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This is Faith Lapidus. And this is Doug Johnson with EXPLORATIONS in VOA Special English.

Today we tell about Yellowstone National Park. It is one of the most beautiful national parks in the United States. However, an ancient and extremely violent volcano created the great beauty of Yellowstone. The ancient volcano that formed Yellowstone is not dead. It is only sleeping. And some experts say it could become very dangerous in the future.Yellowstone National Park is in the western state of Wyoming. It is one of the most unusual places in the world. Extremely hot water shoots out of the ground in several hundred places. Small lakes contain water that is so hot it is dangerous to come too close.

Visitors can watch bubbles coming up through boiling hot mud. They can see rocks that were once liquid and have cooled into strange shapes.

Yellowstone is built on an ancient volcano. A lake of hot liquid rock is about six kilometers under the park. This lake is about 65 kilometers wide. Experts say this lake is under huge amounts of pressure. The pressure and heat cause hot water to shoot out of the ground and mud to boil at Yellowstone.

The Yellowstone volcano has often been called a super volcano because it is so big. Scientists believe major volcanic activity in the Yellowstone area began about two million years ago. This activity created violent explosions and built mountains and valleys. Experts believe this super volcano had three major explosions called eruptions. Each of those three eruptions may have been more powerful than any in recorded history.

Each eruption threw out millions of tons of ash and rock. The last eruption was so huge it covered much of North America with ash. Some of this ash traveled high into the atmosphere and was carried by strong winds around the world.

This cloud of volcanic ash circled the Earth many times. It affected the climate by limiting the amount of sunlight that reached Earth's lower atmosphere and surface. This last eruption formed the mountains and valleys that visitors can see today in Yellowstone.

When a volcano erupts, a huge amount of material explodes out of the volcano. This leaves a giant circular hole in the ground, called a caldera. Experts have known for many years that Yellowstone was formed by volcanic activity. However they could not find the caldera. Many experts searched a huge area in and around Yellowstone Park looking for the remains of the caldera. A few years ago, the National Aeronautics and Space Agency tested new satellite cameras. They offered Yellowstone Park officials photographs of the park taken from space.

A national park expert was extremely surprised when he looked at the photographs taken from space. He immediately saw the caldera they had been searching for. The photographs showed that Yellowstone National Park is the caldera. The caldera is about 70 kilometers long and 30 kilometers wide.

In fact, the park is a system of calderas formed within the past 16 million years. Experts now believe that as many 15 or 20 smaller eruptions also formed calderas.Scientists know the volcanic heart of Yellowstone is deep within the Earth. This area is called a hot spot. It is only one of a few such places on Earth.

Extreme pressure deep in the Earth forces liquid rock up through the hot spot to the lake of hot material that is below the surface. This causes the extreme heat that is found in Yellowstone.

Scientists also know the Yellowstone hot spot is linked to the activity of the North American plate. The North American plate is one of several plates that make up the surface of the Earth. These plates move a few centimeters each year. The hot spot does not move. Very often the action between the hot spots and the plate causes great earthquakes as plates move against each other. The plates often split apart. And often after earthquakes, the hot spot forces liquid rock to the surface. This has not happened for several thousand years.

The Yellowstone hot spot has been linked with the North American plate for as long as 17 million years. At many different times, the hot spot has caused a kind of liquid rock called basalt to explode to the surface.

This basalt rock from the Yellowstone hot spot can be found in the western states of Washington, Oregon, California, Nevada and Idaho. Evidence of this basalt rock can be found in an area as large as 322,000 square kilometers.Yellowstone National Park is the oldest national park in the world. About three million people visit it each year. Its great natural beauty has made it one of the most popular national parks.

Most visitors like to see "Old Faithful," the world's most famous geyser. A geyser shoots hot water high into the air. There are more than 300 geysers in Yellowstone.

Old Faithful is not the biggest or the most beautiful geyser. But it is the most popular. Visitors gather around Old Faithful before each eruption. Experts at the park are able to predict when these will happen. The average time between eruptions is about 90 minutes. Old Faithful shoots water an average of 40 meters into the air. This eruption lasts between two and five minutes. Old Faithful releases up to about 30,000 liters of water into the air each time.

The hot spot deep under the ground produces geysers like Old Faithful. Old Faithful is evidence of the volcanic activity at Yellowstone. But will the Yellowstone volcano erupt again? Most experts think the answer is yes. But no one knows when. The most recent of the three extremely powerful eruptions was about 650,000 years ago.

Experts say at least 30 smaller volcanic eruptions have taken place at Yellowstone. Some of these were perhaps as big as the 1991 eruption at Mount Pinatubo in the Philippines. Several are believed to have been much larger. The most recent of these smaller eruptions took place about 70,000 years ago.Volcano experts say it is extremely difficult to tell when Yellowstone might become an active volcano again. However, earthquakes near a volcano are usually good evidence that a volcano might become active again. For example, Mount Saint Helens in the northwestern state of Washington exploded in 1980. Several earthquakes took place near the volcano before that time. On the morning that it exploded, Mount Saint Helens experienced an earthquake of five point one on the Richter scale.

Yellowstone National Park experiences several thousand earthquakes each year. Most are very small. They cannot be felt. They can only be measured by scientific instruments. However, in August of 1959, an earthquake at Yellowstone measured seven point five on the Richter scale. Twenty-eight people were killed.

It was one of the strongest earthquakes ever recorded in the United States. But Yellowstone's sleeping giant volcano did not erupt.

In 2001, the United States Geological Survey, Yellowstone National Park and the University of Utah signed an agreement.

That agreement established the Yellowstone Volcano Observatory. Under the agreement, the park, the Geological Survey and the university are responsible for improving efforts to study the volcanic system of Yellowstone.

The observatory uses information from many different instruments on the ground and from satellites to study the volcano. The information will help officials decided if Yellowstone's huge volcano is becoming a danger. Then they could warn the public quickly if necessary.

Experts at the observatory say Yellowstone represents some danger to the public. It always has. However, its natural beauty also makes it a treasure that could not be possible without the sleeping giant volcano that is under Yellowstone National Park.This program was written by Paul Thompson. It was produced by Mario Ritter. This is Doug Johnson. And this is Faith Lapidus. Join us again next week for another EXPLORATIONS program in VOA Special English.

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