Chap 7 Earthquakes and
Volcanoes
Chapter Objectives: 3 sections with
8 total objectives
Section 1:
1. Explain how earthquakes
are formed by a buildup within earth’s crust
2. Compare and
contrast primary, secondary and surface waves
3. Recognize earthquake
hazards and how to prepare for them
Section 2:
4. Explain how
volcanoes can affect people
5. Describe how types
of materials are produced by volcanoes
6. Compare how
three different volcano forms develop
Section 3:
7. Explain how
the locations of volcanoes and earthquakes epicenters are related to tectonic
plate boundries.
Elastic rebound
Applying force to a rock causes it to
strain
To change shape
Apply enough force and it will
break/snap – like a piece of lettuce
A vibration occurs when it snaps
Called elastic rebound
This changing shape (deforming)
takes long periods of time
The deforming builds up potential energy in the rocks
If the rock breaks under strain the potential energy is
released
This release causes an
earthquake
Types of Faults
When a rock breaks either side of the
rock might move due to the elastic rebound
On the surface of a break is called a fault
There are three types of faults – type is
determined by the cause of the fault not the change of position or end
result
Normal – caused by rock layers
pulling apart – (as in a divergent tectonic plate) but faults are
smaller and occur just within the crust
Reverse – caused by rock layers pushing together
Slip strike – when the rocks are moved side by side
Faults are caused by the motion of the earth’s plates, sections of Earth’s crust and upper mantle
(Drantch’s note –
you should be able to do a ven diagram and t charts – comparing and contrasting
or describing these waves )
Seismic waves – the energy waves created by earthquakes transmitted thru materials in the earth and at the surface
Earthquake Focus and Epicenter
The
strain along a faiult line is released by movement – it releases potential
energy – which moves out from the fault
This
energy release is an earthquake
The
point where the energy release (movement) first occurs is the focus
The
epicenter is the place on earth’s surface directly above where focus is –
(where the waves started)
Waves are grouped in two ways:
–1. by their time of origin from the start (P
& S)
2. by their
travel location ( interior & surface)
Waves are described in two waves:
1. by
their speed
2. by
their type of vibration
Seismic
waves travel out in all directions from the focus – the greatest damage is by
those that travel along the crust
There
are two types of seismic waves
P
waves – the primary waves – travel the fastest – cause material to vibrate –
rock back and forth – move in the same direction as the waves
S
waves – the secondary waves – cause vibration in materials at right angles from the direction the wave
is moving
Surface waves (those moving outside on the surface) – slowest and largest of the seismic waves – cause the most destruction
Complex movement – meaning they have many forms and effects
Some cause rock and soil to move back and force
Some cause soil to move roll as a wave does in the ocean
Some cause soil to sway – move side to side – these are
the most damaging
Buildings crack and fall down
Bridges and highways collapse
Speed that earthquake waves travel help us to find their location (or distance to the epicenter)
Earthquake
Measurements
A seismograph is a tool used by seismologists
Seismograph - tool used to gather and chart (by tracing) seismic waves from all over the planet
Seismologist – person that studies seismic waves – earthquakes
Magnitude – the strength of seismic wave
(based on the height ( amplitude) of
the line traced on the recording part of a seismograph)
Epicenter Location
When
seismic waves are charted on a seismograph:
The
further apart the different types of waves are from each other helps to
calculate distance from the epicenter
There are two scales to measure earthquakes:
Richter – by the energy strength (amplitude on a seismograph)
Mercalli – by the damage done by the earthquake in a specific location
Major earthquakes can cause much loss of life and destruction
Taiwan – 2400 dead, many injured and more homeless
Current day in Haiti
It leaves people homeless ffor many miles from the epicenter
Ie Mexico city – 400 km from the epicenter
The Richter Scale
Describes
how much energy an earthquake
releases
The
greatest amplitude measured is a 10 on the scale
Each number is 32 times more powerful
then the number before it
A
7.5 is 32 times more destructive (powerful) then a 6.5
Earthquake
Damage
Measured by the Mercalli scale - I thru XII
A Richter
6.7 has caused a Mercalli IX = severe
building damage & cracks in the ground
An earthquake on the ocean floor causes a huge (long length and tremendous volume of water compared to regular beach waves)
In deep water they can be a small height but very fast
About three feet tall
950 km per hour
As they get near land they slow down but get very tall
30 meter in height – nearly one hundred feet tall
An early sign of a tsunami is the the shoreline moves far in from the edge of the beach (it goes away from the place where the people would be)
The waves travel out from the epicenter within and along the ocean
They can travel out from many hundreds of miles
Earthquake dame can be decreased or prevented prevetive building and knowledge
(Drantch note – these preventions were
not in place in Haiti and so much damage occurred)
Is your home
seismic safe
Prevention means:
Heavy
objects on bottom shelves
Secure
any gas appliances
Water
heater, dish wasker, water boiler
Stay
away from windows
Protect
from fire hazards by having gas shut offs
Seismic-Safe
Structures
Have strict building codes which can make buildings (tall
high rise apartments and office buildings) earthquake proof
Steel
and rubber foundations to deal with the vibration
Bendable
pipes so they don’t snap and release gas or water
Bridges and roadways have reinforcement to prevent collapse
How Do Volcanoes Form
Raising magma and hot gases rise up thru
the rock layers form cone shaped mountains called volcanoes
Lava is magma which has flowed out of the vent
(opening ) of the volcano
