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Chapter 3
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Unit 3: The Dynamic Crust
Earthquakes:
 Zones of weakness build up in the earth, creating a
fault.
 The place underground where the break occurs is
called the focus.
 The epicenter is the place above the focus. The
epicenter is on the surface of the earth.
 The intensity of an earthquake is measured using the
Mercalli scale and the magnitude is measured using
the Richter scale.
 A seismograph measures magnitude. Each number
on the scale is 10 times greater in magnitudethan the
number below.
Locating Earthquakes:
 2 different types of energy waves result from
earthquakes. These are called P-waves and Swaves.
 P-waves:
 S-waves:
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 Finding the distance to the epicenter:
1. Subtract the difference between the P-wave and Swave arrival times
2. Make marks on the edge of a piece of paper that
represents that time difference. Use the time scale on
the P and S-wave travel time chart in the ESRTs – p.
11.
3. Slide the paper along the graph until the marks line up
with the 2 lines.
4. Look at the bottom of the graph to find the distance.
 Finding the origin time: The origin time is the moment
that the earthquake actually started. It will be different
than the time that most people detected the quake,
unless they are right at the epicenter!
 When you have the results from at least 3
seismographs, you can use a circle-drawing compass
to locate the epicenter. Put the point of the compass
on a map on the city in which the seismograph was.
Open the compass to the width of the distance from
that city to the epicenter. Repeat for the other 2 cities.
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Earth’s Layers: (See pg 10 in the ESRTs)
Scientists use data from earthquakes to infer the
conditions underneath the surface of the earth. The
earthquake waves travel differently through different types
of substances! Remember, S-waves can ONLY go through
SOLID MATERIAL.
Scientists also look at the meteorites that land on earth.
We can infer that the meteorites represent the material
found deep within the earth.
1. The Crust: The outermost layer of the earth.
 Only 5 km thick under the oceans.
 60 km under continents and mountains.
 The ocean crust is mostly basalt.
 The continental crust is mostly granite.
2. The Mantle: The layer just under the crust.
 We have not been able to yet directly observe
the mantle. The heat and pressure is too
intense for drilling equipment.
 The deepest drill holes are 10 km deep.
 The interface between the Mantle and the Crust
is called the MOHO (short for the Mohoricic
Discontinuity).
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 The mantle is much thicker than the crust! It
goes down to 2900 km below out feet!
 The mantle is denser than the crust.
 There is a small part of the mantle where partial
melting occurs.
 The mantle is probable composed of dark, mafic
minerals.
 Earthquake waves travel faster through the
mantle than through the crust!
3. The Core: The deepest part of the earth – separated
into the OUTER and INNER Core.
 Mostly iron and nickel, we think – just like the
composition of meteorites!
 The OUTER CORE is LIQUID – S-waves can’t
pass through.
 The INNER CORE is SOLID due to the intense
pressure on it.
Earthquake shadow zones occur on various parts of the
earth due to the bending/refraction of earthquake waves!
See p. 62 in the rev. bk.
Earthquakes/Volcanoes/Mountains – all tend to appear
near each other. For example, “The Pacific Ring of Fire”
has a lot of the earth’s active volcanoes, earthquakes and
many mountains.
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Earthquake dangers:
 Tsunamis or tidal waves occur when
earthquakes occur near the water, or under the
ocean.
 Fires occur when gas lines break.
 Soft sediments can collapse in landslides.
Volcanic Dangers:
 Lava and ash can bury cities.
 Toxic gasses can suffocate people.
CONTINENTAL DRIFT
Alfred Wegener thought that our continents were once
joined together. There are many clues on Earth today that
support this theory!
1. The eastern outlines of N. America and S. America
seem to fit against the western shore of Africa.
2. Similar rock formations and mountain ranges are
found on the once-touching parts of these continents.
3. Similar glacial and fossil evidence are found here.
But, it is not just the continents that move!! Now we know
that the ocean crusts are also moving. The mid-Atlantic
ridge or rift is a great example:
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1. The newest rock is in the center of the ridge.
2. The farther you go from the ridge, the older the rocks.
3. There is a mirror image for rock are on each side of
the ridge.
4. There is also a mirror image of magnetic polarity on
either side! (see Rev. Bk. P. 67)
The movements of both the continents and the ocean
crusts are combined in a theory of Plate Tectonics.
 There are a dozen major plates and several more
smaller ones. (See p. 5 in the ESRTs.)
 There are 3 different types of boundaries:
1. Convergent: 2 plates come towards each other.
 If one plate is an ocean plate, it will dive under
the less dense continental plate, producing a
subduction zone and a trench.
 If both are continents, the rock will fold, fault,
and lead to mountain-building – like the
Himalayas!
2. Transform: 2 plates slide past each other – like at the
San Andreas fault.
3. Divergent: This causes a rift – new crust that forms
from the center of a “break” in the crust. The MidAtlantic Ridge is a divergent boundary.
The plate movement along these boundaries is very slow –
only a few centimeters per year! However, this adds up!
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What drives the plate movements?
Convection currents (convection cells) cause heated
materials to rise. (see rev. bk. p 71)
Convection currents in the partially molten part of the
MANTLE, called the ASTHENOSPHERE allows the rigid
LITHOSPHERE (the crust and a small part of the upper
mantle) to move. New crust comes up at the Mid-Ocean
Ridges and crust is destroyed at the Trenches. (See ESRT
p. 5 and Rev Bk p. 68, 70)
Sometimes, plates move over HOT SPOTS and volcanic
features are formed. Find the Hot Spots on p. 5 in the
ESRTs.