Types of lava flows

Depending on the type of lava flow, the resulting rocks look very different. This was a pahoehoe flow.

A lava flow is caused by a volcanic eruption. The lava can flow down the volcano at speeds ranging from 1/24 to 50 meters an hour. They can be 1,300 degrees fahrenheit or more. They are very destructive.

The theory on how volcanoes erupt

The theory is that pressure and other factors in the gas cause some upward pressure. Magma is formed when the earth’s upper crust melts (for some bizarre reason it’s magma under the earth and lava above). The magma falls down with gravity and is pushed up with the pressure of the gases. When the pressure gets high enough the magma shoots out of the volcano turning it into lava.

Pyroclastic Eruptions

Pyroclastic eruptions are formed when water and other gases get dissolved into the magma. The magma rises and the water separates. When the ratio of the gases is more in the magma it disintegrates into a combination of mostly rocks and a little bit of lava, and the volcano erupts explosively.

Pahoehoe Lava

Pahoehoe lava flows very smooth and thin. They are about 1-2 meters thick and go up to 50 meters an hour. They are formed by liquid basalt. They move in “tongues” and can be identified by their smooth glassy skin. When the Pahoehoe cools it has a smooth surface

Sheet lava

Sheet lava is usually 10 to 30 meters thick. They are very fast. They also tend to fill in low lying areas or pool in them. Almost always the top part of the lava cools first. The interior can stay lava for a long time. Sometimes, the liquid lava trapped beneath the hardened crust continues to flow, leaving an empty tube. The top crust can eventually break open, exposing the tube.

A’a flows

A’a flows can come out of the volcano up to 50km per hour. They can push down houses, walls, and forests. When it cools it is sharp and broken into many pieces called clinker. It can be dangerous to walk over because it’s so sharp. The clinkers can range from 1cm to 1m in size.

Block lavas

Block lava is thicker and slower than a’a lava. They move at 1 to 5 meters a day. When they solidify they often make cube like structures that are rather smooth. They are more viscous than a’a lava flows. Block lava is solidified much like sheet lava. Read more about how volcanos form and types of lava.

San Cristobal Volcano in Nicaragua spews gas and steam and bit of rock in a pyroclastic flow.
Tungurahua Volcano in Ecuador is also pyroclastic. At times the rocks shoot out with a lot of force and fall on the houses below.
Some volcanoes erupt just once and then are done. Others continue to erupt for many years.
You can see where air got trapped in the lava. When it cooled, it left a rock with lots of holes in it.
In Ecuador, engineers have built these channels so the lava from Tungurahua has an easy route down. They hope the lava will stay in the channels and not flow over the houses.
There are a lot of factors that cause lava to be different – temperature, chemical composition, and amount of pressure all contribute to different types of volcanic flows.
Just by looking at the lava thousands of years after the eruption, you can see exactly what type of flow it was.

books by Nancy Sathre-Vogel

About Nancy Sathre-Vogel

After 21 years as a classroom teacher, Nancy Sathre-Vogel finally woke up and realized that life was too short to spend it all with other people's kids. She and her husband quit their jobs and, together with their twin sons, climbed aboard bicycles to see the world. They enjoyed four years cycling as a family - three of them riding from Alaska to Argentina and one exploring the USA and Mexico. Now they are back in Idaho, putting down roots, enjoying life at home, and living a different type of adventure. It's a fairly sure bet that you'll find her either writing on her computer or creating fantastical pieces with the beads she's collected all over the world.

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