What is an earthquake?
• An earthquake is a sudden shaking of the
ground caused by breaking rock. They
generate seismic waves which can be
recorded on a sensitive instrument called
a seismograph.
• The record of ground shaking recorded by
the seismograph is called a seismogram.
What is a Seismograph?
• It’s a precision instrument that measures
seismic waves and arrival times. There are
several different types and styles.
Remember lithospheric plate
movement?
• Let’s review how the boundaries react to each
other.
• Divergent boundaries: This is the most
common kind of motion along the mid-ocean
ridges. This is a system of undersea mountain
ranges that extends beneath the world's oceans.
• At divergent boundaries we would find
normal faults
• Convergent boundaries: This is the kind of
motion at subduction zones. This motion
happens where dense oceanic plates collide,
and slide beneath continental plates or less
dense oceanic plates.
•At convergent boundaries we would find
reverse faults.
• Transform boundaries: A good example of this
type of motion is the San Andreas Fault which
runs through California.
The rock slides past each other at these
boundaries. The faults at these boundaries are
called strike-slip faults.
The movement between plates and along faults is not smooth.
They move in jerks, giving rise to earthquakes. The locations of
earthquakes throughout the world delineate the major tectonic
boundaries.
Under Stress
Earthquakes occur on faults.
Because faults have friction, they resist the forces trying to move
the pieces apart. As the forces build, the fault remains locked and
the blocks get deformed because of the increasing stress.
Eventually the stresses get so high that the fault breaks. This
releases the built up stress and allows the sides of the fault to
slide past one another. This is what we call an earthquake.
The blocks return to their locked state until the stresses build up
enough to cause another earthquake.
Types of Stress
• Shear - a sliding or twisting force
• Tension- a pulling force
• Compression- a pushing force
Now that we know how earthquakes
start, what exactly is happening?
• Seismic waves
Earthquakes create seismic waves which shake the ground as they pass. They
sometimes cause buildings to topple. Earthquakes create waves just like waves
of water moving across the ocean and waves of air moving across a field of
wheat.
Consider what happens when a drop of rain hits a pond of water. The drop
disturbs the flat surface of the water and creates waves that travel outward in all
directions from the disturbance. These waves travel on the surface of the pond,
along the interface between the water and the air.
The Slinky and The Rope
Earthquakes generate several kinds of seismic waves including P, for "Primary" and S, for
"Secondary" waves.
P Waves
The P waves move in a compress ional motion similar to the motion of a slinky, while the S waves
move in a shear motion perpendicular to the direction the wave is traveling.
S Waves
• Surface Waves
Surface wave rolls along the ground just like a wave rolls across a lake or an
ocean. Because it rolls, it moves the ground up and down, and side-to-side in
the same direction that the wave is moving. Most of the shaking felt from an
earthquake is due to the surface wave, which can be much larger than the
other waves.
A seismogram is a record of the ground shaking recorded
by a seismograph.
The P waves travel fastest through the Earth so they arrive
at a seismograph first, followed by the S waves and lastly by
the surface waves.
Locating the Shaking
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Seismologists locate earthquakes by measuring the time between
the P and S waves in a seismogram.
After a seismogram "feels" an earthquake, scientists compare the
time difference of these waves to figure out how far away the
earthquake is.
It takes at least three seismograms to locate exactly where the
earthquake is.
One seismograph can only tell how far away it is from that
seismograph. The earthquake could be located anywhere on a circle
of radius equal to this distance and centered on the observation
station. By measuring the S-P times at 3 or more stations these
circles can be drawn around each station and where they meet
indicates the earthquake location.
Once they find where it is, this is what we call the epicenter
Directly below the epicenter (but at different depths) is the
focus of the earthquake.
Measuring A Quake
• There are many ways to measure the size
of an earthquake. Some depend on the
amount of damage caused by the
earthquake while others depend on the
amount of seismic energy emitted by the
earthquake. Some are a combination of
both. There are three popular earthquake
scales.
Mercalli scale
The Mercalli Intensity Scale assigns an intensity or rating to
measure the effects of an earthquake at a particular location.
This is because the amount of damage caused by an earthquake
at a particular location depends on the geology of the location.
The population density and the methods used to construct
buildings near the location are also important in the Mercalli scale.
Earthquake intensities are rated with Roman numerals ranging
from I (not felt) to XII (buildings nearly destroyed).
The Richter magnitude scale was originally
developed by Charles Richter and Beno Gutenberg
to make more quantitative measures of the relative
sizes of earthquakes in southern California. Today,
modified versions of the scale are used to measure
earthquakes throughout the world.
The Richter magnitude is related to the maximum
amplitude of the S wave measured from the
seismogram. Because there is a great range in the
sizes of different earthquakes, the Richter scale uses
logarithms. Thus, a magnitude 7 (M 7) earthquake is
10 times as large as a magnitude 6 earthquake, and
releases over 30 times more energy.
