Science 1 Notes: Volcanoes

 

I.               What is a volcano?

A volcano is basically a vent (hole in the ground) through which magma can rise to the earth’s surface. Lava flowing from fissures (long cracks in the ground) are more common than volcanoes.

Magma is molten rock. Magma, which reaches the surface and flows, is called lava.

Lava refers to both the molten rock itself and to the rocks that it forms. How the lava behaves depends on the amount of silicate (contain Silicon and Oxygen) minerals in the rock.

a)  Lava that contains a low proportion of silicates is thin and runny (has a low viscosity). This lava when it cools forms basalts.

b) Lava with a high proportion of silicates is thick and gooey (viscous).

 

II.         Types of Volcanoes.

A.                       Shield Volcanoes- these are formed from the build up of easily flowing basaltic lavas. They have broad, gently sloping cone that resembles a shield. Mauna Loa on the big island of Hawaii is an example of a shield volcano.

B.                        Cinder Cones - these are common in many volcanic areas. They have steep sides but are rarely taller than 300 meters. They are formed from explosive eruptions that hurl rock, ash and cinders into the air from a single vent.

C.                        Composite or Strata cones –These produce both lava and ash. The volcano is made up of alternating layers of lava, ash, and mud. They build a volcano with a steep sided summit, and more gently sloping sides. Mt. Shasta in California and Mt. Saint Helens in Washington are good examples.

 

III.    Volcanic rocks.

A.                       Volcanic rocks are classified as extrusive igneous (made from fire). Extrusive means that the lava that formed the rocks cooled above ground. Some common types of volcanic rock are:

Obsidian (volcanic glass) –this smooth, black glassy rock is formed from fast cooling lava that is very runny.

Pumice – a very low-density rock with lots of pores (holes) that are formed by escaping gas. Pumice stone is used as a cleaner.

B.                        Intrusive igneous rocks (plutonic) are formed when magma cools slowly beneath the earth’s surface. We only see this rock when the layers of rock above it are eroded away. The Sierra Nevada Mountain range was formed from a large pool of magma that cooled below the earth’s surface. Plate tectonic activity has pushed this rock upwards. The softer layers of rock above it have been eroded away exposing the igneous rock. The main type of rock in the sierras is granite.