Sleeping Giant
      Right now, an eruption is brewing in Yellowstone National Park. Sometime during the next two hours, the most famous geyser. Old Faithful, will begin gurgling boiling water and steam._____1_____
      Old Faithful is not only a spectacular sight; it’s also a constant remainder that Yellowstone sits on one of the largest volcanoes in the world. If you’ve never heard of Yellowstone’s volcano, you’re not alone._____ 2_____ yet it has erupted three times during the last million years. And one of those eruptions spewed enough volcanic ash and other debris to blanket half the United States. 
      Yellowstone’s volcano is sometimes called a “supervolcano,” or extremely large and explosive caldera volcano._____3_____. This supervolcano formed over a hot spot, an extremely hot area in Earth’s mantle. John Valley, a volcano professors, said that as the crust moves across a hot spot, the hot spot melts a section of the plate moving over it,forming “one volcano after another.” 
      The Yellowstone hot spot melts thick continental crust, which may cause catastrophic eruptions. According to experts the eruptions that created each of the three calderas in and around Yellowstone National Park were larger than any other volcanic eruption in recorded history. The most recent eruption, which happened 640,000 years ago, produced at least 1,000 cubic kilometers of ash and debris, which blanketed most of the western half of the United States._____4_____
Geological evidence shows Yellowstone has blown its stack every 700,000 years or so. “If nature were truly that regular and reliable, we would be due for another eruption soon,” said Valley. “However, these processes are subject to variability, so we don’t really know when the next eruption will happen.”
      _____5_____It is the volcanic energy that powers the geysers and hot springs, creates the mountains and canyons, and generates the unique ecosystems that support Yellowstone’s diverse wildlife.

A  Three calderas make up more than a third of Yellowstone National Park.
B  The first Yellowstone eruption, 2 million years ago, released more than double that amount of ash and debris.
C  The volcano is so inconspicuous (不显眼的) that few people know it exists.
D  Then, an enormous fountain will shoot high into the air.
E  While the active geologist processes at Yellowstone do pose some risk to the public, they also make it a unique treasure.
F  Yellowstone National Park attracts the interest of geologists the world over.