Transcript Document

Tectonic Activity
• Volcanoes
– Cone Volcanoes
– Shield Volcanoes
– Composite Volcanoes
– Human effect of Volcanoes
Volcanoes are formed along two types of
plate boundary: destructive and constructive.
Volcanoes occur where molten rock (magma)
comes to the surface of the earth. The magma
rises to the surface through cracks in the crust
called vents.
Cone Volcanoes
When lava is thick and acidic it doesn’t flow far before cooling and solidifying, this causes
cone volcanoes (aka acid cone volcanoes) to have steep sides.
Example: Mount Pelee on Martinique (an island in the Caribbean)
Shield Volcanoes
When lava is runny and thin it can flow a long way before cooling and solidifying, this
causes shield volcanoes to have gentle slopes and wide bases built almost entirely of low
viscosity basaltic lava flows.
Shield volcanoes are the largest of the three types.
The eruptions are generally non explosive due to the low silica content and may last for
years
Example: Mount Kilauea in Hawaii (USA), Galapagos Islands, Snake River Plain in Idaho,
USA. Examples include the Wapi lava field and Hells Half Acre and Iceland (although
Iceland also contains other types of volcanoes.)
Composite Volcanoes
Composite volcanoes are the most deadly of volcano types. They are made of alternate
layers of ash and lava and have steep sides built up by eruptions of intermediate viscosity
andesitic lava and explosive tephra.
Often the lava cools creating a plug which blocks the vent resulting in a huge explosion
blowing out the plug
Example: Mount St Helens in Alaska (USA), Mount Shasta in California, Mount Rainier in
Washington state, and Mount Fuji in Japan.
Human effects of Volcanic activity
The effects of volcanoes can be divided into
primary and secondary effects.
Primary effects are produced directly by the
volcanic activity. Examples include lava
flows, ash-flows, lateral blasts, ash-falls,
and gases. Secondary effects are the result of
primary effects e.g. mud flows
NB Refer to specific examples – Mt St Helens/Mt Vesuvius
Lava Flows
Lava flows are streams of molten rock. Lava flows can erupt relatively non-explosively and
move very slowly (a few meters to a few hundred meters per hour) or they can move rapidly
(typically down steep slopes).
Most lava flows are slow enough that they are seldom a threat to human life. Such flows
generally follow a predictable course. However, lava flows can cause extensive damage or
total destruction by burning, crushing, or burying everything in their paths. Whole villages
have been known to completely disappear beneath lava flows.
To avoid such destruction, controlling a lava flow has become important and successful
deflection has occurred in the past. photos of lava flows.
Pyroclastic Hazards
Volcanic explosions produce volumes of tephra. Tephra is the material blown out of the
volcanic vent when an explosion occurs. Ash-flows, lateral blasts, and ash-falls are the
types of pyroclastic activity that produce tephra, with composite volcanoes and large
calderas the vent sources.
Pyroclastic flows (also called ash-flows) are high speed avalanches of hot ash, rock
fragments, and gas which move down the sides of a volcano during explosive eruptions.
These flows occur when the vent area or ash column collapses. Because pyroclastic flows
can reach 1500 degrees F and travel at high speeds (160-250 kilometres per hour and up),
they are extremely destructive and deadly. Pyroclastic flows are typical of composite
volcano eruptions, but are also associated with large caldera systems.
Volcanic Gases
All magmas contain dissolved gases that are released during and between eruptive episodes.
These gases are predominately steam, followed in abundance by carbon dioxide, compounds
of sulphur and chlorine, and lesser amounts of other gases. While they rarely reach
populated areas in lethal concentrations, gases can be injected to great heights in the
atmosphere by volcanic eruptions, in some cases spreading throughout the globe.
Mud flows (Lahars / Debris flows) are mixtures of water, rock, ash, sand, and mud that
originate from the slopes of a volcano. They can travel over 80 kilometres and commonly
reach speeds of 35 to 65 kilometres per hour.
They contain a high percentage of rock debris look like fast-moving rivers of concrete. Close
to a volcano, they have the strength to rip huge boulders, trees, and structures from the
ground and carry them for great distances. Farther downstream the coarser debris settles to
the bottom of the flow, leaving mud to continue on to cover everything it passes.
Mud Flows are formed when masses of unconsolidated, wet debris become mobilized, and
are commonly start by:
Large landslides of water-saturated debris
Heavy rainfall eroding volcanic deposits
Radiant heat emitted from a volcanic vent suddenly melting snow and ice
Pyroclastic flows on the flanks of a volcano
Breakout of water from glaciers, crater lakes, or from lakes dammed by volcanic eruptions
Historically, mud flows have been one of the most deadly of the volcanic hazards