Transcript Chapter 2

Chapter 2
Internal Energy and Plate Tectonics
Lecture PowerPoint
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Origin of the Sun and Planets
• Solar system began as rotating spherical cloud of gas, ice, dust
and debris
• Gravitational attraction brought particles together into bigger and
bigger particles
• Cloud contracted, sped up and flattened into disk
• Formation of Sun
– Greatest accumulation of matter (H and He) at center of disk
– Temperature at center increased to 1 million degrees centigrade
– Nuclear fusion of hydrogen (H) and helium (He) began,
producing solar radiation
Origin of the Sun and Planets
Figure 2.1
Origin of the Sun and Planets
• Formation of planets
– Rings of concentrated matter formed within disk
– Particles within rings continued to collide to form planets
– Inner planets (Mercury, Venus, Earth and Mars) lost much gas
and liquid to solar radiation, becoming rocky (terrestrial)
– Outer planets retained gas and liquid, as gas planets
Impact Origin of the Moon
– Early impact of Mars-sized body with Earth
– Impact generated massive cloud of dust (from Earth’s crust and
mantle) and gas which condensed to form Moon
– Lightweight gases and liquids lost to space
– Lesser abundance of iron (from Earth’s core) in Moon
Earth History
• Earth began as aggregating mass of particles and gases
– Aggregation took 30 to 100 million years
– Occurred nearly 4.6 billion years ago
• Processes of planet formation created huge amounts of heat
– Impact energy
– Decay of radioactive
elements
– Gravitational energy
– Differentiation into
layers
Figure 2.2
Earth History
• Differentiation into layers
– As temperature rose above 1,000 centigrade, iron melted
– Liquid iron is denser than remaining rock, so sank toward
center of Earth to form inner and outer core
– Release of gravitational energy produced additional heat
– Remaining rock melted, allowing low-density material to rise
– Low-density material formed crust, oceans and atmosphere
• 4.4 billion years ago: large oceans, small continents
• 3.5 billion years ago: life (photosynthetic bacteria)
• 2.5 billion years ago: large continent
• 1.5 billion years ago: plate tectonics
Side Note: Mother Earth
Analogy with Mother Earth, 46-year-old woman:
(1 Mother Earth year = 100 million geologic years)
• First seven years unaccounted for
• 42 years old: life appeared on continents
• 45 years old: flowering plants
• 8 months ago: dinosaurs died out
• Last week: human ancestors evolved
• Yesterday: humans evolved
• Last hour: discovered agriculture, settled down
• 1 minute ago: Industrial Revolution
The Layered Earth
• Differentiated into layers of increasing density
• Center of Earth: Iron-rich core 7,000 km in diameter
– Inner core is solid and 2,450 km in diameter
– Outer core is liquid and has viscous convection
currents, responsible for Earth’s magnetic field
• Surrounding core is Earth’s mantle, 2,900 km thick
– Stony in composition (like chondritic meteorites)
– 83% of Earth’s volume, 67% of Earth’s mass
• Low-density elements have been ‘sweated’ out of the
mantle to form the crust, atmosphere and oceans
The Layered Earth
Figure 2.3
The Layered Earth
Figure 2.4
• Layers can be described in terms of
– Different density (different
chemical and mineral
compositions)
• Crust overlies mantle
– Or different strength
• Lithosphere overlies
asthenosphere
• Lithosphere is rigid (solid
rock)
• Asthenosphere is fluidlike
(plastic rock)
• Mesosphere is solid mantle
below asthenosphere
Side Note: Volcanoes and the Origin of
the Ocean, Atmosphere and Life
• Volcanic gases:
– Hydrogen (H), oxygen (O), carbon (C), sulfur (S), chlorine
(Cl), nitrogen (N)
• Combine to make:
– Water (H2O), carbon dioxide (CO2), sulfur dioxide (SO2),
hydrogen sulfide (H2S), carbon monoxide (CO), nitrogen
(N2), hydrogen (H2), hydrochloric acid (HCl), methane (CH4),
and others
• Dominant volcanic gas is water vapor – more than 90%
Side Note: Volcanoes and the Origin of
the Ocean, Atmospheres and Life
• Volcanic rocks:
– Oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium
(Ca), magnesium (Mg), sodium (Na), potassium (K)
• 4.5 billion years of volcanism has brought light weight
elements to the surface to make up
–
–
–
–
Continents
Oceans
Atmosphere
CHON (carbon, hydrogen, oxygen, nitrogen) elements of life
The Layered Earth
Behavior of Materials
• Gas, solid and liquid are obvious terms, but should be considered
with respect to time
– Over longer time periods, solids may behave as liquid
• Glacier: solid ice, yet flows (ultrahigh viscosity liquid) over
