Volcano Notes
Definition of a volcano: a weak spot in the Earth’s crust where
magma finds its way to the Earth’s surface
A volcano falls into one of these three categories:
1.active – currently erupting or could erupt very soon
2.dormant – sleeping, has a magma source but isn’t erupting
3.extinct – dead, no longer has magma source
Volcanoes can produce or give off three different things:
1. Lava – magma that is on Earth’s surface
- Pahoehoe lava is thin and ropey, more mafic in content
- Aa lava is more jagged and chunky, more felsic in
content
2. Tephra – rock fragments formed by shattering of thick
magma. AKA pyroclastic material
- Dust – smallest pieces
- Ash – a little bigger
- Lapili – 2 inches + in diameter
- Blocks or bombs – size of a car or house
3. Gases – steam, carbon dioxide, and sulfur dioxide.
Sometimes steam can combine with sulfur dioxide to form
sulfuric acid.
Three main types of volcanoes
The type of volcano formed depends on the type of magma
involved. The type of magma also determines the type of eruption
that will occur.
1. Cinder Cone Volcanoes – these form at subduction zones.
These volcanoes have steep sides and the traditional volcano
“look” to them. Cinder cone volcanoes are generally formed
by weak eruptions and usually only erupt once in a lifetime.
Example: Izalco
2. Composite Volcanoes – these are formed by a combination
of both types of lava. These volcanoes give off pyroclastic
material and have extremely explosive eruptions. Composite
cones are the most common kind of volcano.
Example: Mount St. Helens
3. Shield Volcanoes – these look like an ancient shield placed
flat on the ground. It is composed of thinner more mafic
lavas. Shield volcanoes have gentle and gradual eruptions.
These erupt the most often.
Example: Kilauea
Other formations:
Caldera – large crater-shaped basin that results from a big
volcanic eruption. Lakes may be found in these.
Rift Eruptions – where divergent plates are found on the ocean
floor. As the plates move apart, lava is released from the inside the
Earth and pillow lava is formed.
Where volcanoes are found:
90% of all volcanoes are found on plate boundaries.
10% occur in areas called hot spots. Hot spots are holes in the crust
where magma pushes up. Example: Hawaii
What volcanoes produce:
1. Islands – Hawaii is a good example
2. Fertile soils
3. Massive destruction – tephra is given off, mudflows, tsunamis