Category: Astronomy

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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.

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The name “volcano” comes the island of Vulcano, located in the Mediterranean Sea off the coast of Sicily. The Romans believed that Volcano was the chimney to the god Vulcan’s workshop. The island itself was thought to come from the debris that came out of the god’s furnace. The Romans believed that the earthquakes that shook the ground around the island came from Vulcan working in his shop, creating weapons for the gods to make war on one another.

The volcanic activity on the island of Vulcano comes from the northward motion of the African Plate colliding with the Eurasian Plate. This has opened up three volcanic hotspots on the island. There are two old stratovolcano cones at the southern end of the island, and then the most active Fossa cone in the center, and another at the north of the island. Currently about 470 people live on the island of Vulcano, getting their income from tourism.

In mythology, Vulcan was married to Venus, the goddess of love and beauty. The Romans believed that eruptions on Mount Etna in Sicily were caused by Vulcan’s anger at Venus. He works the forge so angrily that the metal turns red hot and sparks and smoke erupt from the top of the volcano.

We have written many articles about volcanoes for Universe Today. Here’s an article about active volcanoes, and here’s an article about shield 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 now-you-see-it, now-you don’t accretion disk (white and blue in the artist’s rendering at left) has tipped astronomers to the birth of a superfast, “millisecond” pulsar that was happening right before their eyes — er, their radio telescopes.

The new finding confirms the long-suspected evolutionary connection between a neutron star and a millisecond pulsar: they are two life stages of the same object.

Anne Archibald, of McGill University in Montreal, Canada and her colleagues announced their discovery in the May 21 online issue of the journal Science.

Pulsars are superdense neutron stars, the remnants left after massive stars have exploded as supernovae. Their powerful magnetic fields generate lighthouse-like beams of light and radio waves that sweep around as the star rotates, and are detectable as pulses on Earth.

Some, dubbed millisecond pulsars, rotate hundreds of times a second. Astronomers believe the fast rotation is caused by a companion star dumping material onto the neutron star and spinning it up.

The material from the companion would form a flat, spinning disk around the neutron star, and during this period, the radio waves characteristic of a pulsar would not be seen coming from the system. As the amount of matter falling onto the neutron star decreased and stopped, the radio waves could emerge, and the object would be recognized as a pulsar.

This sequence of events is apparently what happened with a binary-star system some 4000 light-years from Earth, in the constellation of Sextans just south of Leo. The millisecond pulsar in this system, called J1023, was discovered by the National Science Foundation’s Robert C. Byrd Green Bank Telescope (GBT) in West Virginia in 2007 in a survey led by astronomers at West Virginia University and the National Radio Astronomy Observatory.

The astronomers then found that the object had been detected by National Science Foundation’s Very Large Array radio telescope in New Mexico, during a large sky survey in 1998, and had been observed in visible light by the Sloan Digital Sky Survey in 1999, revealing a Sun-like star.

When observed again in 2000, the object had changed dramatically, showing evidence for a rotating disk of material, called an accretion disk, surrounding the neutron star. By May of 2002, the evidence for this disk had disappeared.

“This strange behavior puzzled astronomers, and there were several different theories for what the object could be,” said Ingrid Stairs of the University of British Columbia.

The 2007 GBT observations showed that the object is a millisecond pulsar, spinning 592 times per second.

“No other millisecond pulsar has ever shown evidence for an accretion disk,” Archibald said. “We know that another type of binary-star system, called a low-mass X-ray binary (LMXB), also contains a fast-spinning neutron star and an accretion disk, but these don’t emit radio waves. We’ve thought that LMXBs probably are in the process of getting spun up, and will later emit radio waves as a pulsar. This object appears to be the ‘missing link’ connecting the two types of
systems.”

The scientists have studied J1023 in detail with the GBT, with the Westerbork radio telescope in the Netherlands, with the Arecibo radio telescope in Puerto Rico, and with the Parkes radio telescope in Australia. Their results indicate that the neutron star’s companion has less than half the Sun’s mass, and orbits the neutron star once every four hours and 45 minutes.

Image caption: Material from distended “normal” star. right, streams onto accretion disk (white and blue) surrounding neutron star, left. Credit: Bill Saxton, NRAO/AUI/NSF

Source: National Radio Astronomy Observatory. Animations are here and here. Warning: that last one may cause dizziness.