Types of Volcanoes

Frequency and Quantity

There are ~ 550 historically active volcanoes About 60 erupt annually The majority are 1) at the edge of continents and

2) underwater

Dormant volcanic cone in Japan

What causes volcanoes?

Underwater Volcanoes

Rift valley

magma

plate plate

Mid ocean ridges

Mid ocean ridge

Mid ocean ridges Are continuous underwater mountain ranges Are formed by rising magma Cause sea floor spreading Form a rift valley between spreading plates

Pillow lava at mid ocean ridge

Hot Spots Hot spots create chains of islands like the

Hawaiian islands or the Aleutian chain.

Hawaiian Islands – hot spot

Land Based Volcanoes

A volcano at the edge of a continent

Sinking plateMid ocean ridge

Melted rock forming magma

?

Three Types of Land Based Volcanoes

Volcanoes take different shapes depending on the amount of SILICA and

DISSOLVED GASSES in the magma High Silica content…

More likely to be explosive!!!

Why? Silica has a thick,

stiff consistency Flows slowly Tends to Harden in

the volcano’s vent

High Dissolved Gas content… More gas comes out, an

eruption can launch ash and debris great distances

However, if lava is viscous and flows quickly, these eruptions may not be that dramatic

What Erupts from a Volcano?

Lava can be thick or

thin.

Blocky lava

PahoehoeAa Pillow lava

What Erupts from a Volcano?Pyroclastic material• Rock fragments created by eruptions

• magma explodes from volcano and solidifies in the air

• existing rock is shattered by powerful eruptions

Volcanic blocks

Volcanic bombs

Lapilli

Volcanic ash

Let’s see it in action http://environment.nationalgeographic.com/

environment/natural-disasters/forces-of-nature.html?section=t

Shield Volcanoes Volcanoes that build up from

many slow, steady, flows of hot lava, are called shield volcanoes. This kind of volcano is low and broad with gently sloping sides. They look like a warriors shield.

They have LOW silica content and LOW dissolved gases.

Unlikely to yield massive explostions

Shield Volcanoes

Cinder Cone Volcano Cinder cone volcanoes form when solid

rock and ash shoot up into the air and fall back around the volcano opening. The cinder cone volcano has steeply sloped sides.

Cinder cone volcanoes have LOW silica content and HIGH dissolved gas content

Cinder cones can have violent eruptions, but are unlikely to send ash or lava too far

Cinder Cone

Composite Volcano Composite volcanoes, erupt with

molten lava, solid rock, and ash. The layers pile up much like layers of cake and frosting. The layers form into symmetrical cones, and the slopes are steep.

Composite volcanoes have HIGH silica content and HIGH dissolved gas.

Composite volcanoes erupt violently and can send ash and lava great distances

Composite Volcano

Composite Volcano

Mt St Helens one day before eruption

Mt St Helens 4 months after eruption

Another before…

…and 2 years later

Now, it is your turn. Pick a type of

volcano and build it out of clay. Then, we are going to

make TOPOGRAPHIC MAPS.