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A volcano is a common geomorphic form which is the mountains formed by the underground molten material and solid debris carried by it, rushing out of the ground. There is a "liquid zone" below the earth's crust around 100-150 kilometres. In this zone, molten silicate materials are containing volatile components of gas under high temperature and high pressure, namely magma. Once it rushed out of the ground from the weak part of the earth's crust, it formed a volcano. Volcanoes are divided into "active volcano", "extinct volcano" and "dormant volcano". Active volcanoes are volcanoes that are erupting or have erupted in the past 10000 years, and those dormant volcanoes, even if they are active but not eruption now, maybe called active volcanoes in the future. The eruption of active volcanoes is also intermittent, but the length of the interval varies.

The United States has more than 10 percent active or potentially active volcanoes on the planet which is one of the most volcanic countries in the world, there are more than 150 of which have erupted in the past 10,000 years. In terms of the number of volcanic eruptions, it is second only to Indonesia and Japan.The United States Geological Survey (USGS) identified 169 volcanoes to be active in the United States, however, there are only 55 active volcanoes that can be considered to pose an extremely high threat to people. The majority of them are located in the Aleutian Islands, Alaska Peninsula, Hawaiian Islands, and the Cascade Mountains in the Pacific Northwest, such as Kilauea, Mount St. Helens, Washington and Yellowstone, Wyoming. Although the Cascade Range volcanoes do not erupt as often as Hawaii or Alaska, they are considered more dangerous. This is because they are more explosive volcanoes and are close to highly populated Washington, Oregon, and California. The Alaska Peninsula and Aleutian Islands contain approximately 80 important volcanic centres, all of which have one or two volcanoes.Alaska has had one or two volcanic eruptions each year since 1900. Over the past 10,000 years, there have been at least 20 devastating volcanic eruptions, including the Novarupta volcanic eruption in the Katmai National Memorial in 1912. And even though most of the state is sparsely populated, volcanic activity still poses many potential dangers.

Geography
The United States is located in the Western Hemisphere and consists of the United States, Alaska and Hawaii. It is on the North American plate, and it faces the Pacific plate in the west. From the Atlantic Ocean to the Pacific Ocean, almost across the entire North American continent, the land area accounts for 7% of the world's total area, ranking fourth in the world. The terrain of the United States is high in the west and low in the east. The coastal area on the east coast has a coastal plain, wide from the south to the narrow from the north to New Jersey. There are also some glacial sedimentary plains in Long Island and other places.

Contributing factors
The majority of active volcanoes are located in the Cascade Mountains and Hawaiian Islands, which results of the subduction of the Pacific plate into the North American plate. A theory called plate tectonics was developed by scientists in the past 25 years to explain the locations of volcanoes and their relationship to other large-scale geologic features. In fact, most volcanoes are located in the edge of the continent, along the island chain or under the sea forming deep mountain ranges instead of distributed on the surface of the earth randomly. This theory described there are twelve large plates on the surface of the earth, which move at a speed of less than one centimeter to ten centimeters each year. The average thickness of these rigid plates is about 80 kilometers. They are spread apart, and slide against each other or collide with each other in the high temperature and flexible interior of the earth with a slow motion.Volcanoes often form either where plates collide and spread or grow in the middle of plates, such as Hawaiian volcanoes..

The most dangerous Volcanoes
Kilauea Volcano, Hawaii

Kilauea Volcano is located in the Kilauea active crater on the southeast slope of Mauna Loa Volcano in the south-central Hawaiian island of the United States. Kilauea is the youngest and most active volcano in the world. Hundreds of thousands of cubic meters of magma are ejected from the crater on the island every day. Eruptions have not stopped since 1983 and there is a huge caldera at the top of the mountain, with a diameter of 4027 meters and a depth of more than 130 meters, including many craters. Kilauea volcano is one of the most dangerous volcanoes because it can cause great disaster to humankind. Damage to trees, wildlife, and buildings on the island of Hawaii.

Mount Saint Helens, Washington
St Helens volcano is located in the northwestern United States Washington State, at an elevation of 2,549 meters, belongs to the Cascade Range. Among the many volcanoes in the Cascade Mountains, St Helens is a relatively young volcano that formed about 300,000 years ago.

The volcano has a recorded large-scale volcanic eruption that occurred in May 1980, killing 57 people and covering large areas of the western United States with ash. The volcanic eruption cut off the mountain 300 meters, and 2.5 cubic kilometers of debris caused by the collapse directly washed into the valley below. After 2004, volcanic activity gradually became active and produced small-scale eruption.

