Author: Fraser Cain

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Mount Vesuvius is a stratovolcano located on the western side of Italy, near Naples. It rises to an altitude of 1,281 meters above sea level; although the height of the volcano changes after each eruption. Vesuvius is best known for its devastating eruption in AD 79, which destroyed the towns of Pompeii and Herculaneum, burying the region under meters of ash, and killing an estimated 10,000 – 25,000 people.

It’s believed that the caldera of Mount Vesuvius started forming around 17,000 years ago and this was enlarged by further eruptions. It started forming because this is a point where the African Plate is being subducted beneath the Eurasian Plate. In the past, the larger cone on Vesuvius was Monte Somma, but it’s now lower than the main cone. The volcano Vesuvius continues to erupt regularly. The last eruption was in 1944, and then 1926, and in 1906. With this history, it’s just a matter of time before it has another eruption.

Although Italians remember the volcano’s history of eruptions, they continue to live on its slopes. There are productive vineyards part way down the mountain, growing in the rich volcanic soil. And an estimated 3 million people live in a region that could be affected if Vesuvius erupts again. Emergency planners in the region have developed a strategy to evacuate 600,000 people from the region if a severe eruption is expected.

We have written many articles about volcanoes for Universe Today. Here’s an article about stratovolcanoes, and here’s an article about Mount Etna; another dangerous volcano in Italy.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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Did you know that volcanoes can make glass? Well, it’s not exactly the kind of glass you’d want to put in your house. It’s called obsidian, and it’s a naturally occurring volcanic glass formed when felsic lava flows from a volcano and then cools without crystal growth.

In a regular eruption, lava pours out of a volcano out onto the surface travels a distance downhill, and then hardens. Although a lava flow might have a thin solid crust, it might take months or even years to fully cool. As it cools, crystals form in the rock. These crystals are larger in the core of the rock, which has taken longer to cool.

In order to get obsidian, the lava erupting from a volcano needs to cool so fast that crystals in the rock don’t have a chance to form. You’ll often get obsidian when lava from a volcano is pouring into a lake or ocean and cools quickly. And glass, unlike crystals, has no regular structure and can therefore fracture in long curved shapes. Obsidian consists mostly of silicon dioxide (70% or more) – that’s the same as window glass. The black color comes from minerals dispersed in the glass, like magnetite or hematite.

If you could hold a piece of obsidian in your hand, it’s usually black or dark grey and very shiny and glasslike. It’s often cracked and broken with sharp edges. You can see than ancient people had many uses for obsidian, since it can hold a very sharp edge. In fact, surgeons to this day have found that obsidian can hold a sharper edge than even the hardest surgical steel.

We have written many articles about lava for Universe Today. Here’s an article about different types of lava, and here’s an article about the temperature of lava.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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Lakes are formed by many different geologic processes, but one of the most dramatic is a maar. A maar is a low-relief crater caused by a phreatomagmatic eruption. This is a situation where ground water comes in contact with lava or magma and the resulting steam causes an explosion. This digs out a hole in the ground, creating the maar crater. And then, it usually fills back in to create a lake. The word “maar” comes from the German word, which is derived from the Latin word “mare” (sea).

Maars can occur anywhere in the world where magma comes in contact with ground water, and can range in size from 10 meters to 8 km across. They’re surrounded by a low rim composed of loose fragments of volcanic rocks and rocks torn from the ground when the explosive eruption happened. They can be 10 to 200 meters deep.

The largest known maar is found on the Seward Peninsula in northwest Alaska, and range in size from 4-8 km across. These maars are so large because the magma encountered large regions of permafrost, creating huge explosions.

A maar is related to a tuff ring. In the case of a tuff ring, the crater edge is raised above ground level. An even more dramatic tuff cone can rise up 300 meters above the surroundings.

Meteor crater in Arizona was once thought to be a maar, but geologists now know that it was created by a meteor impact about 50,000 years ago.

