The Dynamic Crust
The bald men in Albany want you to know:
2.1l The Earth is a dynamic geologic system.
2.1a Earth has an internal source of energy which creates heat.
2.1b The transfer of heat energy within the Earth’s interior results in the formation of regions of
different densities. These density differences result in motion.
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Earthquake Waves
Earthquake Epicenter Location
Earth's Interior
Earth's Liquid Outer Core
Convection Circulation & Seafloor Spreading
Evidence of Plate Motion
Plate Boundaries
Hot Spots
Earthquake Waves
The bald men in Albany want you to know:
2.1l Earthquakes and volcanoes present geologic hazards to humans. Loss of property, personal
injury, and loss of life can be reduced by effective emergency preparedness.
2.1j Analysis of seismic waves allows the determination of the location of earthquake epicenters, and
the measurement of earthquake magnitude.
1. Preparedness
 Build and reinforce structures
 Have a safety plan
 Move out of earthquake prone areas
2.
Seismic Waves
P
Primary
Fastest; Arrive at a seismograph
station first
Compressional; push waves
Can travel through solids, liquids, and
gasses
S
Secondary
Slower; Arrive at a seismograph
station after the P-waves
transform; sise to side waves
Can travel through solids only
Earthquake Epicenter location
Step 1. Determine the arrival times of the p-wave and s-wave.
Based on the sample seismogram the p-wave arrived at 0 minutes
and the s-wave arrived at 5 minutes.
Step 2. Calculate the time difference. S-wave arrival time minus
P-wave arrival time equals time difference. Based on our
sample we would have 5 minutes - 0 minutes = 5 minute time
difference
Step 3. Now turn to the Earthquake Time Travel Graph on page 11 in your
reference tables. Place a sheet of paper along the Travel Time axis of the
graph as shown in diagram 4 (only part of the graph is shown here) and mark
the time difference calculated in step 2.
Step 4. Now slide the piece of paper across the graph until the bottom mark
lines up with the p-wave line and the top mark lines up with the s-wave line.
Make sure the edge of the paper is parallel to the vertical lines on the graph.
Step 5. Now read straight down the edge of the paper until it crosses the
Epicenter Distance axis. This is the answer. Based on the sample seismogram
the epicenter distance is 3,400 km.
Step 6. Open a compass to the determined distance. Place the center of the
compass on the seismograph location and draw a circle around that point. The
earthquake epicenter must be somewhere on that circle.
Step 7. Repeat for two other seismograph locations.
Where the three circles meet is the location of the
epicenter.
Earth’s Interior
The bald men in Albany want you to know:
2.1j Properties of Earth’s internal structure (crust, mantle, outer core, and inner core) can be inferred
from the analysis of the behavior of seismic waves (including velocity and refraction). This analysis
leads to the inference that Earth’s interior is composed of layers that differ in composition and states
of matter.
2.1l Compared to continental crust, ocean crust is thinner and denser.
Drilling
We have only drilled part way through the crust.
Evidence
Meteorites
Xenoliths
Seismic Waves
Layers of the Earth's Interior
Thicker
Granitic
Continental
Lower Density
I
N
T
Crust
E
R Lithosphere
Oceanic
I
O
R
Rigid Mantle
Asthenosphere
Stiffer Mantle
Outer Core
Inner Core
Thinner
Basaltic
Higher Density
Earth’s Liquid outer core
The Earth’s outer core is believed to be a liquid. Whenever there is an
earthquake there is a shadow zone where no S-waves are detected. Something
inside the Earth is stopping the S-waves. We know that S-waves can not
travel through liquids; therefore it is believed the outer core of the Earth is
liquid. Although P-waves travel through the outer core they seem to be
refracted (bent). The amount they are bent could be accounted for by an Iron
rich liquid in the outer core.
Convection Circulation & Seafloor Spreading
The bald men in Albany want you to know:
2.1k The outward transfer of Earth’s internal heat drives convective circulation in the mantle that
moves the lithospheric plates comprising Earth’s surface.
2.1l The lithosphere consists of separate plates that ride on the more fluid asthenosphere and move
slowly in relationship to one another. New ocean crust continues to form at mid-ocean ridges.
2.1n Many of Earth’s surface features such as mid-ocean ridges are a consequence of forces
associated with plate motion.
Convection Circulation
Density differences in
the Earth’s mantle
cause convection
circulation. This
circulation is believed
to be the driving force
behind plate motion.
Seafloor Spreading
As the crust of the Earth is pulled apart at mid-ocean ridges, new rock forms.
The rock closest to the mid-ocean ridges is younger than the rock farther
away. Rocks on opposite sides of the ridge also show the same pattern of
magnetic reversals.
Evidence of plate motion
Evidence of Plate Tectonics
1.
Puzzle Like Fit
2.
Fossil Correlation
3.
Rock Correlation
4.
Earthquake and Volcano
Location
5.
Paleoclimate
Continents appear to fit together.
The same fossils are found on
opposite continents.
The same rocks are found on
opposite continents.
Earthquakes and volcanoes occur in
specific areas that represent the
plate margins.
Fossilized ferns in Antarctica and
evidence of glaciers in Africa
suggest the continents have moved
plate boundaries
The bald men in Albany want you to know:
2.1l The lithosphere consists of separate plates that ride on the more fluid asthenosphere and move
slowly in relationship to one another, creating convergent, divergent, and transform plate boundaries.
2.1n Many of Earth’s surface features such as mid-ocean ridges/rifts, trenches/subduction
zones/island arcs, mountain ranges, hot spots, and the magnetic and age patterns in surface bedrock
are a consequence of forces associated with plate motion.
2.1o Plate motions have resulted in global changes in geography and climate.
3.
Plate Boundaries
a) Divergent
Where two plates are moving away from
one another. Ex. Mid Atlantic Ridge.
b) Convergent
Where two plates are moving toward one
another. Can be Continent-Continent,
Ocean-Continet, or Ocean-Ocean. Ocean
trenches and volcanic mountains are
associated with these boundaries.
c) Transform
Where two plates are sliding past one another. Ex.
San Andreas fault system in California.
Hot spots