Mount Rainier, Washington
The scientists believed that Mount Rainier is one of the most dangerous volcanoes in the Cascade Range although it has not erupted since 1890 and it often occur earthquakes. There are 26 glaciers in Mount Rainier which contain more than five times the snow and ice content of all other Cascade volcanoes, a huge lahal will be triggered by enough water that generated by a small part of ice which is melted by volcanic activity. Also, the nearby cities is a threat of Mount Rainier eruption, such as Seattle and Washington, the melting of the glaciers caused by the eruption would be fatal to more than 150,000 people living in the former Lahal (landslide) deposit.

Mount Hood, Oregon
The altitude of Mount Hood is 11,250 foots which is an active volcano. The seismic activity increased in the 20th century although it has been relatively dormant for the past 15 years. The Mount Hood is close to two major cities, Portland Oregon and Washington State Washington therefore the dangers of it is similar to the Mount Rainier.

Three Sisters, Oregon
The type of volcanoes of Three Sisters are two Stratovolcano and one shield volcano. It is one of the most dangerous volcanoes in the United States that is unique. The highest peak of Three Sisters is 10,363 foots, and the activities of southern mountains has increased. An eruption from one mountain may cause a chain reaction on another which makes it dangerous. Moreover, the eruption of volcano can wipe out towns and attractions because they are located near the resort.

Yellowstone Super Volcano
The Yellowstone Super Volcano belongs to Caldera with an elevation of 9203 foots. This super volcano has erupted more than 1,000 cubic kilometers of magma, the last super-eruption occurred 2.1 million years ago and erupted 2,450 cubic kilometers of magma. If Yellowstone Park erupted on that scale today, volcanic ash would cover most of the United States and could cause global temperatures to drop.

Mount Shasta, California
The type of this volcano is Stratovolcano which has not experienced a major eruption for more than 250 years. Mount Shasta is dangerous because its unique geological structure created a hard rock, blocking the rock's air vents, until an explosive eruption was formed. More importantly, its eruption may destroy nearby towns, and it is so powerful that it may collapse the mountain.

Eruptive History
Mauna Loa: It has erupted 15 times since 1900, with eruptions lasting from less than 1 day to as many as 145 days and the most recent eruption is on March 25, 1984.

Kilauea Volcano:


 * 1) A massive eruption of Kilauea volcano in 1960 filled a new land of about 2 square kilometers by the sea.
 * 2) Another eruption in 1986 added 6.8 square kilometers of new land to the Big Island.
 * 3) They erupted again in July and August in 2002.
 * 4) In 2014, the lava erupted from Kilauea volcano flowed to nearby residential areas and destroyed the houses in the town of Pahowa.
 * 5) After the Kilauea eruption in 2016, lava flowed to the Hawaii Volcanoes National Park and devoured the forest, the magma flows from the crater and flows into the Pacific Ocean. The cooled and solidified magma will form new land.
 * 6) Volcanic eruptions continue to intensify in 2018 from May toJuly has caused many injuries.

Mount Hood, Oregon: This volcano is dormant for 1000 years, it has erupted in 1781 and lasted for a decade.

Mount Shasta, California: The last eruption occurred in 1786.

Mount Saint Helens, Washington: It erupted in the late 1700s, 1800-1857, 1980-1986, and 2004-2007.

Effects of Volcanoes

 * 1) The eruption of  these volcanoes can damage the infrastructure and property such as houses, roads and fields, it can also threaten the health and safety of residents, for example, falling ash would make residents hard to breathe. In 1980, the eruption of Mount St. Helens destroyed a huge forest and those people are tens of miles from the volcanic source have been killed, the major river has been choked and ridge has been destroyed. Additionally, in 2018, Kilaues Volcano has erupted which destroyed more than 700 structures and the lava covered 35.5 square kilometers, including houses, roads, farms and highways. Moreover, Airborne ash caused a huge damage to aircraft which worse hundreds of millions of dollars and disrupted the businesses and lives for people who lived in Washington and Alaska, According to research, the loss of life caused by noxious gas emissions in California and the prevalence of human respiratory ailments in Hawaii.
 * 2) The lava can cause the death of plants and animals. In 1980, there are around 24,000 animals dead which results from the eruption of  Mount Saint Helens volcano, including hares, deer, bobcats, black bears and lions.
 * 3) Volcanoes can also affect weather. The eruption of volcanoes can emit a mixture of gases and particles into the air when volcanoes erupted, these gases have cooling effect such as ash and sulphur dioxide, particles of dust and ash and carbon dioxide which is a greenhouse gas to promote global warming, causing "greenhouse effect".