Which of the Following States Was Formed by Continuous Volcanic Lava Flow

What is a volcano?

A volcano is an opening in the Earth's surface through which lava, volcanic ash, and gases escape. Ancient people believed that volcanoes were under the control of the gods. In fact, the word volcano comes from Vulcan, the Roman god of fire.

Currently there are about 1,500 potentially active volcanoes worldwide. 169 of these are in the United States.

Video courtesy of Spotmatik Ltd / Shutterstock

Volcanic eruptions can cause death and widespread devastation. They can also bring benefits and provide:

• Valuable mineral deposits like gold, aluminium, and nickel.
• Nutrient-rich fertile soils from volcanic ash.
• Geothermal energy—much of Iceland's energy comes from geothermal sources.
• Lava flows that create land—in 2018, a new island appeared off the coast of Hawaii as a result of lava from Kilauea eruptions.

Anatomy of a Volcano

All volcanoes have some things in common:

Did you know?

What is the difference between magma and lava?

• Magma is molten rock stored in the Earth's crust.
• Lava is molten rock that has reached the Earth's surface through a volcanic vent.

Where volcanoes are found

The lithosphere is the outermost layer that surrounds the Earth. It consists of the crust and part of the mantle. This is broken into extremely large slabs called tectonic plates. For example, the North American Plate (includes most of North America, Greenland, and part of Siberia) is 75,900,000 square kilometers (29,305,000 square miles). These move around on the ductile layer beneath.

Look at the map above to see how the Earth's surface is divided into seven major plates. Volcanoes are mostly (but not always) found where these tectonic plates meet, called the plate boundaries.

About 75 percent of the Earth's active volcanoes are in the Ring of Fire. This 40,000-kilometer (25,000-mile) chain of volcanoes and seismically active sites circles the edges of the Pacific Ocean.

Video courtesy of National Geographic Partners

How volcanoes form

We already know that volcanoes are formed mostly, but not always, at the boundaries of tectonic plates. Let's look at the mechanisms behind this.

• An island arc volcano is a type of subduction zone volcano.

  • Island arc volcanoes occur when one plate descends, or subducts, under another plate.
  • Subduction allows water from the subducting plate to be driven upward, off the subducting plate and into the mantle wedge.
  • This lowers the melting point of the mantle, and it melts to form magma.
  • This magma will rise and leak into the crust forming a volcano.
  • This process can create a chain of volcanic islands.

  Examples of island arcs are the Japanese islands and the Aleutian islands of Alaska.

• 

  Cleveland Volcano, Aleutian Islands.
  Image credit: NASA

• About five percent of all known volcanoes form in the middle of plates, not at their edges. These intraplate volcanoes are caused by hot spots, unusually hot areas deep within the Earth.

  Magma rises from the hot spots and erupts as lava through cracks in the Earth's surface forming volcanoes.

  As a plate moves slowly across a hot spot, a chain of volcanoes or volcanic islands can form.

• 

  The islands of Hawaii and Samoa were formed in this way. Plates move about 5 centimeters a year so this is a slow process.

  Satellite view of Hawaii archipelago (USA).
  Image credit: NASA

• Volcanoes called rift volcanoes are formed in spreading centers. In these zones, plates move away from each other, decreasing the pressure on the underlying mantle, allowing it to rise and melt, forming magma. Spreading centers can be found under the sea or on land.

  On land
  When spreading centers develop within continents, they form new plate boundaries and trigger volcanic activity. Spreading may have created East Africa's volcanic Great Rift Valley.

• 

  Hills covered by volcanic ashes, Great Rift Valley, Tanzania, East Africa.
  Image credit: Aghezzi

• Under the sea
  The mid‐ocean ridge is a continuous, underwater seam of mountains and volcanoes that form where divergent tectonic plates meet. It stretches all around the globe and is more than 64,000 kilometers (40,000 miles) long. The tectonic plates are spreading apart allowing soft molten rock to bubble up between the plates. Once this hardens it forms the seafloor. This process can also form volcanic islands. For example, Iceland was formed where the mid-ocean ridge met with a mantle plume – a 'hotspot' of abnormally hot rock in the mantle – and eruptions of lava built volcanoes and filled rift valleys.

  Rift volcanoes generally issue nonviolent streams of lava, instead of explosively erupting. They also tend to be shield volcanoes with gently sloping slides.

• 

  Thingvellir Valley and Lake Thingvallavatn, Thingvellir National Park, Thingvellir, Iceland.
  Image credit: Emory Kristof

• When two plates come together, one of the plates may slide under another in a process called subduction.

  • Heat from deep in the Earth melts rock in the descending plate. At the same time, water is driven from the subducting slab into the overlying mantle wedge, lowering the melting point of the mantle, and melting it to form magma.
  • The molten rock rises through the plate above it and can burst out of the surface of the Earth as lava, gradually forming a volcano.
  • Subduction volcanoes tend to be explosive stratovolcanoes. Hundreds of volcanoes rise from subduction zones encircling the Pacific Ocean. This famous belt is known as the Ring of Fire.

