Download Volcanoes

Volcanoes
The Global Distribution of
Volcanoes
Mid-Ocean Ridges
• mid ocean ridges
• The following video talks about how midocean ridges form volcanoes, and
“repave” the Earth’s surface.
I. Volcanoes Beneath the Sea
• A. Evidence of volcanic
rocks on the oceans floor
indicate there are more
volcanoes under water
than on land.
• B. Mid-Ocean Ridge – A
continuous mountain range
extending through the
North and South Atlantic
Oceans, the Indian Ocean,
and the South Pacific
Ocean.
I. Volcanoes Beneath the Sea
(cont’d)
• C. Rift Valley – The deep central cleft in
the mid-oceanic ridge.
• D. Magma – Naturally occurring molten
rock material generated within the Earth.
•
i. The magma rises and forms
submarine (underwater) volcanoes.
•
ii. Some places along the mid-ocean
ridges have so much volcanic activity, they
can form islands.
II. Volcanoes on Land
• A. The Cascades mountain range along
the western U.S. are formed by volcanoes.
• B. The “Ring of Fire” is a notable belt of
volcanoes that is spread throughout the
Pacific Ocean.
•
i. The Pacific plate is sliding beneath
continental plates and that forms the “Ring
of Fire.” These plates are called
convergent plates.
II. Volcanoes on Land (cont’d)
• ii. When a plate travels downward, it
creates heat. Water escapes the melting
rock, and eventually magma is formed.
The magma rises because it is less dense
than the existing rock.
• iii. 4/5’s of volcanoes on land are formed
when one plate slides under another.
III. Volcanoes Formed by Rifting
on the Continents
• A. Some volcanoes (like the ones in
Eastern Africa) are because the plates are
pulling away from each other. This is
called a divergent plate.
•
i. The plate is stretched and broken,
and it opens up a rift valley. This rift valley
allows magma to escape.
III. Volcanoes Formed by Rifting
on the Continents (cont’d)
• B. There is evidence of continental rifting
along the Eastern U.S. too.
IV. Volcanoes at Hot Spots
A. Hot spot – A fixed source of abundant
rising magma that forms a volcanic
center that has persisted for tens of
millions of years.
B. The Hawaiian Islands are evidence of
hot spot volcanoes.
Convergent Plate Boundaries
Convergent Plate Boundaries
Divergent Plate Boundaries
• Most are at mid-ocean ridges
• Mantle melts as pressure is reduced
– Melt is basaltic in composition
• Cooling basalt forms new oceanic crust
Activity 2
Volcanic Landforms
• 1. Topography of Volcanic
Regions
•
i. Contour lines - A line on
a map that connects points of
equal elevation of the land
surface.
•
a. Contour lines never
cross.
•
b. The closer together
the contour lines, the steeper
the slope.
• Details and pictures of contour
lines
Topographic Maps
• Topographic map - A
map showing the
topographic features
of the land surface.
Volcanoes
• National Geographic volcanoes
II. Magma Composition
• A. Magma is a mixture of liquid, melted
rock, and dissolved gases.
• B. The most abundant chemical elements
in magma are silicon and oxygen.
• C. Silicon and oxygen are the building
blocks of most of the common minerals.
• Lava Flows and Pyroclasts
II. Magma Composition (cont’d)
• D. Silica - the combination of the most
common elements, oxygen and silicon,
that are found in a volcano. The “oxide” is
SiO2.
•
a. More silica means more viscous,
or more of a violent explosion.
•
b. High silica will not form as much
lava as low silica, but it probably will cause
the pyroclastic flow to be huge.
III. Types of Volcanoes
• A. Composition Cone - A volcano that is
tall and has steep slopes because the lava
does not flow easily.
•
a. A volcano that is constructed of
alternating layers of lava and pyroclastic
deposits.
• B. Shield Volcano - The are generally the
largest in the world. They are not that tall
as the lava slowly moves out of them.
III. Types of Volcanoes (Cont’d)
• A. Shield volcanoes are not explosive.