Transcript chpt 8Earthquake and volcanoes

Earthquakes
Volcanoes
Earthquakes, volcanoes and
plate tectonics
 Elastic
Rebound: when rocks are put stress
they may break causing elastic rebound
 Rocks
change shape or deform slowly over a
long period of time
 Energy
is released and vibrations move
through rock which is known as earthquakes
 The
surface of a break where rocks move as
a result of elastic rebound is called a fault
 Faults
can be either normal, reverse or
strike-slip depending on whether they are
pulled apart, move together, or move past
each other (shear forces)
Earthquakes
release waves and are
transmitted through the earth
These
waves
waves are called seismic
 When
the potential energy is released from
strained rocks seismic waves are released
 The
point at which the the energy is released
is called the focus
 The
point above the earthquake focus (at the
surface) is called the epicenter
These seismic waves travel from focus
throughout the earth’s interior(3 waves released)
 P-waves or primary waves are the fastest waves
to move through the interior


They move back and forth in the same direction as
the waves are moving
S-waves or secondary waves move at a slower
pace and move in an up and down pattern at
right angles to the direction of the wave
 L-waves or surface waves arrives last and cause
the most damage for buildings and structures at
the surface (at surface they move side-to-side)

 Seismograph
is an instrument that records
seismic waves
 Seismic
waves are recorded by a pen on a
pendulum that increases a line in a graph
according to the magnitude of the
earthquake
 Move
all heavy objects to lower shelves
 Secure your gas hot-water heater and
appliances
 Seismic safe structures assured by building
codes helps to build structures so they can
ride out vibrations of earthquakes
 Steel and rubber supports help riding out the
vibrations
 Spiral re-enforcment rods on cement pillars
help keep bridges and buildings up during
earthquakes
 Small
seismic activities are better than no
seismic activity in an earthquake prone area
 If
an earthquake prone area has no
earthquake activity it could lead up to one
large slippage where damage at surface is
great
 How
do volcanoes form
 Rising magma, solids, and gases spew out
onto earth’s surface to form cone-shaped
mountains are called volcanoes
 Magma (molten rock) that reaches the
surface through vents is called lava
 Volcanoes have circular holes near their
summits are called craters
 Tephra are bits of rock or solidified lava
dropped from the air (could be ash, cinders,
or larger rocks called bombs or blocks)
 Volcanic
islands form when oceanic crust and
mantle collides
 Older
denser oceanic crust subducts or sinks
beneath less dense mantle, where it melts
forming a magma pool and rises to form
volcanic islands
 Pyroclastic
flows are massive avalanches of
hot glowing rock flowing on a cushion of
intensely hot gases
 Composition
of magma influences how
destructive a volcano can be
 The more silica in the magma, the thicker
the magma and more chance that it will have
a violent eruption
 Iron and magnesium rich magma is more
fluid and erupts quietly (low silica content)
 Water vapor and gases trapped in magma by
silica rich magma leads to violent eruptions
 Basaltic
lava (high in iron and magnesuim
and low in silica) flow in broad flat layers
 Shield
volcanoes have broad bases with
gently sloping sides
 Hawaiian
islands are an example
 Lower
altitude volcanic mountains that form
as a result of layers volcanic ash, lava, and
cinders (usually less than 300 m in height)
 Moderate
 Gases
to violent eruptions occur
are important to formation of cinder
cone volcanoes
 Steep
sided mountains composed of
alternating layers of lava and tephra
 Erupt
violently releasing large quantities of
ash and gas
 Then
lava layers flow in between the tephra
layers (mountains formed because of
subduction zones and magma rise to surface)
 Cascade
Mountains are composite volcanoes
 Very
fluid magma can ooze from cracks or
fissures in earth’s crust
 Low viscosity of lava allows it to flow like
water across the surface
 Flood basalts form lava plateau like the
Columbia River Basalt flows
 Built up in some areas 2 miles thick
extending from Canada to California to
Wyoming
 Where
volcanoes form: most volcanoes form
along plate boundaries
 Divergent plate boundaries: Where plates
move apart long cracks form (rifts)
 When plates move apart, stress is placed on
the crust that allows cracks to form where
magma rises to those weaken fractures
 Fissures are formed where magma flows as
lava reaches the surface (primarily at rifts)

Basalt is most common rock at rifts zones
 Dense
oceanic crust dives under continental
crust at convergent boundaries

When one plate dives under another plate, basalt
and sediment are carried deep under earth’s
surface

The material eventually melt and rises through
weakness and cracks above to the surface

Pacific rim volcanoes like the Cascade Mts are
examples
 There
are areas on earth’s surface that is
hotter than others where there is a pool of
magma below
 Plates moves over these hot spots and
allows the magma to rise and reach the
surface
 Volcanoes are formed above these hot spots
 Hawaiian islands form in the middle of
convergent and divergent zones
 80%
of earthquakes occur along the Pacific
Rim of Fire
 Earthquakes are a result of pressure and
stress built up by moving plates at
convergent and divergent boundaries