Volcano types and projectiles

Eruptions and prediction

Lava types Lava is the material that erupts from volcanoes, and has

many different types. Mafic lava is rich in iron and magnesium, and

usually found on oceanic crust. Felsic lava contains silica and is usually from

melted continental crust. Pahoehoe lava is a rapidly cooling lava that

gives a ropey, wrinkly look. Aa is jagged and blocky. Pillow lava is circular and cools quickly.

Lava types (continued)

Aa lava flowing in Hawaii Pillow lava on the mid-ocean ridge

Eruption differences Volcanoes that erupt can do so

explosively or quietly. Explosive eruptions tend to

happen on continental volcanoes because there are many trapped gases in the lava.

Quiet eruptions tend to happen on oceanic volcanoes (including island volcanoes) and are because mafic lava is very hot and thin, allowing gases to escape easily.

A 10 meter high lava fountain from an explosive eruption

Volcanic rock fragments Tephra particles are rock

thrown from volcanoes during eruptions.

It is classified in the following ways, and are all based on the size of the particles.

Tephra is also called pyroclastic material.

Tephra classificationSize Name

0 – 0.25 mm Volcanic dust

0.025 mm – 2 mm Volcanic ash

2 mm – 64 mm Lapilli

64mm > Volcanic bombs

64mm > Volcanic blocks

Volcano cone features Shield volcanoes cover a wide area and result

from quiet eruptions. Mafic lava forms a gradually sloped cone.

Cinder cones cover smaller areas but are higher and steeper, made from solid fragments ejected from a volcano.

Composite cones/Stratovolcanoes are formed from both types of eruptions, usually alternating.

Volcano cones

Shield volcano (top left)Cinder cone (top right)Stratovolcano (left)

Craters and calderas The vent on the top of a

volcano is called a crater. A caldera is the remnants

of a volcano whose cone has collapsed.

Krakatau is the most well-known caldera, sporting a diameter of 6 km.

Volcanic eruption prediction Predicting volcanic eruptions is somewhat possible and

semi-reliable. Small earthquakes near volcanoes usually occur before an

eruption, and the more frequent they are, the more likely a nearby volcano will erupt.

Gas emission from craters may indicate an imminent eruption as well.

Geologists also watch for surface bulges on a volcano, usually a sign that lava is building pressure in the crater.