Moment Magnitude Scale
• Most current system used
•Uses the Magnitude system
•Can be used for all earthquakes near and far
•Combines several aspects of both Mercalli and Richter scale.
Recorded difference
Magnitude 1
Magnitude 2
Intensity damages
Jump to Tsunamis
What are Volcanoes?
• A volcano is an opening in the Earth that
erupts gasses, ash, and lava.
• Can you name some?
• Kilauea (kee low AY ah)- most active in
world, located in Hawaii
• Hekla, located in Iceland
• Mount St. Helens in Washington State
• Soufriere Hills in Montserrat; one of the
largest recent eruptions
Where do they form??
Effects of Eruptions
• What effects can erupting volcanoes have?
• Lava flow destroys everything in path
• Pyroclastic flow (extremely hot cloud of volcanic
ash and debris) can start fires with anything it
contacts.
• Falling volcanic ash can collapse buildings,
block roads, and cause major respiratory
problems that can lead to death.
• Can cause acid rain
How do volcanoes form?
• Magma forced upward from pressure of surrounding
rock. The magma is less dense than the rock that
surrounds it.
• The magma exits through openings in the earth called
vents.
• Often these vents are found at divergent boundaries.
• At subduction zones of convergent boundaries, vents
are created for the volcano to form.
• Hot spots (like the Hawaiian Islands) also give birth to
volcanoes. The area is especially hot and close to the
magma. Eventually the magma makes it to the surface.
Do all volcanoes erupt violently?
• NO!
• Some volcanoes erupt explosively, and
some erupt quietly.
• The type of eruption depends on the
amount of trapped gasses, water vapor,
and silica (the elements silicon and
oxygen) in the magma.
What type of boundary do you think will
induce large amounts of water vapor?
• Convergent boundaries (at subduction
zones)
WHY??
Magma composition
• Two major types: Silica poor, and silica rich.
• Silica poor result in quiet eruptions: we call this
basaltic magma. This pours out of the vent and
runs down the volcano.
• Silica rich result in explosive eruptions: we call
this granitic magma. This is thicker, and traps a
lot of gas causing pressure to build. This results
in an explosion of magma.
• There is a third which is in between the two
called andesitic (named after the Andes)
Types of volcanoes
There are three major types of volcanoes:
Shield, Cinder Cone, and Composite.
Shield volcanoes
Mauna Loa, Hawaii, is an excellent example of a shield volcano
Shield Volcanoes
• This type of volcano can be hundreds of
miles across and tens of thousands of feet
high. The individual islands of the state of
Hawaii are simply large shield volcanoes.
Shield volcanoes have low slopes and
consist almost entirely of hardened
lava. Lava flows from one or more
vents without erupting violently. They
almost always have large craters at
their summits.
Shield volcano in cross section
Cinder Cone Volcanoes
Cinder Cone volcanoes
• These volcanoes consist almost entirely of
loose, grainy cinders, and almost no lava. This
means the lava is thrown from the volcano and
solidifies in the air, and is combined with bits of
rock. This composition is called tephra.
• Cinder cones are small volcanoes, usually only
about a mile across and up to about a thousand
feet high. They have very steep sides and
usually have a small crater on top.
Composite Volcanoes
Composite Volcanoes
• Many are located in populated areas and have wellknown names: Vesuvius, Krakatoa, Fujiyama, and Mount
St. Helens. These volcanoes are typically tens of miles
across and ten thousand or more feet in height. As
illustrated in the previous figure, they have moderately
steep sides and sometimes have small craters in their
summits. Volcanologists call these "strato-" or composite
volcanoes because they consist of alternating layers of
solid lava flows mixed with layers of sand- or gravel-like
volcanic rock called tephra cinders or volcanic ash.
Two more terms…
- Volcanic neck
- Caldera
Volcanic Neck: A massive pillar of rock more
resistant to erosion than the lavas and
pyroclastic rocks of a volcanic cone.
Caldera: The Spanish word for cauldron, a basin-shaped volcanic
depression; by definition, at least a mile in diameter. Such large
depressions are typically formed by the subsidence of volcanoes.
Crater Lake occupies the best-known caldera in the Cascades.
Kaguyak volcano, Alaska. This lake-filled caldera formed atop
a former stratovolcano (note remnant of upper part of older
cone at right center). The caldera formed about 1,100 years
ago and is 2.5 km in diameter. The prominent peninsula and
small island consists of lava domes erupted after the caldera
formed.
Kinds of lava flows…
• Pahoehoe (pa HOY hoy) - Pahoehoe lava is
characterized by a smooth, billowy, or ropy surface. A
ropy surface develops when a thin skin of cooler lava at
the surface of the flow is pushed into folds by the faster
moving, fluid lava just below the surface.
• Aa (AH ah) – These are flows that have a rough rubbly
surface composed of broken lava blocks called clinkers.
• Pillow lava - Lava that forms from an underwater
eruption and is characterized by pillow-shaped masses.
Pahoehoe (pa HOY hoy) Lava
Aa lava flow
Pillow lava