Plate Tectonics
Plate Tectonics
Continental Drift
• Alfred Wegner—1st to
propose the theory of
continental drift
• His theory– Continents
were once a single
landmass then, over
time, they broke up and
drifted to where they
are today
Wegner’s Evidence of Continental Drift
1. Fossil Evidence
2. Rock formations
3. Climatic Evidence
4. Geographic Evidence
Fossil Evidence
Let’s do a puzzle!
• Fossil puzzle…match up
the fossils on the
continents to view
Wegner’s evidence of
continental drift
Rock
Formations
• Just like fossils,
similar rock
formations
were found on
different
continents
Climatic Evidence
• Shows that land used to have different
climates than is natural to have in the current
position of the land
• Example: Tropical plant fossils
were found in Antarctica
Geographic Evidence
• Shapes of continents look like they fit together and
have similar geographical features
FYI
• *As continents rift or as mountains form,
populations of organisms are separated.
When
• populations of organisms are separated, new
species may evolve from existing species.
• *Similar rock formations and fossil evidence
supported Wegener’s evidence.
Mid-Ocean Ridges
--a place where crust is spreading an creating
new ocean floor
--a place where old crust is recycled (this process
makes oceanic crust younger than continental
crust)
ACTIVITY!!!!!!!!!
Lets make a mid-ocean ridge!
Sea-Floor Spreading
• Harry Hess hypothesized that the valley at the
center of mid-ocean ridges was a crack in the
Earth’s crust
• At this crack, magma rises up to form new sea
floor which cools and forms new sea flooring
This process is known as sea-floor spreading
Layers of our
Planet
Layers of our Planet
• Lithosphere – the solid, outer layer of Earth
that consists of the crust and the rigid upper
part of the mantle
• Asthenosphere – The solid, plastic or puttylike layer of the mantle beneath the
lithosphere. Made of mantle rock that flows
very slowly, which allows tectonic plates to
move on top of it
Theory of Plate Tectonics
• This is the theory that explains how large pieces of
lithosphere called plates, move and change shape
Tectonic Plates
• Scientists have identified about 15 major plates:
Plate Tectonics Activity
Types of Plate Boundaries
Divergent Boundaries
• These types of boundaries move AWAY from
each other
• Examples: Mid-ocean rifts, the Red Sea
(between the African and Arabian Plates)
Convergent Boundaries
Plates collide together at these types of boundaries
Examples:
• Areas of subduction
• Mountain ranges
• Ocean trenches
trench
Transform Boundaries
• A boundary at which two plates slide past each
other horizontally
Example:
San Andreas Fault in
California
The
Supercontinent
Cycle
• The process by which
supercontinents form
and break apart
• This happens
because tectonic
plates of the Earth
are always moving,
though very slowly
450 Million Years ago
• Near the end of
the Precambrian
time Earth’s
continents were
separated…not
as much land
was above water
as today
250 Million Years Ago
• By the end of the Permian Period, Pangaea had
formed
150 Million Years Ago
• Pangaea had split into two continents, Laurasia and
Gondwanaland
60 Million Years Ago
• Continents were moving toward their current
positions on Earth
Future?
• Scientist propose that in about 250 million years
the earth might look like this:
Earthquakes
What is an Earthquake?
• Earthquake--Occur when rocks under stress
shift along a fault (energy released)
• Fault—break in rock where one block
moves relative to another
Let’s Build a Fault
Model!
Damage from Northridge Earthquake in Southern
California--1994
Anatomy of an Earthquake
• Focus--1st motion of earthquake in Earth,
along fault
• Epicenter—point on Earth’s surface directly
above focus
• 90% of continental earthquakes have
shallow focus—most damaging
Parts of an Earthquake
Elastic Rebound
Elastic Rebound—
sudden return of
deformed rock to
undeformed
shape
Elastic rebound
causes earthquakes
Fence offset
due
to Elastic
Rebound
Seismic Waves
• When rocks on a fault move, they release
energy as seismic waves
• Seismic waves travel outward from the focus
in all directions
Body wave—seismic wave that travels thru a
medium
–two types: P-wave & S-wave
Seismic Waves
• P (primary) waves are the fastest & travel thru
solids, liquids, & gases
Seismic Waves
• S (secondary) waves are 2nd fastest & travel
thru solids only. Although slower they can
cause the most damage
Seismic Waves
Surface wave—
a seismic wave that
travels along surface
of medium. These
are S & P waves that
reach the surface &
can cause greatest
damage
Water Wave and Slinky
Activities!
WAYS TO MEASURE AN EARTHQUAKE
• There are two major scales to measure an
earthquake:
–Mercalli Scale: measures earthquake
intensity (by amount of damage caused)
–Richter Scale: measures earthquake
magnitude (strength)
Studying Earthquakes
• Seismology—study of earthquakes &
seismic waves
• Seismograph—records vibrations in ground
• Seismogram—tracing of EQ motion,
recorded by seismograph
Bill Nye on the Richter Scale
http://www.youtube.com/watch?v=1qbg7orb1lc
Bill Nye on Waves
Earthquake recording station/seismograph clip
http://www.youtube.com/watch?v=dx4OqT0PYnU&lr=1
SEISMOGRAM
Order of recording: P-waves, S- waves, then surface
waves
•Shows S-P lag time
P-Wave
S-Wave
S-Wave
S-Wave
•To find S-P lag time, subtract where the P wave starts from
where the S wave starts (S-P = Lag Time)
What is S-P lag time for El Paso,TX?
