Hot Spots

Hotspots are not always found on the ridges of tectonic plates, but above matle plumes, where the heat of the Earth's mantle makes a column of hot material that increases until it get's to the crust, which is inclined to be slimmer than in other places of the Earth. The temperature of the plume makes the crust melt and create pipes, which can expel magma. Because the tectonic plates move but the mantle plume stays in the same place, each volcano becomes dormant after a period of time and a new volcano is then created as the plate moves over the hotspot. The Hawaiian Islands are thought to be produced in such a way, aswell as the Snake River Plain, with the Yellowstone Caldera being the section of the North American plate presently above the hotspot.
Picture URL:http://images.google.ie/imgres?imgurl=http://www.greenpeace.org/raw/image_full/russia/en/photosvideos/photos/lake-troitskoe-in-the-volcano.jpg&imgrefurl=http://www.greenpeace.org/russia/en/photosvideos/photos/lake-troitskoe-in-the-volcano&h=323&w=430&sz=72&hl=en&start=30&usg=__BJhvZFxicsT6lhmkHQ2eU3UTWSg=&tbnid=g5YGJQDdW_bfmM:&tbnh=95&tbnw=126&prev=/images%3Fq%3Dvolcano%26start%3D20%26gbv%3D2%26ndsp%3D20%26hl%3Den%26sa%3DN
The picture above is an example of a dormant volcano.