pictures with the class so that they have a true sense of what a volcano looks like.
The third objective will be to address what causes volcanoes. Do this together as you work to build the volcano by asking questions and have them act out what it is to be a volcano. Young learners like to use their bodies, so have them get on all fours on the floor. Then tell them that inside of them is hot liquid, just like in a volcano, which is going to come out. Have them begin to rise up from the floor as the hot liquid rises inside of them. When they're ready to erupt tell them the lava is coming out of them and have them jump up and throw their hands toward the ceiling, simulating an eruption.
The fourth objective will be to consider how after an eruption a volcano changes the landscape. This can be a difficult concept for young learners to grasp so be patient and give them lots of hands-on work to arrive at the answer. First, finish building your classroom volcano. Then have it go through the eruption process. When it is done ask the children to talk about how the volcano has changed and how it has changed the landscape around it.