Posted on by Fraser Cain

Composite Volcano

Mount Fuji - a composite volcano

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Geologists have identified 3 major types of volcanoes. There’s the shield volcano, formed from low viscosity lava that can flow long distances. There are cinder cone volcanoes, which are made by the eruption of lava, ash and rocks that build up around a volcanic vent. But the last type is the composite volcano, and these are some of the most famous volcanoes (and most dangerous) in the world.

A composite volcano is formed over hundreds of thousands of years through multiple eruptions. The eruptions build up the composite volcano, layer upon layer until it towers thousands of meters tall. Some layers might be formed from lava, while others might be ash, rock and pyroclastic flows. A composite volcano can also build up large quantities of thick magma, which blocks up inside the volcano, and causes it to detonate in a volcanic explosion.

Composite volcanoes are fed by a conduit system which taps into a reservoir of magma deep within the Earth. This magma can erupt out of several vents across the composite volcano’s flanks, or from a large central crater at the summit of the volcano.

Some of the most famous volcanoes in the world are composite volcanoes. And some of the most devastating eruptions in history came from them. For example, Mount St. Helens, Mount Pinatubo, and Krakatoa are just examples of composite volcanoes that have erupted. Famous landmarks like Mount Fuji in Japan, Mount Ranier in Washington State, and Mount Kilimanjaro in Africa are composite volcanoes that just haven’t erupted recently.

When large composite volcanoes explode, they can leave behind a collapsed region called a caldera. These are deep, steep-walled depressions which marked the location of the volcano. And it’s in this region that a new composite volcano will build back up again.

Another name for composite volcanoes are stratovolcanoes.

We have written many articles about composite volcanoes for Universe Today. Here’s an article about the recent eruption of Mount Redoubt in Alaska, and here’s an article about Mount Etna.

You can learn more about composite volcanoes from the USGS.

And we have recorded an entire episode of Astronomy Cast just about volcanoes. Listen to it here, Episode 141: Volcanoes, Hot and Cold.

Posted on by Fraser Cain

What is a Volcanic Neck?

Devils Tower, a volcanic neck.

Remember that strange rock formation in Close Encounters of the Third Kind. It looked like the top of a toothpaste tube, but made of solid rock. That’s a volcanic neck, and it has nothing to do with space aliens. In reality, a volcanic neck is the solidified magma trapped inside a volcano. After millions of years, the softer outer layer of the volcano erodes, and all that remains is the volcanic neck. The structure in Close Encounters is Devil’s Tower, located in Wyoming.

Volcanic necks are somewhat rare because when a magma plug forms within a volcano, it often leads to an explosive eruption, like what happened with Krakatoa, or more recently with Mount St. Helens. The plug is broken up and ejected as ash and rock in a split second. But if the pressure isn’t great enough to actually detonate the top of the volcano, the plug cools and hardens deep within the Earth.

There are some very famous volcanic necks around the world. Probably the most famous is Devils Tower in Wyoming. It rises 386 meters above the surrounding landscape, a lone prominence of rusty red rock. I’ve actually stood beside it, and it’s one of the most impressive geologic features I’ve ever seen.

The type of erosion will define the shape of the volcanic neck. For example, glaciers will erode away one side of the volcanic neck, but leave a long tail behind.

We have written many articles about volcanoes for Universe Today. Here’s an article about the largest volcano in the Solar System, and here’s an article about the largest volcano on Earth.

You can also find out more information about volcanic necks from the USGS.

We have also recorded an episode of Astronomy Cast dealing with volcanoes on Earth and across the Solar System. Check out Episode 141 – Volcanoes, Hot and Cold.

Posted on by Fraser Cain

Mantle Plume

Hotspot

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One of the mysteries of Earth science is hotspots. While most volcanoes are found at plate boundaries, where two tectonic plates are rubbing against each other, volcanic hotspots can be anywhere, even in the middle of continents. What causes volcanic hotspots? One theory is the idea of a mantle plume.

A mantle plume is kind of like what’s going on inside a lava lamp. As the light heats up the wax in a lava lamp, it rises up through the oil in large blobs. These blobs reach the top of the lamp, cool and then sink back down to be heated up again.

Inside the Earth, the core of the Earth is very hot, and heats up the surrounding mantle. Heat convection in the mantle slowly transports heat from the core up to the Earth’s surface. These rising columns of heat can come up anywhere, and not just at the plate boundaries. Geologists did fluid dynamic experiments to try and simulate mantle plumes, and they found they formed long thin conduits topped by a bulbous head.

