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Unit 10—Earth Science
Chapter 28 The Changing Earth
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28.1 Understanding Earth
 Geology is the study of rocks and materials that make up Earth and the processes that shape it.
 In 1666, Nicholas Steno (1638-87) noticed that shark’s teeth resembled mysterious stones called
“tonguestones” that were found in local rocks.
 Steno’s explanation helped him develop ideas about how rocks and fossils form.
 These ideas are used in a technique called relative dating.
 Relative dating is a way to put events in the order in which they happened.
 In the picture above on the right, what happened? In what order?
 The approximate age of each layer of a rock formation can be determined by applying Steno’s
idea called superposition.
 A stack of newspapers illustrates superposition.
 Superposition means that the bottom layers of rock are older than the layers on the top.
 Original horizontality states that sediment particles fall to the bottom of a basin, such as a
riverbed, in response to gravity and result in horizontal layers.
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Chapter 28 The Changing Earth
 Lateral continuity is the idea that layers of sediment extend in all directions when they form and
before they become rock layers.
 The idea of lateral continuity states that layers of rock are continuous unless a geologic event
like a river interrupts them or an earthquake offsets them.
Example of cross-cutting: the vein depicted is
younger than the rest of the rock around it
 Another important idea, developed by Scottish geologist James Hutton (1726-97), is that the
"present explains the past."
 The idea of cross-cutting relationships states that a vein of rock is younger than the rock that
surrounds a vein (see picture at the right above).
 Sometimes rock pieces called inclusions are contained in another rock.
 During the formation of a rock with inclusions, sediments or melted rock surrounded the
inclusion and then solidified.
 Therefore, the inclusions are older than the surrounding rock.
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Chapter 28 The Changing Earth
 Over geologic history, many animals and plants have lived and become extinct.
 Their remains have become fossils.
 The idea of faunal succession states that fossils can be used to identify the relative age of layers
of a rock formation.
 William Thompson Kelvin (1824-1907), known for proposing the absolute temperature scale
that came to be named after him, meticulously calculated Earth’s age to be between 10 million
and 100 million years.
 Lord Kelvin’s calculation was not accurate because he did not realize that Earth has internal heat
from the core and radioactive decay (above at left).
 Earth’s age was estimated by measuring the radioactive decay of uranium to lead.
Unit 10—Earth Science
Chapter 28 The Changing Earth
 With improved techniques and evidence from tree rings and glaciers, the age of Earth is
estimated to be about 4.6 billion years.
 The picture below better illustrates the layers of Earth. Earth’s surface is covered with a thin
crust.
Unit 10—Earth Science
Chapter 28 The Changing Earth
 The lithosphere is primarily the Earth’s crust plus the upper layer of the mantle.
 The asthenosphere is layer of the Earth’s mantle just below the crust. It is made up of flowing
material that allows the plates to move.
 The geologic time scale is a model of the Earth’s geologic history that divides time into eras.
 Much of geologic time was understood by studying the fossil record.
 The same geologic events that occurred in the past still occur today (volcanoes and earthquakes,
extinctions, etc.).
 Fossils are any direct or indirect remains of prehistoric life.
 Scientists who study the fossil rock record are called paleontologists.
 Scientists who study the Earth-shaping processes and Earth structures are called geologists.
 Geologic time websites:
 http://www.fossils-facts-and-finds.com/clock_of_eras.html
 http://funnel.sfsu.edu/courses/geol350/timescaleinayear.pdf
 The division of Earth’s interior into crust, mantle, outer core, and inner core is based primarily
on the study of volcanic eruptions.
 There are two kinds of crust:
 continental
 oceanic
Continental Crust
Oceanic Crust
Average Thickness
10-80 km
5-10 km
3
Density
2.75 g/cm
3.0 g/cm3
Oldest Known Age
3.5 billion years
200 million years
Composition
mostly granite
basalt
Unit 10—Earth Science
Chapter 28 The Changing Earth
 The rocky material of the mantle
moves in very slow convection
currents.
 This movement is related to
density and temperature
differences in the mantle.
