Download Plate tectonics and Volcanoes

 The lithosphere (earth’s crust and rigid upper mantle)
is broken into about 12 major plates and several small
ones
 Oceanic:
 under oceans
 Thin
 3-4 miles thick
 Basalt
 Continental:
 “land”
 20-30 miles thick
 granite
 The lithospheric plates interact with one another at
plate boundaries (where two plates meet).
 Plates can:
Converge or come together
2. Diverge or move apart
3. Slide past one another horizontally
1.
 Oceanic-continental
 When a oceanic plate collides with a continental plate
the thinner more dense oceanic plate subducts; it is
overridden by the thicker less dense continental plate.
 Features: volcanic mountains along the continental
plate and a trench where the oceanic plate subducts.
 Example: Andes Mts.
 http://geology.com/nsta/convergent-boundary-
oceanic-continental.gif
 Ocean-Ocean
 When two oceanic plates meet one of them subducts.
 Features: a trench,
Volcanic island arc
 Example: Aleutian
Trench and Islands,
Mariana Trench and
Islands
 http://geology.com/nsta/convergent-boundary-
oceanic-oceanic.gif
 Continental-Continental
 When two continental plates collide both are less dense
than the mantle so neither of them subducts.
 The colliding continental masses are folded and uplifted
to form mountains.
 Features: tall, folded
mountains
 Example: Himalayas
 Continental
 When continental lithospheric crust begins to separate a
long, narrow depression called a rift valley forms.
 The rising magma puts pressure on the crust producing
fractures
 Features: rift valley
 Example: East African Rift Valley
 Oceanic
 Two oceanic plates are separating and a rift (a fault-
bounded valley) forms.
 New crust is created
 Features: A ridge of underwater mountains
 Example: Mid-ocean Ridge
 When two plates are not moving toward each other or
apart but are sliding past one another horizontally.
 Features: a fault
 Example: San Andreas Fault
 Convection is the tranfer of energy through movement
of a fluid.
 The mantle is solid but part of it the asthenosphere
can flow – it is like a soft, pliable plastic
 Convection current is set up by the transfer of energy
from the hot, lower mantle and the cooler, upper
mantle.
 Ridge push – during formation of a ridge
asthenosphere rises; the weight of the uplifted ridge
pushes an oceanic plate toward the trench at the
subduction zone.
 Slab pull – a sinking region of mantle convection
current may suck an oceanic plate downward; weight
of the subducting plate helps pull the trailing
lithosphere into the subduction zone.
Other Tectonic Mechanisms
 Hot Spots – volcanoes that occur due to unusually hot
regions within the mantle
 Form along plate boundaries.
 Form at hot spots
Volcano Formation
 Rising magma gathers in an area of weak rock called a magma chamber
 The magma breaks through a fissure or crack as it is forced to the
surface.
 The gas-rich magma moves up
a circular tube called a vent to
the surface.
 Eruptions of lava and
pyroclastic material build
a volcanic mountain
Volcanic Material
 Lava: molten rock – the composition of the magma
determines the type of eruption. High viscosity lava
erupts more violently than low viscosity lava.
 Gases: SO2, H2S, CO2, HCl, HF – can produce
volcanic smog, acid rain and diminished daylight
 Pyroclastic material: ejected particles – from the size
of fine dust to large boulders
 Cinders – lapilli 2-64mm
 Blocks - >64 mm
 Bombs – semimolten blocks
Types of Volcanoes
 Shield: accumulation of fluid
basaltic lava; low cone
 Cinder: accumulation of
cinders
 Composite or stratovolcano:
acculmulation of alternating
layers of ash and lava
Dangers
 Earthquakes
 Mudslides; lahar
 Pyroclastic flows
 Gases
Pyroclastic falls
 Tephra falls back to earth due to gravity
Pyroclastic flows
 Hot gas and rock flows downslope at high speed
 http://www.youtube.com/watch?v=yvG_N7eqMWk
Lahar or mudflows
 Mud is created by tephra and water and flows down
slope; can also contain debris
 http://www.youtube.com/watch?v=kznwnpNTB6k
Lava flows

Lava flows down slope
Aa – rough textured lava
pahoehoe – smoother flowing
block-lava flows
Aa
pahoe pahoe
ravioli
Landslides
 Debris travels down slope at high speed