Volcanoes 

Volcano Super Volcano 
Normal volcanoes are formed by a column of magma - molten rock - rising from deep within the Earth, erupting on the surface, and hardening in layers down the sides. Super volcanoes, however, begin life when magma rises from the mantle to create a boiling reservoir in the Earth's crust.
More than 80 percent of the Earth's surface -- above and below sea level -- is of volcanic origin. The last super volcano to erupt was Toba 74,000 years ago in Sumatra. Ten thousands times bigger than Mt St Helens.
About 500 active volcanoes are known on Earth, not counting those that lie beneath the sea. Yellowstone Park - is one of the largest supervolcanoes in the world. The last eruption was 640,000 years ago.
The largest eruption in the world this century occurred in 1912 at Novarupta on the Alaska Peninsula. The term "supervolcano" implies an eruption of magnitude 8 on the Volcano Explosivity Index, meaning that more than 1,000 cubic kilometers (250 cubic miles) of magma (partially molten rock) are erupted.
The May 18, 1980 eruption of Mount St. Helens (Washington) was the most destructive in the history of the United States. The most recent such event on Earth occurred 74,000 years ago at the Toba Caldera in Sumatra, Indonesia.
It cooled the whole Earth by 21 deg. F. 
Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 feet above sea level, its top being over 28,000 feet above the deep ocean floor. From its base below sea level to its summit, Mauna Loa is taller than Mount Everest. A supervolcanic eruption occurs somewhere in the world approximately every 50,000 years or so.
The United States ranks third, behind Indonesia and Japan, in the number of historically active volcanoes (that is, those for which we have written accounts of eruptions). There are about 40 known supervolcanoes around the world, most are extinct.
Seven volcanoes in the Cascades have erupted since the first U.S. Independence Day a little more than 200 years ago. The force of a supervolcanic eruption at Yellowstone would be the equivalent of 1,000 Hiroshima bombs exploding every second.
Mount Rainier, in Washington State, is the tallest (4,392 meters, 14,410 feet) volcano in the Cascade Range but it is only the third most voluminous volcano after Mounts Shasta and Adams . The supervolcanic crater at Yellowstone is over 80 kilometres long by 45 kilometres wide - large enough to hold the world's biggest city, Tokyo (population 18 million).
The Three Sisters area (Oregon) contains 5 large volcanic cones of Quaternary age-- North Sister, Middle Sister, South Sister, Broken Top, and Mount Bachelor. Temperatures would drop by up to 12 degrees in the northern hemisphere and up to 16 degrees in the southern hemisphere causing mass starvation as the life-giving monsoon rains would fail.
Some of the Earth's grandest mountains are composite volcanoes -- sometimes called stratovolcanoes. The total worldwide death toll from a Yellowstone super-eruption has been estimated at 1 billion.
Scientists use the term magma for molten rock underground and lava for molten rock (and contained gases) that breaks through the Earth's surface.  
Pyroclastic flows, which can reach 1500 degrees F and move at 100-150 miles per hour, are capable of knocking down and burning everything in their paths.  

Links on these 

http://volcano.und.edu/vwdocs/vw_exchange.html 
http://www.huttoncommentaries.com/ECNews/ECN1.html#CalderaActivity 
http://www.livescience.com/forcesofnature/050308_super_volcano.html 
http://pubs.usgs.gov/fs/2005/3024/
http://vulcan.wr.usgs.gov/LivingWith/VolcanicFacts/misc_volcanic_facts.html#earths_surface