years
• Elastic deformation is recoverable – object returns to original
shape
• Ductile deformation is permanent – stress applied over long time
or at high temperatures
• Brittle deformation is permanent – stress applied very quickly to
shatter or break object
The Layered Earth
• Permanent stress occurs
when yield stress is
reached
• Rocks at Earth’s surface
(low temperature, low
pressure): brittle
• Rocks in asthenosphere:
“soft plastic”
• Rocks in deeper mantle:
“stiff plastic”
Figure 2.5
The Layered Earth
•
•
•
•
•
Asthenosphere is plastic
About 250 km thick
Comes to surface at mid-ocean ridges
Lies more than 100 km below surface elsewhere
Allows Earth to be oblate spheroid (flattened during
rotation; like Solar System during formation)
• Allows continents to ‘float’ atop the mantle, by
principles of isostasy
Isostasy
• Isostasy: Less dense materials float on top of more dense materials
(i.e. iceberg floating in ocean); buoyancy principle
• Earth is a series of density-stratified layers
• Core – densities up to 16 gm/cm3
• Mantle – densities from 5.7 to 3.3 gm/cm3
• Continents (crust) – densities
around 2.7 gm/cm3
• Oceans – densities around
1.03 gm/cm3
• Atmosphere – least dense
Figure 2.4
Isostasy
Examples of isostasy:
• Impoundment of water in Lake Mead behind Hoover
Dam caused area to sink 175 mm over 15 years
• Scandinavia is currently rising (about 200 m so far)
– Had been depressed under weight of ice sheets
during last Ice Age (since 10,000 years ago)
– Ground ruptures and earthquakes are present
– Viking ship buried in the harbor mud of Stockholm
was lifted above sea level
– Another 200 m of uplift is likely
Internal Sources of Energy
• Impact energy
– Tremendous numbers of smaller bodies hit the Earth early
after its formation, converting energy of motion to heat
• Gravitational energy
– As Earth pulled to smaller and denser mass, gravitational
energy was released as heat
Heat from both of these early sources is still flowing to
the surface today, as heat conducts very slowly
through rock
Internal Sources of Energy
Radioactive isotopes
• Unstable radioactive atoms (isotopes) decay and release heat
• Early Earth had much larger amount of short-lived radioactive
elements and therefore much greater heat production than now
• Radioactive decay process:
– Measured by half-life: time for half the atoms of a radioactive
element (parent) to disintegrate to decay (daughter) product
– Half-lives against time is negative exponential curve
Internal Sources of Energy
Figure 2.8
Figure 2.7
Internal Sources of Energy
• Sum of internal energy from impacts, gravity and
radioactive elements (plus tidal friction energy) is
very large
• Internal temperatures have been declining since early
Earth maximum, but still significant enough to cause
plate tectonics, earthquakes and volcanic eruptions
Internal Sources of Energy
Age of Earth
• Oldest Solar System materials are 4.57 billion years
old
– Measured using radioactive elements in Moon rocks and
meteorites
• Oldest Earth rocks (found in northwest Canada) are
4.055 billion years old
• Oldest Earth materials (zircon grains from Australian
sandstone) are 4.37 billion years old
Internal Sources of Energy
Age of Earth
• Earth must be younger than 4.57 billion years old
materials that formed the planet
– Seems that Earth has existed as coherent mass since about
4.54 billion years ago
– Probably took 30 million years (0.03 billion years) for
Earth to form
• Collision that formed the Moon seems to have
occurred between 4.537 and 4.533 billion years ago
• Earth must be older than 4.4 billion years old zircons
• Conclusion: Earth has existed about 4.5 billion years
In Greater Depth: Radioactive Isotopes
Elements defined by number of positively charged protons
– Isotopes are different forms of the same element with different
numbers of neutrons
– Radioactive isotopes are unstable and release energy through
their decay process to more stable isotopes
• Knowing the half-life of radioactive isotopes allows us to use their
quantity as a clock to date rocks
In Greater Depth: Radioactive Isotopes
• Nuclear fission:
– Parent atom sheds particles to become smaller daughter atom
– Alpha particle: two protons and two neutrons (helium atom)
– Beta particle: electron
– Gamma radiation: lowers energy level of nucleus
Figure 2.9
In Greater Depth: Radioactivity Disasters
• Chernobyl disaster of 1986, in Ukraine
– Explosion released 185 million curies of radioactivity,
affecting much of Europe
– 50 deaths in area, with many more later deaths from cancer