When the top of a mantle plume reaches the base of the Earth’s lithosphere, it flattens out and melts a large area of basalt magma. This whole region can form a continental flood basalt, which only lasts for a few million years. Or it can maintain a continuous stream of magma to a fixed location; this is a hotspot.

As the lithosphere continues to move through plate tectonics, the hotspot appears to be shifting its position over millions of years. But really the hotspot is remaining in a fixed location, and the Earth’s plates are shifting above it.

Two of the most famous places that might have mantle plumes underneath them are the Hawaiian Islands and Iceland.

We have written many articles about volcanoes and the interior of the Earth for Universe Today. Here’s an article about the difference between magma and lava, and here’s an article about magma chambers.

Here’s a great resource on mantle plumes, and here’s another.

We have recorded an entire episode of Astronomy Cast about volcanoes around the Solar System. Listen to it here: Episode 141: Volcanoes, Hot and Cold.

Posted on by Stephanie Bender

Continental Crust

The crust is the top layer of the Earth’s Surface. Did you know that there are 2 types, though? One is called the Oceanic Crust, and the other, the Continental Crust. As its name suggests, the Oceanic Crust is the top layer of Earth that forms the ocean floor. The Continental Crust, however, will be our focus.

We walk on top of and dig down through the Continental Crust when we plant or drill. Even if there is an unstable surface at the very top, like sand, the deeper parts of the Crust are made of harder rocks. The large land masses, continents, have bases made from sedimentary, igneous, or metamorphic rocks, as well as any combination thereof. This shield rock is the oldest known; it’s been tested, dated, and found to have been here for 3,960,000,000 years!

Geologists, scientists who study the Earth, believe that shield rock was created when hot molten iron, known as magma cooled. If their math’s correct, it happened around the time these rocks formed, almost 4 billion years ago, right? Some of those rocks were so big it took a long time for them to cool. So, even if the rocks were formed 3.9 billion years ago, they might not have cooled for quite some time. Many estimate that the Continental Crust wasn’t completely hard for another 60,000,000 to 160,000,000 years.

The top portion of this rock has another name, platform rock. The oldest-known platform rocks are approximately 600,000,000 years old, and can be found in central North America. The sedimentary rock ranges from 1,000 to 2,000 meters thick; that is equivalent to more than a half mile to 1.25 miles. When we put the top and bottom portions of the Continental Crust together, we get what scientists call, a craton. Most cratons are stable and haven’t been damaged by earthquakes or volcanoes for hundreds of millions of years.

Around the edges are the continental margins, mostly created by sedimentary rock originally found in the oceans. How is that possible, you ask? Well, it’s due to earthquake and volcanic activity. In this case, it’s mainly due to a phenomenon called, subduction. You see, the Earth fits together like a puzzle; and, if you try to place the wrong piece into a spot where it fits, but isn’t quite right, what happens? Another piece might pop out of place. Sometimes, a continental margin works its way under the oceanic crust. When that occurs, the oceanic layer ends up on top of the continental margin. This is subduction. The most well-known place for this is along The Ring of Fire, an area that covers the edges along the Pacific Ocean. This is why so many and such violent earthquakes, volcanic eruptions, and tsunamis occur in that part of the world.

Universe Today has a wealth of information on this and other related topics. Here are just 2 of those available. The first is entitled,
Earth, Barely Habitable?.

The second is called, Interesting Facts About Planet Earth.

Universe Today also hosts Astronomy Cast, a science program that covers a variety of subjects. Episode 51: Earth, explains this subject in greater detail.

The Encyclopedia of Earth , by Michael Pidwirny has some excellent information, too.

Sources:
USGS
Science Daily

Posted on by Fraser Cain

Yellowstone Eruption

Welded tuff at Yellowstone National Park.

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Millions of people visit Yellowstone National Park every year, but how many think about the fact that they’re standing on top of one of the largest volcano calderas on Earth? Within the last 17 million years, there have been more than 100 large eruptions within the Yellowstone caldera, and thousands of smaller lava flows and steam explosions. In fact, the last great Yellowstone eruption happened about 70,000 years ago, and it only seems like a matter of time before it all happens again. Don’t panic, though, geologists monitor Yellowstone carefully, and they don’t think any large eruptions will happen soon.

The Yellowstone calderas measures 55 km wide by 72 km long, and rises to an elevation of 3,142 meters at its tallest point – Mount Sheridan. The constant uprise of the region created a plateau where there used to be a mountain range. These eruptions and uplift helped create the eastern Snake River Plain.

