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Two important types of volcano are difficult to recognize, especially when they are very large. The first of these is the \"giant caldera\" (see left). Calderas, which are simply circular depressions, are found on the summits of many volcanoes. \"Giant\" calderas are the largest of these: huge craters up to many tens of miles across. Giant Calderas form by collapse (see animation) in gigantic eruptions that spew volcanic rocks out hundreds or even a thousand miles in all directions. Sometimes the calderas are so filled with lava and volcanic ash that there is no recognizable depression at all. These can only be found by carefully locating the big fractures or \"faults\" in the ground that mark the edges of the caldera. One such caldera nearly fills Yellowstone National Park. In other cases, the edges of the caldera remain as large cliffs or ridges surrounding the central depression. However, the depression is so large that a person standing in the middle of it could hardly see the edges and would only recognize them if they were pointed out. These giant calderas can best be seen in images taken from space, like the one of the Valles Caldera in New Mexico (below,left).
The second type of volcano which is difficult to recognize either from the ground or from space is the \"fissure eruption.\" (shown left). In this type, there is no central crater at all. Instead, giant cracks open in the ground and expel vast quantities of lava that spread far and wide to form huge pools that can cover almost everything around. When these pools of lava cool and solidify, the surface remains mostly flat. Since the source cracks are usually buried, there is often nothing \"volcano-like\" to see--only a flat plain. The fissure eruption shown above at right was at Los Pilas volcano in Nicaragua in 1952.
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