 Hot material is less dense and
rises.
 Cold material is denser and
sinks.
Unit 10—Earth Science
Chapter 28 The Changing Earth
28.2 Plate Tectonics
 The theory of plate tectonics explains the movement
of continents and other geological events like
earthquakes and volcanoes.
 The term tectonics means construction or building.
 The theory of plate tectonics, stated in 1965, refers
to the movement of giant pieces of solid rock on
Earth’s surface called tectonic plates.
 In 1915, Alfred Wegener (1880-1930), a German meteorologist, wrote a book titled The Origin of
Continents and Oceans.
 Wegener gathered evidence that supported his idea that all the continents had been connected.
 Evidence supporting the fact that the continents had once been a single land mass exists—rock
and fossil correlation can be made where the continents appear to fit together.
 The ancient supercontinent (the single land mass) was known as Pangaea
Unit 10—Earth Science
Chapter 28 The Changing Earth
Sea Floor Spreading
 In the early 1960s, Henry Hess (1906-69),
a geologist and former commander of a
Navy ship equipped with an echo sounder,
used the profile of the sea floor to
propose that it was spreading at the midocean ridges.
 An echo sounder is used to make a profile
of the sea floor.
 Around the same time, Robert Dietz (1914-95), a
scientist with similar ideas, coined the term seafloor spreading.
 Sea-floor spreading describes the sea floor on
either side of a mid-ocean ridge as moving away
from the ridge and creating a rise or valley.
 Mid-ocean ridges such as the Mid-Atlantic Ridge
and the East Pacific Ridge are areas on the ocean
floor that contain the youngest oceanic crust.
 When Pangaea began to break apart, the MidAtlantic Ridge was formed in the Atlantic Ocean.
 Hot fluid from the mantle
(called magma) enters the
rise or valley and cools,
creating new sea floor (also
called oceanic crust).
 Hydrothermal vents on the
ocean floor near the midocean ridges provide
evidence that convection
currents exist in the
asthenosphere.
Unit 10—Earth Science
Chapter 28 The Changing Earth
Magnetic Patterns
 Over geologic time, the magnetic polarity of
Earth has switched.
 Scientists believe the poles switch because
of a magnetic interaction between the
planet’s inner and outer core.
 In the 1950s and 1960s, scientists
discovered that the rocks of the sea floor
have a very interesting magnetic pattern.
 Stripes of rock with a north-south
orientation (normal) alternate with stripes
of rock with a south-north orientation
(reversed).
 The stripes you see in the figures below are
an interpretation of a magnetic profile.
 A zone of weakness or a break in Earth’s crust is known as a fault.
 A fault is a crack in a rock mass along which there has been movement of the rock layers.
 Most present-day faults occur at plate boundaries.
Unit 10—Earth Science
Chapter 28 The Changing Earth
 There are three main kinds of plate boundaries:
 divergent
 convergent
 transform
 The tectonic plates that cover Earth’s surface are pieces of the lithosphere that fit together and
float on the asthenosphere (a part of the mantle).
 There are a number of large tectonic plates on Earth’s surface, and smaller plates are being
identified all the time.
 These plates continue to slowly move around the earth.
 Can you identify which of the plates are only made of oceanic crust?
Unit 10—Earth Science
Chapter 28 The Changing Earth
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Convergent plate boundaries occur where two plates approach each other.
One result of two plates converging is subduction.
Subduction is the process by which one tectonic plate is pushed under another.
Trenches form as evidence that one plate is sliding under another plate.
A deep oceanic trench marks the boundary between a subducting and an overriding plate at a
convergent boundary.
Earthquake and volcanic activity are both found at convergent plate boundaries.
Volcanic island arcs, trenches, and mountain building are events that occur at convergent plate
boundaries.
If oceanic crust is converging with continental crust, a trench will form.
The Mariana Trench, Caribbean Islands, and the Himalayan Mountains are examples of features
found at convergent plate boundaries.
A transform plate boundary is a place where masses of rock slide past each other, moving in
opposite directions.
