Around 1912, a German scientist named
Alfred Wegener theorized that all of the Earth's continents were
once joined together in a single, large landmass. He further proposed that
the continents have separated and collided as they have moved around over
the last few million years. He called this theory continental drift. He
provided several pieces of evidence to support his theory:
Continent Shapes- The continents
appear to be shaped in such a way that they would fit together nicely,
like a jigsaw puzzle.
Rock Formations- There are rock
formations on different continents that match up beautifully when the
continents are put back together.
Fossils- There are fossils found on
different continents that would also match up nicely if the continents
were all once together.
of the time mostly thought Wegener was crazy!
In the 1950's, scientists discovered
some surprising evidence in support of Wegener's theory. While mapping the
ocean floor, scientists discovered two important, and unexpected things:
the age of the rocks that make up the ocean floor gets older as you move
away from the ridges at the center. This meant that the youngest rocks
were found near the ridges, and the oldest rocks near the continents.
Below is a graph of the
rock ages for the map on top.
there are stripes of alternating magnetic polarity on each side of
the ridge. When the molten rock hardens, the magnetic minerals in the rock
align themselves with the Earth's magnetic field. Scientists discovered
that the Earth's magnetic field has reversed itself many times, at
intervals of roughly every 100,000 years. The pattern they observed makes
sense if the ocean floor is being formed at the ridge and gradually pushed
outward in both directions.
discoveries gave rise to the now respectable science of Plate Tectonics.
This is the theory that the Earth's seemingly solid crust is actually made
up of several pieces, or plates, that move around independently.
The places where the different plates
meet, called plate boundaries, are where the tectonic action really
is. There are three basic types: convergent, divergent, and transform
Boundaries: This a when two plates
are moving toward each other, as shown above.
the two plates are of relatively low, and similar densities, the plates
will form a Collision Boundary.
this scenario, the crust is forced upward by the collision, resulting in mountain
building. The diagram above shows how this type of collision between
forced the formation of the
one of the plates is more dense than the other, as happens when oceanic
and continental crust meet, then the more dense plate will be forced under
the less dense plate. This forms a trench, or deep valley, where
the plates meet. This is called subduction, and is shown in
the diagram above. This often results in a chain of volcanoes running
parallel to the trench.
Boundaries: As you might expect, this
is essentially the opposite of a convergent boundary. This occurs when two
plates are moving away from one another, as shown above. This is
seen at mid-ocean ridges and rifts.
Boundaries: This type of boundary
forms when two plates are sliding past one another. The diagram
above illustrates this motion. The most popular example of this is the San
Andreas Fault in
of the different boundaries and their locations are found on page
5 of the Earth
Science Reference Tables, shown below. Notice the key that shows
the different boundaries and their symbols.
The movement of the plates is driven
by convection currents in the mantle. These currents cause the
solid plates to float along on top of the semi-molten mantle material.
there is an opening in the middle of a plate that allows the molten
material to flow through it. This is called a hot spot, and usually
results in a chain of volcanic islands that form as the plate moves over
the hot spot. The Hawaiian Islands are a great example of this.
As you studied volcanoes, igneous,
metamorphic and sedimentary rocks, and earthquakes, you learned how these topics
are related to plate tectonics. In this chapter we take a closer look at plates
and plate motion. We will pay particular attention to plate boundaries and the
possible driving mechanisms for plate motion.
The history of the concept of plate
tectonics is a good example of how scientists think and work and how a
hypothesis can be proposed, discarded, modified, and then reborn. In the first
part of this chapter we trace the evolution of an idea - how the earlier
hypothesis of moving continents (continental drift) and a moving sea floor
(sea-floor spreading) were combined to form a theory of plate tectonics
1. Continental drift was proposed by
Alfred Wegner in the early 1900s based on the apparent fit of continental
coastlines, similar fossil plants and animals on widely separated continents,
distribution of Paleozoic glaciations and paleoclimatology, and apparent polar
2. Wegner proposed that all continents
had once been connected in a supercontinent called Pangaea, that broke apart
to form the present continents. Wegner thought the continents moved across
stationary oceanic crust. His ideas received little support when
proposed because he could provide no mechanism that allowed continents to plow
through ocean crust.
