Volcano Block
Written by Fraser Cain
You probably imagine a volcanic eruption with rivers of lava pouring out the top of a volcano, but that's not all the comes out during an eruption. You'll get volcanic gasses, clouds of ash and even large chunks of rock called volcanic blocks. These can range in size from the tiny to boulders several meters across. And they can be thrown hundreds of meters through the air during a violent eruption. Volcanic blocks make a very dangerous rain.
To understand where volcano block come from, how it could be possible for car-sized chunks of rock to rain from the sky during an eruption, consider the structure of the volcano itself. A volcano is built up with many eruptions. In some cases, the path to the magma chamber is relatively clear, and the lava flows out easily, but in other cases, when the lava is very viscous (thick and syrupy), it will clog up the volcanic vents that reach the surface.
During a new eruption, pressure will build up underneath the block, until the rock fractures since it can't hold the pressure back. This quick release will throw the fractured pieces into the air. In an eruption of Kilauea, Hawaii in 1924, volcanic blocks weighing 14 tonnes were thrown through the air. And Mount Vesuvius in Italy has hurled blocks weighing 2-3 tonnes more than 200 meters.
Another way that blocks are formed is when lava is pouring into the ocean during an eruption. The violent mixing of lava with sea water can hurl cooled chunks of lava into the air. These can rain back down inland dozens of meters from the shore.
Geologists classify a volcanic block as any chunk of rock bigger than 64 millimeters thrown out during an eruption. It's important to distinguish volcanic blocks from volcanic bombs. Volcanic bombs are chunks of lava that have hardened solid while being thrown in the air. They take on strange forms when they hit through ground.
Here's an article about the difference between magma and lava, and here's an article about different types of lava.
We have written many articles about the Earth for Universe Today.
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.
Filed under: Astronomy
