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.

Cinder Cone Volcanoes

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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.

Albedo of the Earth

The albedo of the Earth is 0.367.

That’s the simple answer, now here’s the more complex one. Astronomers use the term “albedo” to define the amount of light that an object in the Solar System reflects. For example, if a planet was perfectly shiny, it would have an albedo of 1.00; it would reflect 100% of the light that hit it. If a planet was perfectly dark, it would have an albedo of 0, and so it would reflect 0% of the light that struck it.

The object with the highest albedo in the Solar System is Saturn’s moon Enceladus, with an albedo of 99%. On the other hand, asteroids can have albedos as low as 4%. The Earth’s moon has an albedo of about 7%. Can you imagine if we had Enceladus for a moon? Now that would be bright.

The albedo of the Earth is very important because it helps define the temperature of the planet. Fresh snow has an albedo of 90%, while the ocean has a very low albedo; land areas range from 0.1 to 0.4.

NASA’s Terra and Aqua satellites are constantly measuring the albedo of the Earth with their MODIS instruments, to help detect any evidence that the albedo is changing over time.

We have written many articles about the Earth for Universe Today. Here’s an article about how scientists track Earthshine on the Moon. And here’s a more detailed article about the albedo of the Moon.

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.