3. Paleomagnetism is the study of the
ancient magnetic fields of the earth. Magnetized minerals preserve a record of
the direction of the magnetic pole and their distance from it at the time of
their formation. Paleomagnetic data revived interest in continental drift by
demonstrating polar wandering and supporting the reconstruction of Pangaea.
4. Other recent evidence for
continental drift includes better continental margin fits, similar rock
contacts and age relations between continents when fitted together, glacial
movements indicated by striations, and sources of boulders in ancient tills,
and similar geologic sequences including metamorphic rocks in Brazil and
5. The idea that the sea floor spread
away from mid-oceanic ridges and was subducted beneath a continent or island
arc as a result of mantle convection was proposed by Harry Hess in the early
6. Sea-floor spreading explains processes at the mid-oceanic ridges as the
result of rising mantle: the existence of the ridge itself, high heat flow,
basaltic volcanism, a rift valley and shallow-focus earthquakes.
7. Sea-floor spreading explains
processes at the oceanic trenches as the result of descending oceanic crust:
existence of the trench itself and volcanism.
8. Sea-floor spreading explains the
young age of the sea floor, loss of older oceanic crust, and increasingly
older oceanic crust away from the ridge crest.
9. Plate tectonics is the theory that
the earth's surface is divided into a few large, thick plates that move and
change size. It combines the older ideas of continental drift and sea-floor
spreading. Plates are formed by lithosphere (crust and uppermost mantle) and
are carried along by the asthenosphere to a depth of about 200 km. New lithosphere is added
along the ridges at the trailing edge of the plate and lost to subduction.
Plate boundaries are either diverging, converging or transform.
10. Sea floor magnetic anomalies were
symmetrical with respect to the mid-oceanic ridge crests and matched the
pattern of magnetic reversals discovered previously in stacked lava flows. Spreading
rates are 1 to 6 cm/year. The hypothesis also allows prediction of the sea
floor age based on magnetic anomalies that can be tested with samples
recovered by deep-sea drilling.
11. Diverging plate boundaries
experience extension that produces normal faults, shallow-focus earthquakes,
rift valleys, basaltic volcanism, crust thinning, uplift, and creates new
ocean basins. Whether rifting causes uplift, or vice versa is unclear.
12. Transform boundaries allow plates
to slide past one another. These boundaries exhibit strike-slip motion and may
connect two ridge segments, a ridge and a trench, or two trenches. The
straight course of these faults resolves mechanical constraints caused by
divergence along curved boundaries.
13. Ocean-ocean convergence is
characterized by andesitic to basaltic island arcs and trenches
14. Ocean-continent convergence
exhibits an active continental margin associated with young volcanic and some
metamorphic mountain belts and trenches.
16. Continent-continent convergence passes through the stages exhibited by
ocean-continent convergence, but results in a suture zone of young mountains
in continental interiors marking the former subduction site, thickened
continental crust, and shallow focus earthquakes. Ex. the Himalayas. Collision
zone not subduction zone -- no trench.
18. Plate tectonics explains
consistently: distribution of basaltic and andesitic volcanoes, shallow-,
intermediate-, and deep-focus earthquakes, young mountain belts, mid-oceanic
ridges, oceanic trenches, and fracture zones.
19. Convection currents in the
asthenosphere cause mantle movement. The overlying plates are carried
along with mantle movement.
