Now you decide to go look around some more, and you find an exhibit on different types of lava and different types of eruptions. It says that there is one type of eruption that happens very suddenly because the magma is not thick. It comes out of the earth very fast and is full of glass. When it cools it forms a rock full of tiny holes and is called pumice.

Another type of eruption is when there is thicker magma. The gas pressure breaks up the rock into different pieces. These rock pieces are called pyroclasts. When it is cooled, it is called pyroclastic rock.

The last type of eruption is the quieter type, where very fluid magma comes from dikes. Dike are underground tubes of lava that usually go in a sideways direction from the volcano, tunneling through layers of rock that are already there. When there are many dikes in one area it is called a dike swarm.

Photograph by J.P. Lockwood in March 1983/USGS

A Dike

 

Lava that comes out of these dikes and looks a lot like rope is called Pahoehoe, which is Hawaiian for smooth, ropy lava. There is another type of lava that comes from these dikes carries clinkers which are broken lava blocks. When they get to the front of the flow, they tumble down the front. The flow then goes over the clinkers, and it creates A'a, which is Hawaiian for rocky, bumpy lava, and would hurt to walk on with bare feet.

Photograph by J.D. Griggs on July 16, 1990/USGS

Pahoehoe

 

An A'a flow of lava progresses over an old Pahoehoe flow

 

 There are some other types of eruptions. The first one is called Strambolian. This is when large clots of molten lava burst from the summit crater. It looks somewhat like fireworks.

Strambolian Eruption

The next type is called Vulcanian. This is when there is a dense cloud of gas that is full of ash. The ash forms a whitish cloud near the upper level of the cone.

Vulcanian Eruption

Another type is Vesuvian. This also has lots of smoke that is full of gas that forms a cauliflower-shaped cloud.

Vesuvian Eruption

There is also a type called Pelean. This is when gas, dust, ash, and lava fragments come from the central crater, fall back and the lava slowly flows down the sides of the volcano.

Phreatic is another type. This is when there is explosive and expanding steam that is caused when cold ground and surface water comes into contact with hot rack or magma. This is usually a very weak type of eruption, but not always.

The last and most powerful is called Plinian. There is very explosive, vicious lava in this type of eruption. The eruption of Mount St. Helens and Pinatubo were of this type.

What's that? You see another exhibit called volcanic products so you decide to go take a peek.