• 28 sec – 0 sec = 28 sec lag time
What is S-P lag time for San Francisco, CA?
• 120 sec – 0 sec = 120 sec lag time
What is S-P lag time for Seattle, WA?
• 165 sec – 0 sec = 165 sec lag time
Which city is closest to the epicenter?
• The city with the shortest lag time is closest to the epicenter, so it’s El Paso, TX
Locating Earthquakes Through Triangulation
• S-P lag times are
analyzed to find the
epicenter of
Earthquakes using 3
stations
• Why 3 stations?
Epicenter
• Because all we know
is distance, so it takes
3 circles to narrow it
down to one point
Let’s stop here and
practice finding S P lag
time
Fault Zones
• Fault zone—region w/ lots of faults that
typically form at plate boundaries
• Fault zones and earthquakes can occur away
from plate boundaries as well
Fault Zones—New Madrid
• New Madrid fault line—New Madrid, MO—
120 miles southward on I-55
• 1811/1812—most widely felt series of E.Q. in
U.S. history (2000+)
• Five were 8.0+
• Ancient fault zone deep in crust of Mississippi
River region
Tsunamis
• Tsunami—giant ocean wave that forms
after volcanic eruption, submarine
earthquake or landslide
• May form when ocean floor rises due to
earthquake
Tsunami animation
http://embc.gov.bc.ca/em/tsunamis/causes
_2.htm
Japan’s Tsunami Before and After…(Google Earth)
Volcanoes
Erupting with fun!
Magma vs. Lava
• Magma is liquid rock produced under the Earth’s
surface but Magma becomes Lava as soon as it
leaves the Earth and flows outside.
Lava
Magma
Magma
Magma can form under three conditions:
1. If rock temperature rises above its melting point
then it will melt into magma
2. Rock can melt when too much pressure is
removed from rock that is above its melting point
3. The addition of fluids, such as water, may lower
the melting point of some rock and cause it to
melt
Volcanism
• Volcanism is any activity that includes the
movement of magma onto Earth’s surface
• A volcano is a vent or fissure in Earth’s surface
through which magma and gases are expelled
Location and Formation Of Volcanoes
Most volcanoes
occur in convergent
and divergent
boundaries of
tectonic plates
A major zone of active volcanoes is located in the
“Pacific Ring of Fire” which is also a major earthquake zone
Locations of Volcanoes
Some tectonic boundaries include:
• Subduction Zones
• Mid-Ocean Ridges
• Hot Spots
Subduction Zones
A subduction zone is where one tectonic
plate moves under another
Mid-Ocean Ridges
• A long, undersea
mountain chain that
has a steep, narrow
valley at its center.
• Forms as magma
rises from the
asthenosphere
• Creates new ocean
floor as tectonic
plates move apart
(sea floor spreading)
Hot Spots
• Areas of volcanism within the interiors of
lithospheric plates
• The Hawaiian Islands were created by a hot spot
• See HOT demo!
Types of Volcanic Eruptions
• Quiet volcanoes have mafic magma
which is rich in magnesium and iron.
Mafic magma is dark in color with
runny lava
• Explosive volcanoes have felsic magma
which is rich in light-colored silicate
materials and is thick and sticky.
Compare water boiling (quiet volcano) to thick
spaghetti sauce boiling (explosive volcano)
Pyroclastic
Materials
• “Pyro” means fire
• Pyroclastic materials
include fragments of
rock that form during
volcanic eruptions
Types of Pyroclastic Materials
(from smallest to largest
1. Volcanic dust
2. Volcanic ash
3. Lapilli—means little
stones…they normally fall near
the vent
4. Volcanic bombs—clots of lava
that are thrown out of the
volcano & become round or
spindle shaped as they go
through the air
5. Volcanic blocks—solid rock that
is blasted through the
vent…some are as big as a
house!!
Read “What Volcanoes
Can Produce”
Types of Volcanoes
1.
2.
3.
4.
Shield volcanoes
Cinder Cones
Composite Volcanoes (Stratovolcanoes)
Calderas
Shield Volcanoes
• Wide base
• Gently sloping sides
• Forms from quiet eruptions
Cinder Cone Volcanoes
• Steep slopes
• Form from explosive eruptions and are
made of pyroclastic materials
Composite Volcanoes
(Stratovolcanoes)
• Made with alternating layers of hardened lava flows
and pyroclastic materials
• This type of volcano alternates quiet and explosive
eruptions
Calderas
• This type of volcano is created when the magma
chamber below a volcano empties and the cone
collapses
• The collapse leaves a large basin-shaped depression
(crater)
• Some calderas eventually fill with water to become
lakes
Read “What Are Volcanoes?”
and answer questions