Transcript Different Kinds of Volcanoes

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.
http://www.classzone.com/books/earth_sci
ence/terc/content/visualizations/es0902/es
0902page01.cfm
http://earthguide.ucsd.edu/eoc/teachers/t_t
ectonics/p_subduction.html
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
5-100 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. Composite
B. Shield
C. Cinder
D. 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