The Alaska Volcano Observatory updates activity continuously. The contents can be adapted to several age groups.
This award-winning Volcano World at the University of North Dakota provides the history and hints for interpretation of basic volcanic activity on our globe. The use of remote-sensing tools is explained.
What Happens When Lava Meets Ice?
This Science Channel video looks at what happens when you pour molten lava on ice. (Hint: it's not what you'd expect.) They interview the scientists involved, describe how they create the lava, and show some other tests they are running. You can find a longer video of just the lava pour here.
Remember kids, don't try this at home!
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Virtual field trips can be made to key volcanoes--active and extinct. For instance, the Scripps Oceanography Institute sponsors virtual/real expeditions to the volcanoes in Costa Rico.
Volcano Cafe tracks the activities of volcanoes around the world. Postings range from current eruptions (especially in Iceland) to volcano formation and geology to a look at when one might erupt and why it's so hard to pin it down.
Vinegar Plus Baking Soda Volcano Eruption in a Vacuum Chamber
Here's an interesting take on the classic classroom demonstration.
While using a full vacuum is a little overkill, using a partial vacuum can better demonstrate the pressure difference and explosivity when lava emerges from a volcano. Makes it a great classroom project for older students, allowing them to make predictions, take measurements and write up their results.
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This category of sites lets students monitor changes from the safety of their computer. Students could follow such changes for a whole school year, thereby engaging in sustained inquiry.