Unit 10—Earth Science
Chapter 28 The Changing Earth
 At divergent plate boundaries, the plate move away from one another.
 Rift valleys and ocean ridges are found at divergent plate boundaries.
 A mid-ocean ridge is an ocean mountain range that results from divergent plate
boundaries.
 Convection currents in the mantle distribute heat from the core to Earth’s surface.
 The movement of tectonic plates is related to the distribution of heat by convection currents in
the mantle.
 The tectonic plates continue to move today, but the movement is very slow (not noticeable in a
lifetime). Tectonic movement cannot account for recent melting of glaciers and polar ice.
Unit 10—Earth Science
Chapter 28 The Changing Earth
28.3 Earthquakes
 As tectonic plates move, friction causes the
rocks at plate boundaries to stretch or
compress.
 Like a stretched rubber band or a
compressed spring, these rocks store
energy.
 When the rocks break, change shape, or
decrease in volume, the stored energy is
suddenly converted to movement energy
and an earthquake occurs.
 A seismograph measures earthquakes, and
seismologists use seismic waves to study
Earth’s internal structure.
 This is similar to how a doctor uses X rays to
look at bone structure.
 Since earthquake waves travel at different
speeds through different materials, useful
information about the composition of
Earth’s interior can be gathered.
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The majority of earthquakes occur at the boundaries of tectonic plates.
Individual earthquakes also occur where there is a fault.
A scientist who studies earthquakes is called a seismologist.
A fault is a place in Earth’s crust such as a crack or a transform plate boundary.
If scientists have data from at least 3 different seismograph stations, the epicenter of an
earthquake can be determined.
 P-waves (primary waves) travel the fastest and move through fluids (water) so these are
received first by seismograph station.
 S-waves (secondary waves) travel a bit slower and do not travel through water.
 P-waves travel in a push/pull direction, while S-waves move sideways.
 If only the arrival times of the P-waves and S-waves are known, only the distance to the
earthquake can be determined.
The focus is the place within Earth where an earthquake originates.
The epicenter is the place on Earth’s surface directly above the point at which an earthquake
originates. The waves from the earthquake hit the epicenter first.
The Richter scale, a scale from 1-10, is used to rate earthquakes based on the amount of energy
released. Its rating is based on readings from seismographs
The Mercalli scale has 12 descriptive categories that rate the damage caused to buildings, the
ground, and people. One earthquake may have different Mercalli ratings in different areas.
The Moment Magnitude scale rates the total energy. Its rating is based on total energy emitted
and damage caused.
Unit 10—Earth Science
Chapter 28 The Changing Earth
Worldwide Earthquakes:
 The west coast of the United States experiences frequent earthquakes because those regions
are near the San Andreas fault and a plate boundary.
 The Midwest and eastern United States rarely experience earthquakes.
 The most useful information in predicting the occurrence of an earthquake at a particular
location is the seismic history of the area.
 Geologic hazard maps are prepared. These compile seismic history to give the geology of an
area.
 Minor earthquakes release stored energy in small, less destructive amounts.
 Rocks in areas that do not experience frequent small earthquakes may have a lot of stored
energy.
 When this potential energy is finally converted to kinetic energy, the earthquake could be big.
Unit 10—Earth Science
Chapter 28 The Changing Earth
Earthquake Safety
Get outside in a wide
open area.
Sit down to avoid
falling.
If you are inside,
drop, cover, and
hold (drop down,
cover your head,
hold on to
something sturdy).
Stay away from
windows.
Description
Great
Major
Strong
Moderate
Light
Minor
Very minor
Frequency of Earthquakes (Worldwide)
Richter Scale Magnitude
Average # per year
8 or over
1
7-7.9
18
6-6.9
120
5-5.9
800
4-4.9
6,200
3-3.9
49,000
1-2.9
9000/day
 Another risk from earthquakes is the possibility of a tsunami developing.
 A huge wave generated by an underwater earthquake or landslide is called a tsunami.
 The speed at which this wave travels can be about 700 kilometers per hour.
Unit 10—Earth Science
Chapter 28 The Changing Earth