We have written many articles about volcanoes for Universe Today. Here’s an article about the parts of a volcano, and here’s an article about dormant volcanoes.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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A volcano hotspot is a region on the Earth’s surface that has experienced volcanism for a long time. A good example of this is the Hawaiian Islands. Each of the islands in the long chain were created by the same volcano hot spot. The volcano built up an island that extended above the surface of the ocean, and then plate tectonics carried the island away, creating an extinct volcano. But there’s always a new volcano being created by the same hot spot.

There are dozens of volcano hot spots around the world, with the Hawaiian Island chain just being the most well known. Others include the Azores hotspot, the Canary hotspot and the East Australia.

About 30 km below the surface of the Earth is the mantle, a region where temperatures can reach thousands of degrees Celsius. But that’s under the continents. Underneath the oceans, the mantle is only 10 km down or less. Molten rock can seep out of the mantle and form vast magma chambers beneath the Earth’s crust. This magma finds its way to the surface, creating volcanoes.

Geologists believe that volcano hotspots are created when a narrow stream of hot mantle convects up from the Earth’s core-mantle boundary. This stream is known as a mantle plume.

Another theory is that hotspots are created when asteroids impact the Earth. The shockwave of the impact causes seismic waves to ripple through the Earth and create a hotspot on the exact opposite point on the Earth from the impact. This is known as the antipodal pair impact theory.

One of the most dramatic volcano hotspots wasn’t here on Earth but on Mars; the hotspot that created the largest volcano in the Solar System – Olympus Mons. Scientists believe that plate tectonics ceased on Mars billions of years ago, but the same volcanic hotspot kept pushing up magma. This allowed Olympus Mons to continue growing for billions of years, and reach its current height of 27 km.

We have written many articles about volcanoes for Universe Today. Here’s an article about dormant volcanoes, and here’s an article about extinct volcanoes.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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When it comes to liquids, viscosity is a measurement of how thick or syrupy it is. Water has low viscosity, while corn syrup, for example, is highly viscous. You can measure lava in terms of viscosity as well. And the lava viscosity defines the size and shape of a volcano. Even though lava is 100,000 times more viscous than water, it can still flow great distances.

When lava has low viscosity, it can flow very easily over long distances. This creates the classic rivers of lava, with channels, puddles and fountains. You can also get bubbles of lava filled with volcanic gasses that burble and pop on the surface of the lava. And over time, volcanoes made from low lava viscosity are wide and have a shallow slope; these are known as shield volcanoes. Classic examples of shield volcanoes are Mauna Kea and Mauna Loa in Hawaii, as well as Olympus Mons on Mars.

When lava has a high viscosity, it’s very thick and doesn’t flow very well at all. Instead of rivers of lava, you can get crumbling piles of rock flowing down hill. It can also clog up the volcanic vent and form blocks that resist the flow of lava. Viscous lava will trap pockets of gas within the rock, and not let them pop as bubbles on the surface. But most importantly, highly viscous lava is associated with explosive eruptions and dangerous pyroclastic flows.

An example of a low viscosity (fast flowing) lava is basaltic lava. This flows quickly out of a volcano at a temperature of about 950 degrees Celsius. This flows out for great distances creating shield volcanoes or flood basalt fields. An example of high viscosity lava is felsic lava, like rhyolite or dacite. It erupts at lower temperatures, and can flow for tens of kilometers.

We have written many articles about lava for Universe Today. Here’s an article about lava flows, and here’s an article about the temperature of lava.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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When volcanoes erupt, they can blast out lava, hot gasses, rocks and clouds of ash. Some of this ash rises up into the air and can travel for hundreds of kilometers in the air. Other ash pours down the sides of the volcano in great pyroclastic flows. When this ash cools, hardens and forms rocks, this material is called volcano tuff.

Geologists have a catch all name for rock ejected out of a volcano during an eruption: tephra. That can include tiny ash particles or large rocks. Particles smaller than 2 mm in diameter are considered ash. And when this ash is compacted down into rock, then you get volcanic tuff.