In the last 17 million years, there have been 142 caldera-forming eruptions in Yellowstone. This is an eruption large enough that a significant amount of lava, ash or rock were released – usually as an explosive eruption. Three of these eruptions have been classified as “super eruptions”, where up to 2,500 cubic km of ash and rock exploded out of the volcano. Just for comparison, Mount St. Helens, which erupted in 1980, only released 1 cubic km of material… so 2,500 times that in a single eruption. One of these super eruptions would have devastated most of North America, and cooled the climate of planet Earth for decades. The oldest of these Yellowstone eruptions happened 2.1 million years ago, which created the Huckleberry Ridge Tuff. The next oldest happened 1.3 million years ago, and the most recent super eruption happened about 640,000 years ago.

And since that last super eruption, there have been numerous smaller (but still powerful eruptions) non-explosive eruptions. The most recent lava flow has been estimated to have occurred about 70,000 years ago, and a steam explosion created a 5-km crater 13,800 years ago. The only eruptions that happen at Yellowstone today are the numerous geothermal vents around the caldera. These mix with water to create the famous geysers, like Old Faithful. These geysers indicate that Yellowstone is still a very active region, and more eruptions are likely.

Geologists are continuing to monitor the Yellowstone caldera, including the speed at this the caldera floor is rising up. Like Hawaii, Yellowstone is created by a single volcanic hotspot located under the Earth. The North American Plate is slowly moving over top of the hotspot, creating a long chain of calderas. The current caldera in Wyoming is the current location of the hotspot. Geologists have measured that the caldera floor is rising upwards at almost 7 cm per year. Fortunately, they find no evidence that we’re due for another super Yellowstone eruption. Of course, these things are difficult to predict.

We have written many articles about volcanoes for Universe Today. Here’s an article about about a Yellowstone-like formation on Mars, and an article about how extreme life in Yellowstone might offer hope for the search for life on Mars.

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.

Source: Wikipedia

Posted on by Fraser Cain

Barcena Volcano

Barcena

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Barcena is a volcano located on the island of San Benedicto, the third largest island of the Revillagigedo Islands. The whole island is only about 4.8 km by 2.4 km and Barcena takes up a good chunk of the southern end. Barcena rises to an elevation of 332 meters, forming a volcanic crater.

There has only been on eruption from Barcena in recorded history, but it was a big one. On August 1, 1952, Barcena had a severe Vulcanian eruption measuring 3 on the Volcanic Explosivity Index. It released huge pyroclastic flows that rolled over the entire island, covering it in ash and pumice to a depth of 3 meters. Within less than 2 weeks, it had created a new volcanic cone more than 300 meters high. A second series of eruptions started up later in the year, releasing magma that broke out of the cone and flowed into the ocean. By late 1953, the volcano went dormant again.

The eruption wiped out all the plants and wildlife on the island, making the San Benedicto Rock Wren extinct. Within a few years the plants and wildlife made a return, although the island still looks barren.

We have written many article about volcanoes for Universe Today. Here’s an article about Tacana, a tall stratovolcano that straddles the border between Mexico and Guatemala. And here’s an article about Paricutin, a volcano that suddenly appeared in a farmer’s cornfield.

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.

Posted on by Fraser Cain

Basalt

Lava fountain in Hawaii.

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Basalt is a hard, black volcanic rock with less than 52% silica. Because of this low silica content, basalt has a low viscosity (thickness), and so it can flow for long distances after erupting from a volcano. During an eruption, a basalt lava flow can easily move more than 20 km away from a vent. Basalt is the most common rock type in the Earth’s crust. In fact, most of the ocean floor is made up of basalt.

Basalt is made up of dark colored materials like pyroxene and olivine, but it also contains lighter minerals like feldspar and quartz. These crystals form because the lava cools slowly after erupting out of a volcano. Although a lava flow might look cool shortly after an eruption, it might take months or even years to cool all the way through. The crystals are bigger in the middle of a cooled lava flow because that part had longer to cool. If a lava flow cools quickly, like when it falls into a lake or ocean, it becomes a glass-like rock called obsidian. This is because the crystals in the rock don’t have time to form.

Shield volcanoes are made up entirely of basalt lava eruptions. A good example of this are the volcanoes Mauna Loa and Mauna Kea on the Big Island of Hawaii. Over hundreds of thousands of years, they have built up tall volcanoes that are extremely wide because of the fast flowing basalt lava.

Geologists have found large outpourings of lava covering hundreds of kilometers of land called flood basalt. The largest of these is known as the Siberian Traps in northern Russia. This is a region of 1.5 million square kilometers covered by basalt.

We have written many articles about volcanoes for Universe Today. Here’s an article about obsidian, and here’s an article about different types 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.