20. Mantle convection may result in
mantle plumes or hot spots. They are stationary with respect to moving plates and produce
hot spots, such as Yellowstone, Iceland and the Hawaiian Islands. Mantle
plumes may also be responsible for the initial fracturing of the lithosphere
causing divergence. (e.g. Red Sea
||2 plates move away from each
||Mid-ocean ridges, rift valleys
- basaltic magma creating new crust
||2 plates slide past one another
||San Andreas Fault, mid-ocean
3 types of convergent
boundaries - destroying crust
|ocean crust - ocean crust
||2 plates move together
- older, denser crust - subducts
- curved volcanic island arc
- andesitic to basaltic magma
- ex. Japanese island arc, Aleutian Islands
- shallow, intermediate, deep earthquake
|ocean crust - continental crust
||2 plates move together
- oceanic crust - subducts - denser
- andesitic magma
- metamorphic rock
- ex. Andes Mtns., Cascades
- shallow, intermediate, few deep
|continental crust - continental
||2 plates move together
- No subduction - no trench
- Double crust thickness
- Mountain range which may have marine
fossils from oceanic crust that became narrow (Think India plate
- shallow earthquake focus
Earthquakes & Volcanoes
Earthquakes, volcanoes and mountain
ranges tend to happen in similar areas.
The map above shows frequent earthquake activity as the bands of dots.
Most of the world's active volcanoes (triangles) are along the edges of
tectonic plates (the lines).
Types of Plate Boundaries
Convergent- plates collide into each
Where an oceanic and a continental plate collide, the denser oceanic
plate will be forced under (subduction) the other.
- plates spread apart
- caused by magma upwelling from deep in the Earth
- usually found in the oceans along with mid-ocean ridges
P & S
EQ Waves as X-Rays
Transform- two plates slide past each
In this aerial view of the San Andreas Fault (transform) the trees
in the orchard (dots) have been offset by the slipping of the plates.
The Pacific Plate is to the left and the North American to the right.
This picture shows how things like fences, roads, rivers and
buildings can be offset by the sliding of the plates.
The exception to this arrangement are "Hot Spots" which are
plumes of hot material (rather than belts) in the middle of plates.
These spots stay stationary while the plate moves above it. The spot
melts through the plate like a blow torch and produces a volcano above
it. As the plate moves, the spot melts through another spot producing a
chain of volcanic islands. Hawaii is an example of a hot spot island
The movement of the plates is caused by convection
currents deep within the Earth. The force that moves the plates around
the earth are convection currents inside the mantle.
- Hotter Mantle material rises while cooler material sinks.
- The crust is split and diverges where the material rises and
- The plates converge and subduct where the material is sinking.
The different types of plate boundaries are caused by a combination
of the direction of convection as well as they type of crust:
continental or oceanic.
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- Puzzle Fit of the continents to form Pangaea (see below)
- Fossil Evidence (see below)
- Glacial Evidence (see below)
- Coal in Antarctica- coal is formed in tropical swamps. Coal was
formed when Antarctica was closer to the equator.
- Magnetic Stripes
on the ocean floor (this one's going to take some explaining so it
gets its own page)
- Mountain Chains appear where they should if continents are
Puzzle Fit- if the continents were cut
out of a map, most of the landmasses will fit together to form a larger
supercontinent, which is called Pangaea.
Fossil Evidence- in the picture above,
fossils of many land-living have been found on opposite shores. When
Pangaea is re-assembled, the fossils match up.
Glacial Evidence- when Pangaea is
re-assembled, there is evidence of a single ice sheet (at least for this
episode) affecting many of the southern continents. When viewed this
way, this sheet leaves consistent evidence of a single glacier. When
viewed on the current continents, it is inconsistent and even highly
improbable. For example, India, which is north or the Equator, has
glacial evidence coming from the south!
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An earthquake is an event where two pieces of crust
shift against each other. The rumbling felt is from the rocks slipping,
sticking and breaking. The vibrations are called seismic waves. There
are different types of seismic waves that vibrate in different ways.
The focus is the spot within the earth
where the earthquake began.
The epicenter is the spot on Earth's
surface closest to the focus.
A fault is a crack along which the
during an earthquake, several types of waves are generated. The
vibrations felt are actually called seismic waves that are traveling
through the Earth.
- Primary wave- travels phastest so it arrives at
seismic stations phirst.
- Push-pull wave: rock vibrates forward and backward
in the same direction that the wave travels ("parallel
- Pass through solids and liquids (magma).
- Secondary wave- arrives at a seismic station second.
- Slow wave- not as fast as the P-wave.
- Shake wave (shear wave)- vibrates side-to-side.