Tuff can range in texture, chemistry and mineral properties. Some tuff is very soft and can be easily dug with hand tools. Other tuff has been keep under pressure and cemented together to the point that it’s as hard as obsidian. Since there’s always been volcanism on Earth, volcanic tuff can be found around the Earth, in many different places and rock layers. Some is exposed on the surface, while others are buried by other eruptions or eroded material.

One of the most dramatic events is a “nuee ardente”. This is a glowing avalanche of hot ash cascading down the side of a volcano at speeds greater than 100 km/hour. When the ash avalanche comes to a stop, all this ejecta will compact together to form welded tuffs. There are large regions like this in Yellowstone National Park.

Ancient people used the soft nature of volcanic tuff to make buildings. They could carve out bricks from the soft rock to make walls.

We have written many articles about volcanoes for Universe Today. Here’s an article about volcanic ash, and here’s an article about volcanic rock.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

When a volcano erupts, it’s spewing forth lava, ash and hot rock. But where does this material come from, and how does it get to the surface? A volcano conduit is the pipe or vent at the heart of a volcano where material wells up from beneath the surface.

The surface of the Earth is relatively cool, but things get hotter as you descend beneath the ground. When you get about 30 km down (beneath the continents), you reach the Earth’s mantle. This is region of the Earth where rocks can be heated to more than 1,000 degrees C. Because of this high heat and pressure, liquid rock squeezes out of the mantle and collects in magma chambers beneath the Earth’s crust. The magma is “lighter” than the surrounding rock, so it floats to the surface, finding its way though cracks and faults in the crust. Eventually it reaches the surface and erupts as a volcano.

The volcano conduit is the pipe that carries this magma from the magma chamber, up through the crust and through the volcano itself until it reaches the surface. Stratovolcanoes, the largest kind of volcano, can have entire networks of volcano conduits inside them, and they can have eruptions from the central crater at the top, or from volcanic vents on the side.

After an eruption, the lava can cool and harden in the volcano conduit forming a hard plug. In some cases the plug causes the volcano to build up additional pressure and have an explosive eruption. In other cases, the volcano goes extinct, and the hard plug is all that remains when the rest of the volcano erodes away. Some of the most beautiful natural structures are these volcanic necks perching up above the surroundings.

We have written many articles about volcanoes for Universe Today. Here’s an article about dormant volcanoes, and here’s an article about extinct volcanoes.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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Mount Etna is a stratovolcano on the east side of the Island of Sicily. Standing 3,329 meters tall, Etna is the second largest volcano in Europe, and the highest mountain in Italy south of the Alps. But more importantly, Mount Etna is one of the most active volcanoes in the world, in an almost constant state of eruption.

Etna is classified as a stratovolcano (also known as a composite volcano). This is where many different kinds of eruptions over time have built up the huge mountain. You can have layers of lava, rock and ash, and many volcanic vents reaching the surface and capable of erupting. Many of the largest, most dangerous volcanoes in the world are stratovolcanoes (Mount St. Helens, for example).

Geologists believe the Etna started erupting about 300,000 years ago. In the last 35,000 years or so the mountain has had many explosive eruptions with pyroclastic flows cascading down its banks. Ash from Mount Etna eruptions has been found in Rome, located 800 km away. The successive eruptions have also caused calderas on the mountain to collapse creating depressions. There are now almost constant eruptions on Etna, with severe eruptions happening every 20 years or so.

You would think that the Italians would be nervous about having an active volcano in their back yard, but people actually live on the slopes of Etna. There are vineyards and orchards spread across its flanks; that’s because the rich volcanic soil is so good for planting. For example, in 2007 an eruption brought rivers of lava flowing down the slopes of Etna into an uninhabited valley. Villagers in the city of Catania on the island of Sicily could watch the eruption. Only an airport was closed during the eruption.

We have written many articles about Mount Etna for Universe Today. Here’s an article about images of Etna captured by 4 different satellites. And here’s an article about Mount Saint Helens.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.