What is a lava tube? A lava tube is a tunnel made by flowing lava that sets on the outside but continues to flow on the inside. They can range from very small to enormous.
For lava tubes to form, you obviously need lava. Not all volcanic eruptions produce lava. Some of the bigger, more explosive volcanic eruptions don’t produce as much lava. The pressure builds up in them until they explode, and then they blast rock, ash, and debris into the air. They can also create pyroclastic flows, which are when superheated gas and ash pour down the side of the volcano, almost like a river. It can travel at over 150 km per hour and can burn or destroy everything in its path. Pompeii was buried by a pyroclastic flow. This kind of volcanic eruption doesn’t produce lava or lava tunnels.
Lava is magma that has reached the surface. It is a very hot stream of almost liquid, molten rock. It is made up of minerals and gases. These minerals have become molten because of the immense pressure in the Earth. When something is pressurized, its melting point is raised, which means it can get very, very hot without melting. This is the rock deep in the Earth. As the rock rises, the pressure drops, and the melting temperature of the rock drops to a point lower than the temperature it is at, so it melts. The minerals are not completely melted, and the magma is a viscous fluid. Sometimes the magma comes to the surface and flows out of the volcano. At this point, it becomes lava.
There are three types of lava, and they contain different minerals, which affect their flow in different ways. There is basaltic lava, which has lots of iron, magnesium, and calcium. It is the hottest type of lava, generally between 1000 and 1200℃, which means it can generally travel the furthest from its source. Then there is andesitic lava, which has a moderate amount of all these minerals and is cooler than basaltic lava by about 200℃. Last, there is rhyolitic lava, which has a lot of potassium and sodium, but not much iron, magnesium, and calcium. It is about 650 to 800℃, which makes it the coolest of all the types of lava. This is the most viscous of all the types of lava.
Lava tubes are made by basaltic lava because it has the lowest viscosity, which means it can travel the furthest. It also has a surface called pahoehoe, which sets very easily. When the lava comes out of the ground, it contains a lot of gas, which slowly rises to the top. This makes the lava more like foam, and it allows heat to be carried away from the outer layer of the lava more quickly, cooling it. As the lava flows away from the volcano, the outer surface continues to cool, forming a fairly hard roof. Underneath this, the rest of the liquid lava continues to flow. If the slope the lava is flowing down is not very steep, then the lava will slowly cool down and solidify until the whole tube is full of solid rock. However, if the slope is steeper, the lava will continue to flow, ending up in the sea, or going into a hole in the ground. That will leave the lava tube completely empty, giving us the tubes that we have today. Lava in lava tubes actually flows further than lava exposed to the air because the roof of the tube provides insulation, which stops the lava from cooling and keeps it flowing. Observations in Hawaii have shown that the temperature of lava in a lava tube only drops by a few degrees over the entire length of the tube.
Some lava tubes can be over 20 meters wide, and they can stretch for tens of kilometers. They also give a good sign of what volcanic activity has occurred because subsequent lava flows will follow the same path. Volcanologists can see the height that the lava flow reached marked on the wall of the tube. There are a lot of lava tubes on other planets that we can see as well. There are some on the moon and many on Mars. A lot of planets, such as Mars, don’t have tectonic plates, so volcanoes there can grow to enormous sizes. Olympus Mons, on Mars, is 2.5 times taller than Mount Everest. It also puts out a lot of lava, which can be seen from the lava tubes. They travel a long way out from the volcano. The lower gravity also means they can be bigger, and some of these tubes are hundreds of meters wide. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Lava_tube
https://volcano.oregonstate.edu/faq/what-lava-made
https://education.nationalgeographic.org/resource/magma
https://perlan.is/articles/what-is-lava
https://www.usgs.gov/news/volcano-watch-how-does-pahoehoe-flow