- Solids wave- only travels through solids.
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In order to do the calculations that help find the distance to
epicenter and the time of the earthquake, you’ll need to do
math with time. It is difficult to do time math in a calculator,
and by hand it does not work exactly the same way as regular
math. Regular math is what they call “base 10” which means
that whenever you count past 9 you must move over one place to
the tens column. Time is base 60. You can count 59 seconds and
then you go to the minutes column.
For example, if you want to subtract 82 minus 17 in regular base
ten numbers you would “borrow” a ten and start by taking 7
away from 12.
is the same as
8712 (70 plus 12)
Time math works almost the same way except
instead of taking over ten from the neighboring column you’ll
take one minute and convert it into 60 seconds.
3:13:25 (“3 hours, 13 minutes, and 25
3:12:85 (3 hrs,12 min, 85 secs is the same as
How to Use the P-Wave and
S-Wave Travel Time Chart
& S-Wave Chart (opens a new window so your pop-up blocker may
The P-line shows how much time it take a P-wave to
travel a certain distance. So if you need to know how much time it takes
the p-wave to travel 2,000km, it is just over 4 minutes (about 4:05 ).
The S-wave works the same way: for 2,000km it takes 7:20 .
To find the distance to epicenter:
You are in charge of watching the seismic station
tonight when the seismograph detects an earthquake. The earthquake
didn’t happen where you are- you can’t even feel it. As a result,
you don’t know what distance or direction the earthquake happened. The
P-wave and S-wave are separated by 4:05 (4 minutes, 5 seconds). You need
to find a spot on the graph where the P-line and the S-line are
separated by 4:05 .
- Take a scrap piece of paper, line it up along the left edge of
- Put a small tick mark on your scrap paper at zero, and a small
tick mark at 4:05 .
- Slide the scrap paper up along the chart until it the two tick
marks just touch the P and S lines. BE SURE THAT YOUR SCRAP PAPER
IS PERFECTLY STRAIGHT UP AND DOWN (use the lines on the grid as a
- Now that you have found the right spot on the graph, drop a line
straight down to the bottom of the graph to read the distance-
To Find The Time That The Earthquake Occurred
When a seismograph detects an earthquake that happened
at some distance, (2,600km for example) you know that the earthquake
happened some time in the past and it took time for the waves to reach
your station. But how long ago? All you need to do is answer the
question “how long does it take a P-wave to travel 2,600km?
- Find 2,600km on the bottom of the chart.
- Go straight up until you reach the P-line and read the time from
the left of the chart: 5:00 (5 minutes).
- Now compare times: if you detected the earthquake at 3:17:00 and
it took 5:00 then the earthquake happened 5 minutes before 3:17:00
or 3:12:00 .
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The intensity or strength of an earthquake is measured
in two main ways:
- The Richter Scale
- measures the amount of energy that an earthquake releases
- Each number of magnitude is 10x stronger than the number
- The Mercalli Scale
- Measures the amount of damage from an earthquake
- Ranges from I to XII
- Based on common earthquake occurrences such as
"noticeable by people" "damage to
buildings" chimneys collapse" "fissures open in
Seismic waves as “x-rays”
- P-Waves travel through solid and liquid
- S-Waves travel only through solids
- Seismic waves travel faster through denser material.
- Because of this, the path traveled by a seismic wave is bent
towards the surface.
Properties of the material (such as density and
pressure) that the waves pass through can be inferred by the speed and
angle that the waves travel.
The layers of the earth are determined by the jumps in velocity and
“echoes” of seismic waves.
The MOHO is a boundary between the crust and the upper
mantle where the velocity of waves jumps up sharply. This sharp increase
in velocity is called a discontinuity.
A shadow zone occurs on the opposite side of the earth from an
earthquake because of the liquid outer core. S-Waves are stopped all
together while the P-Waves are refracted (bent) to create a zone where
no waves are picked up at all. This zone is between 102° and 143°around
the earth from the earthquake.
Lab research and studies of meteorites suggest that the
core is made of Iron and Nickel (FeNi).