Posted on by Fraser Cain

What are Active Volcanoes?

Strombolian eruption

Geologists classify volcanoes into three distinct groups: dormant, extinct and active volcanoes. Dormant volcanoes haven’t erupted in a long time, but they could again; extinct volcanoes have erupted for thousands of years and might be dead. Active volcanoes, on the other hand, erupted recently, and they’re probably going to erupt again soon.

There are approximately 500 active volcanoes in the world today, not including those underneath the oceans. In fact, as you read these words, there are probably 20 volcanoes erupting right now. Between 50-70 volcanoes are erupting every year, 160 have erupted in the last decade. And there are about 550 that have erupted since the beginning of recorded history.

The definition of an active volcano is difficult to pin down, since single volcanoes can have networks of volcanic vents across their flanks. And Iceland, there can be eruptions along volcanic fields hundreds of kilometers long. At Mexico’s Michoacan-Guanajuanto field, there are 1,400 cinder cones, maars and shield volcanoes coming from a single magma chamber.

And these are just the volcanoes on land. Scientists estimate that 3/4 of the lava that reaches the Earth’s surface happens underwater at the submarine midocean ridges.

So when does a volcano become dormant or extinct? A volcano is active if it’s currently erupting or showing signs of unrest. The Smithsonian Global Volcanism Program defines an active volcano as having erupted within the last 10,000 years. A volcano finally goes extinct when there’s no lava supply in the magma chamber beneath the volcano.

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.

Posted on by Fraser Cain

A’a Lava

A'a lava

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There are several different kinds of lava, depending on the chemical composition and temperature of the molten rock that erupts from a volcano. The smooth variety is called pahoehoe, and the rougher variety is known as a’a (pronounced ah-ah). A’a is a Hawaiian word meaning “stony with rough lava”.

If you’ve ever been to the Big Island of Hawaii and gone for a hike, you’ve seen a’a lava. It’s incredibly rough and jagged black rock that takes forever to walk across; and tears your shoes apart as you go.

During an eruption, a’a lava comes out of the volcano as a very thick (viscous) lava that travels very slowly. The inside of an a’a lava flow is thick and dense. Surrounding this thick dense core is a sharp spiny surface of cooling rock. These fragments of rock are carried on the top of the a’a lava flow and make a crunching grinding sound as the lava flows downhill.

Once the lava flow stops, it can take weeks or even years for the lava to harden completely. The interior dense core hardens in place with the jagged fragments surrounding it. This is why old a’a flows are so sharp and jagged.

A’a flows move slowly – you could easily outrun one – but they move fast enough to tear down buildings, cover roads, and destroy vegetation.

The smoother pahoehoe lava can turn into a’a lava as it gets further downhill. This happens because of the delicate balance of gas content in the lava, the changes in lava viscosity, and the rate of deformation as the lava flows and cools. Once this balance changes, the pahoehoe can change into a’a. Of course, a’a lava never changes back into pahoehoe.

We have written many articles about volcanoes for Universe Today. Here’s an article about lava tubes on Pavonis Mons… on Mars. And here’s an article about the dark lava floor of crater Billy.

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.

Posted on by Fraser Cain

Cinder Cone Volcanoes

Cinder cone Paricutin. Image credit: USGS

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Picture a volcano in your mind. You’re probably thinking of a cinder cone volcano, the simplest type of volcano. Cinder cone volcanos have steep sides with a bowl-shaped crater at the top.

Cinder cone volcanoes grow from a single vent in the Earth’s crust. Gas-charged lava is blown violently out of the volcano’s central vent, and the ash and rocks rain down around the vent. After multiple eruptions, the volcano takes on the familiar cone shape, with the erupted rubble forming the steep slopes. Cinder cones rarely grow much taller than 300 meters above their surroundings, and they’re common in western North America, and wherever there’s volcanic activity.

Although they can be solitary structures, cinder cones are often associated with other kinds of volcanoes, like shield volcanoes and stratovolcanoes (or a composite volcano). For example, geologists have discovered more than 100 cinder cones on the sides of Hawaii’s Mauna Kea, one of the biggest volcano in the world. Each cinder cone comes from a vent that opened up on the sides of the volcano.

One of the most famous cinder cone volcanoes erupted out of a Mexican corn field in 1943. The volcano erupted for 9 years, and quickly built up the cinder cone to 424 meters, and covered 25 km2 of fields in lava flows and rubble. Nearby towns were eventually buried in ash by the eruptions.

We have written many articles about volcanoes for Universe Today. Here’s an article about the biggest volcano on Earth, and here’s one about the largest volcano in the Solar System.

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.