Tephra is rock and solidified magma that rains down from the sky after a volcanic eruption (explosion) –it comes out in two forms
Can be:
dust like material called cinder (like ashes)
large rocks called bombs or blocks
Where Plates
Collide
Volcanoes can form from collisions of the Earth’s crust
At plate subduction
zones the older and denser oceanic
plate (crust with mantle) is pushed
under the less dense newer plate
The friction
at this site melts the crust forming pools of magma
Some
Caribbean islands formed this way
Montserrat
island
Eruptions
on a Caribbean Island
Soufrierre Hills Volcano – on island of monseratte
1995 but exploded
was thought dormant
Ash
rose to 10,000 meters
Covered the capital and many other
cities
Magma
avalanches and hot boiling mudflows covered cities and destroyed the port
1998
explosion volcanic ash reached 14,000 meters
ash
covered entire island
heavy
rain followed and caused mudflows
Pyroclastic flows – massive avalanches of hot rock flowing ontop of a cushion
of intensely hot gases
Can
reach speeds of 200/mph
Volcanic
Risks
Maps can be made to demonstrate risk of volcanic activity
by location around a volcano and its island
Volcanoes can cause destruction but also can deposit new rock
New rock deposition occurs many ways
What Determines How a Volcano Erupts
The
magma composition (what is made of) determines the type of eruption
Lava with lots of silica (silca and
oxygen) is thicker and resists flow
Lava
with iron and magnesium (less silica) flows easily
The
amount of water vapor in the lava will also influence (effect) lava eruption
Sticky,
silica rich magma traps much water vapor
The
trapped steam (of water vapor) tends to build up pressure (like a container of
soda) and can cause a powerful eruption (explosion)
In
the Lesser Antilles Volcano
Plate
subduction brought water into the mantle which got trapped in magma
Shield Volcanoes
Basaltic
lava – flows in broad, flat layers (Mona
Loa)
(high in iron and magnesium) – thinner less dense vs silica
magma high in silca – thicker denser
Shield volcanoes are basaltic lava
Gentle slopes of
basaltic layers
The largest types of volcanoes
Form when magma is forced between layers from the
depths
Or when there are divergent
plates
Rising magma gathers gases on its way to the surface
Moderate to violent explosions of:
Volcanic ash, cinders & lava blown high in the air (hot lava quickly cools in mid air)
This tephra forms small cone
300 meters high and usally form groups with with other larger volcanoes
tends to be short lasting because of the sudden gas release
Sunset
Crater
Steep
sided with alternating layers of tephra and and lava
They occur where
one plate sinks below another
Noising
eruptions lead to tephra deposits
Quiet eruptions lead to the gentle lava flows
Examples
are:
Mt. St. Helens
Soufriere Hills
Mt. Rainer
Fissure Eruptions
Magma
that is highly fluid, low viscosity (it flows easily- more like liquid ice
cream then ketchup) can ooze from cracks and fissures
This
magma is associated with fissure eruptions
A
flood basalt is a low flat ooze that has hardened with millions of years of
erosion
It is called a lava plateau
The Columbia River Plateau
Keep this simple – don’t over analyse or over study the
below material- use flash cards (know the differences and similarities without
a lot of memorizing)
Just
have some favorite facts in case you want to write a story
Choose a composite volcano, a cinder volcano, and a shield
volcano
– be able to describe its:
Eruptive
force, silica content, gas content, eruption products
Good examples are: Krakatoa, Paricutin, Kilauea
Earth’s Moving Plates
Earthquakes are the vibrations within the
tectonic plates as they move apart, together or slide past each other
These conditions can also cause
volcanoes
Where Volcanoes
Form
Most volcanoes
form at plate boundaries
Plate
movement can help explain the formation of volcanoes
Divergent Plate
Boundries
As tectonic plates move apart (divergent plates) cracks or
rifts form between them
The rifts contain
fractures in the crust which serve as an opening thru which magma can rise
Rift zones are the
most common place where volcanoes arise
Fissure eruptions occur at rift zones
They forms
lava which is basaltic – the most common rock in Earth’s crust
Convergent
Plate Boundries
Also common place is along convergent plate boundries
Dense oceanic plates
sink and melt below the less dense other plate
Mantle heat helps to
melt the baslt and sediment of the oceanic plate
The melted
basalt (magma) is then is forced upward as a volcano
Where the Pacific
plate collides with other plates surrounding it is called THE
RING OF FIRE
A belt of volcanos surrounding the Pacific Ocean
Hot
Spots
Hot spot
is where large bodies of magma are forced upward thru the crust.
The Hawaiian Islands
are forming as the pacific plate passes over this hot spot
The hot spot
stays in one place – the plate slides over it
The breaking and elastic rebound as plates move together causes earthquakes
Three causes of earthquakes are:
Convergent boundaries – plates moving together
Divergent boundaries – plates moving apart
Transform boundaries - plates grinding past each other side by side
Earthquake
Locations
Earthquakes happen most often in a common set of belts
80%
happen in the same belt where volcanoes form – the Pacific ring of fire
There
is a definite relationship between earthquake epicenters and tectonic plate
boundaries
Earth’s
Plates and Interior
Seismic waves provide much information for researchers about
Earth’s interior and tectonic plates
The
layers of the earth have been mapped depending on the speed at which seismic
waves pass thru the layers
The
asthenosphere was noted because seismic waves slow when the reach the base of
the lithosphere
The
asthensophere is a warmer, softer, molten layer that the colder brittle rocky
plates slide on.
What is
Driving Earth’s Plates
The mantle is heated by the core
As
it becomes hot and less dense it rises and forms convection cells which move
the plates
Also
as the plates they may also pass over hot spots which cause volcanic activity
Yellowstone National
Park – old faithful is over a hot spot in the North American Plate