• 

  Volcanoes of Bromo National Park, Java, Indonesia.
  Image credit: Manamana

Types of volcano

• Cinder cone
• Shield
• Composite
• Caldera
• Lava dome
• Submarine

• Video courtesy of Wead / Shutterstock

  Cinder cone volcanoes are relatively small, steep, cone-shaped hills made up mostly of partly burned ash and lava cinders. Cinder cones tend to be explosive volcanoes but they can also produce lava.

  Although larger volcanoes form very slowly, a cinder cone can develop rapidly. A good example is the Paricutín volcano in Mexico, which grew from a crack in a corn field to a cone over 300 meters (984 feet) high over the course of one year in the 1940s. Cinder cones also have a shorter lifespan than slower-growing types of volcano.

• Photograph by J. Baylor Roberts

  Cinder cone volcanoes are relatively small, steep, cone-shaped hills made up mostly of partly burned ash and magma cinders. Cinder cones tend to be explosive volcanoes but they can also produce lava.

  Although larger volcanoes form very slowly, a cinder cone can develop rapidly. A good example is the Paricutín volcano in Mexico, which grew from a crack in a corn field to a cone over 300 meters (984 feet) high over the course of one year in the 1940s. Cinder cones also have a shorter lifespan than slower-growing types of volcano.

• Photograph by Saro17 / Getty Images

  Cinder cone volcanoes are relatively small, steep, cone-shaped hills made up mostly of partly burned ash and magma cinders. Cinder cones tend to be explosive volcanoes but they can also produce lava.

  Although larger volcanoes form very slowly, a cinder cone can develop rapidly. A good example is the Paricutín volcano in Mexico, which grew from a crack in a corn field to a cone over 300 meters (984 feet) high over the course of one year in the 1940s. Cinder cones also have a shorter lifespan than slower-growing types of volcano.

• Video courtesy of Rick Ray

  Shield volcanoes get their name from their shape which resembles a warrior's shield lying on the ground. The shape comes from streams of lava that flow from a summit vent or group of vents and then cool down, forming smooth, gentle slopes. Their summits are nearly flat.

  Examples are Kilauea in Hawaii and Piton de la Fournaise on Reunion Island in the Indian Ocean. Areas of the world with the most shield volcanoes include Hawaii, the Galapagos Islands, and Iceland.

• Photography by USGS / Getty Images

  Shield volcanoes get their name from their shape which resembles a warrior's shield lying on the ground. The shape comes from streams of lava that flow from a summit vent or group of vents and then cool down, forming smooth, gentle slopes. Their summits are nearly flat.

  Examples are Kilauea in Hawaii and Piton de la Fournaise on Reunion Island in the Indian Ocean. Areas of the world with the most shield volcanoes include Hawaii, the Galapagos Islands, and Iceland.

• Photography by CORBIS / Getty Images

  Shield volcanoes get their name from their shape which resembles a warrior's shield lying on the ground. The shape comes from streams of lava that flow from a summit vent or group of vents and then cool down, forming smooth, gentle slopes. Their summits are nearly flat.

  Examples are Kilauea in Hawaii and Piton de la Fournaise on Reunion Island in the Indian Ocean. Areas of the world with the most shield volcanoes include Hawaii, the Galapagos Islands, and Iceland.

• Video courtesy of fitopardo / Getty Images

  Composite volcanoes, also known as stratovolcanoes, are steep-­sided, symmetrical cones formed when pyroclastic eruptions (explosions of ash, cinders, and rock fragments) alternate with lava flows and layers build up over time. They usually have a central vent or a cluster of vents in a crater at their summit.

  Mount Fuji in Japan, Mount St. Helens in Washington State, and Mount Etna in Italy are all composite volcanoes.

• Photograph by pxhidalgo

  Composite volcanoes, also known as stratovolcanoes, are steep-­sided, symmetrical cones formed when pyroclastic eruptions (explosions of ash, cinders, and rock fragments) alternate with lava flows and layers build up over time. They usually have a central vent or a cluster of vents in a crater at their summit.

  Mount Fuji in Japan, Mount St. Helens in Washington State, and Mount Etna in Italy are all composite volcanoes.

• Photograph by Fomin Sergey / EyeEm

  Composite volcanoes, also known as stratovolcanoes, are steep-­sided, symmetrical cones formed when pyroclastic eruptions (explosions of ash, cinders, and rock fragments) alternate with lava flows and layers build up over time. They usually have a central vent or a cluster of vents in a crater at their summit.

  Mount Fuji in Japan, Mount St. Helens in Washington State, and Mount Etna in Italy are all composite volcanoes.

• Video courtesy of BBC Universal / Getty Images

  A caldera is a large oval or circular hollow that can form soon after an eruption. If the magma chamber has emptied over a short period of time, it won't be able to support the weight of the mountain above it. This leads to a collapse above the magma chamber and the caldera forms.

  The caldera may be encircled by steep cliffs and filled with lakes. Calderas can also form on composite volcanoes and on the flat tops of shield volcanoes.

• Photograph by Cavan Images

  A caldera is a large oval or circular hollow that can form soon after an eruption. If the magma chamber has emptied over a short period of time, it won't be able to support the weight of the mountain above it. This leads to a collapse above the magma chamber and the caldera forms.

