Volcanoes What is a Volcano? A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface. Volcanoes usually occur at Divergent plate boundaries, Convergent plate boundaries and “hotspots”

Divergent Plate Boundaries At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates.

Divergent Plate Boundaries cont. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.

Convergent Plate Boundaries Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be very viscous due to its high silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.

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Hotspots "Hotspots" is the name given to volcanic provinces postulated to be formed by mantle plumes. These are postulated to comprise columns of hot material that rise from the core-mantle boundary. They are suggested to be hot, causing large-volume melting, and to be fixed in space. Because the tectonic plates move across them, each volcano becomes dormant after a while and a new volcano is then formed as the plate shifts over the postulated plume. The Hawaiian Islands have been suggested to have been formed in such a manner.

Lava enters the Pacific at the Big Island of Hawaii

Different Kinds of Volcanoes What makes volcanoes different?

How are these three volcanic cones different ? Look closely and think of 2 ways that they are different from one another.

Cinder Cones Cinder Cone Characteristics Small in size: less than ½ mile at base. Steep sides Lava is ash and cinders Eruptions are short explosive event

Composite Cones (Also called stratovolcanoes ) Composite Cone Characteristics Big volcanoes: ½ - 2 miles at base Broad base with steep summit. Thin runny lava alternates with thick, chunky ashy lava. Mostly granitic composition lava Eruptions alternate between explosive with ash and cinders, and quiet with runny lava. Majestic “postcard” volcanoes like St. Helens.

Shield Volcanoes Shield Volcano Characteristics Huge, enormous, broad cone miles at base. Rounded dome, rather flat summit. Thin, runny lava, usually basaltic. “Quiet” eruptions; lava oozes out rather slowly. Hawaiian Islands.

All Three Cone Types Together A. CompositeB. ShieldC. CinderD. Cinder

Quiz Time!! For each of the following slides, decide if they are composite, cinder or shield cones.

1. What kind of Volcanic Cone ?

2. What kind of Volcanic Cone ?

3. What kind of Volcanic Cone ?

4. What kind of Volcanic Cone ?

5. What kind of Volcanic Cone?

6. What kind of Volcanic Cone?

7. What kind of Volcanic Cone?

8. What kind of Volcanic Cone?

9. What kind of Volcanic Cone? (view from the air looking down on summit)

10.What kind of Volcanic Cone? (There are two types in photo)

Check your answers