  The caldera may be encircled by steep cliffs and filled with lakes. Calderas can also form on composite volcanoes and on the flat tops of shield volcanoes.

• Photograph by National Parks Service

  A caldera is a large oval or circular hollow that can form soon after an eruption. If the magma chamber has emptied over a short period of time, it won't be able to support the weight of the mountain above it. This leads to a collapse above the magma chamber and the caldera forms.

  The caldera may be encircled by steep cliffs and filled with lakes. Calderas can also form on composite volcanoes and on the flat tops of shield volcanoes.

• Video courtesy of Stefan Schuetz / Getty Images

  Lava domes are formed when very thick lava erupts through the volcano's vent and then piles up around it. Like lava flows, there isn't enough pressure for the volcano to erupt explosively. But unlike lava flows, where the faster-moving lava can flow for miles, the lava is so thick that it doesn't flow very far.

  These steep mounds can grow to become several hundred meters high and be more than 1,000 meters (3,280 feet) in diameter.

• Photograph by Ulet Ifansasti / Getty Images

  Lava domes are formed when very thick lava erupts through the volcano's vent and then piles up around it. Like lava flows, there isn't enough pressure for the volcano to erupt explosively. But unlike lava flows, where the faster-moving lava can flow for miles, the lava is so thick that it doesn't flow very far.

  These steep mounds can grow to become several hundred meters high and be more than 1,000 meters (3,280 feet) in diameter.

• Photograph by Mike Poland / USGS

  Lava domes are formed when very thick lava erupts through the volcano's vent and then piles up around it. Like lava flows, there isn't enough pressure for the volcano to erupt explosively. But unlike lava flows, where the faster-moving lava can flow for miles, the lava is so thick that it doesn't flow very far.

  These steep mounds can grow to become several hundred meters high and be more than 1,000 meters (3,280 feet) in diameter.

• Video courtesy of Ocean Today / Smithsonian Institution / NOAA

  There are considerably more volcanoes underwater than on land. Most eruptions on the seafloor occur from fissures, for example along mid-ocean ridges and around Iceland. Other eruptions form cone­-shaped seamounts or sea mountains.

  A seamount will occur where magma erupts through the seafloor and forms new seafloor. If the seamount grows out of the ocean it is known as a volcanic island. Hawaii is an example of a volcanic island.

• Photograph by NOAA / NSF / WHOI

  There are considerably more volcanoes underwater than on land. Most eruptions on the seafloor occur from fissures, for example along mid-ocean ridges and around Iceland. Other eruptions form cone­-shaped seamounts or sea mountains.

  A seamount will occur where magma erupts through the seafloor and forms new seafloor. If the seamount grows out of the ocean it is known as a volcanic island. Hawaii is an example of a volcanic island.

• Photograph by NOAA Okeanos Explorer Program, INDEX-SATAL 2010

  There are considerably more volcanoes underwater than on land. Most eruptions on the seafloor occur from fissures, for example along mid-ocean ridges and around Iceland. Other eruptions form cone­-shaped seamounts or sea mountains.

  A seamount will occur where magma erupts through the seafloor and forms new seafloor. If the seamount grows out of the ocean it is known as a volcanic island. Hawaii is an example of a volcanic island.

Did you know?

Composite volcano eruptions are the most catastrophic. Lava pours out from fissures or cracks in the volcano's walls, strengthening the cone and forming a seal as it hardens. Pressure gradually builds up within the cone and the result is a violent explosion of pyroclastic material from the volcano's vents.

How volcanoes erupt

We know that volcanic eruptions can be

• fairly calm lava flows, calm enough for scientists to stand near and observe, or
• violently explosive, powerful enough to blow mountains apart.

What decides how violent the eruption will be?

This is decided by the physical properties of the magma:

• Viscosity: how thick or runny it is
• Gas contents: the amount of dissolved gas in it

Viscous magma

Viscosity describes how runny the magma or lava is. Viscosity is affected by the amount of silica (SiO₂ also known as silicon dioxide) in the magma.

Silica molecules form long chains in the magma; these get tangled together, making it difficult for the magma to slide past itself. The more silica, the more viscous the magma, the slower it will move.

Dissolved gases

In the Earth's crust the magma is at a higher pressure than at the Earth's surface, so it is possible for gases (water vapour and carbon dioxide) to be dissolved in it.

As lava moves up towards the Earth's surface, the gas tries to escape because the pressure on it is decreasing. Think about what happens when you shake a bottle of soda!

Did you know?

The 1883 eruption of Krakatoa measured 6 out of 8 on the Volcanic Explosion Index with an estimated force comparable to 200 megatons of TNT (trinitrotoluene, an explosive). The atomic bomb that devastated Hiroshima had a force of 20 kilotons. Krakatau was nearly ten thousand times more explosive than the bomb.

Types of Eruption

The following summarizes how the properties of viscosity and the amount of gases dissolved in the magma control what kind of eruption happens and what kind of volcano results.

Select each eruption type to find out more about it.

johnstonsompere.blogspot.com

Source: https://www.nationalgeographic.org/forces-nature/volcanoes.html