– Possibly caused by misinterpretation of small earthquake
• Can such a thing occur in nature? Depends on relative
amounts of U-238 and U-235:
– Most uranium ore is U-238, about 0.7% is U-235
– Uranium ore used in reactors is enriched to 2-4% U-235
– Because U-235 decays more rapidly than U-238, at some point
in the past all uranium ore would have had about 2-4% U-235
– Sites in West Africa were natural nuclear reactors about 2.1
billion years ago, at about 400 degrees centigrade temperatures
Plate Tectonics
Tectonic cycle:
• Melted asthenosphere flows upward as magma
• Cools to form new ocean floor (lithosphere)
• New oceanic lithosphere (slab) diverges from zone of formation
atop asthenosphere (seafloor spreading)
• When slab of oceanic lithosphere collides with another slab, older,
colder, denser slab subducts under younger, hotter, less dense slab
• Subducted slab is reabsorbed into the mantle
• Cycle takes on order of 250 million years
Plate Tectonics
Tectonic cycle:
Figure 2.11
Plate Tectonics
• Lithosphere of Earth is broken into plates
• Plate Tectonics: Study of movement and interaction
of plates
• Zones of plate-edge interactions are responsible for
most earthquakes, volcanoes and mountains
• Divergence zones
– Plates pull apart during seafloor spreading
• Transform faults
– Plates slide past one another
• Convergence zones
– Plates collide with one another
Plate Tectonics
Lithosphere of Earth is broken into plates separated by:
divergence zones, transform faults, convergence zones
Figure 2.12
Development of the Plate Tectonics
Concept
• 1620: Francis Bacon noted parallelism of Atlantic
coastlines of Africa and South America
• Late 1800s: Eduard Suess suggests ancient
supercontinent Gondwanaland (South America,
Africa, Antarctica, Australia, India and New
Zealand)
• 1915: Alfred Wegener’s book supports theory of
continental drift – all the continents had once been
supercontinent Pangaea, and had since drifted apart
• Theory of continental drift was rejected because
mechanism for movement of continents could not,
at the time, be visualized
Development of the Plate Tectonics
Concept
• 20th century: study of ocean floors provided wealth
of new data and breakthroughs in understanding
– Lithosphere moves laterally
– Continents are set within oceanic crust and ride along
plates
• Theory of plate tectonics was developed and widely
accepted
In Greater Depth: Earth’s Magnetic Field
• Earth’s magnetic field acts like giant bar magnet,
with north end near the North Pole and south end
near the South Pole
• Magnetic pole axis is now inclined 11o from
vertical (tilt has varied with time) so that
magnetic poles do not coincide with geographic
poles (but are always near each other)
• Inclination of magnetic lines can also be used to
determine at what latitude the rock formed
• Magnetic field is caused by dynamo in liquid
outer core:
– Movements of iron-rich fluid create electric
currents that generate magnetic field
Figure 2.13
In Greater Depth: Earth’s Magnetic Field
Problematic details of Earth’s magnetic field await resolution:
• Strength of magnetic field waxes and wanes
• Magnetic pole moves about geographic pole irregularly,
crossing 5o to 10o of latitude each century
• Magnetic polarity reverses
– Every several thousand to tens of millions of years
– Orientation of magnetic field switches from north (normal)
polarity to south (reverse) polarity
– Reverses take few thousand years to complete, with
complex field present during switch
• Changes in magnetic field are preserved in most volcanic and
some sedimentary rocks
Magnetization of Volcanic Rocks
• Magnetic patterns of ocean floor first observed in
mid 20th century – very important to theory of plate
tectonics
• Why does the ocean floor have a magnetic pattern?
– When lava cools to below 550oC (Curie point), atoms in
iron-bearing minerals line up in direction (polarity) of
Earth’s magnetic field
• Polarity of Earth’s magnetic field can be either to
north or to south and depends on time in Earth’s
history
Magnetization of Volcanic Rocks
• Successive lava flows stack up one on top of
another, each lava flow recording Earth’s polarity
at time it formed
• Each lava flow can also be dated using radioactive
elements in rock to give its age
Figure 2.14
Magnetization of Volcanic Rocks
• Magnetic patterns of ocean floor
• What does magnetic polarity of
lava flows tell us?
– Plotting the polarity of different lava
flows against their ages gives us a
record of the Earth’s polarity at
different times in the past
– Timing of polarity reversals (north
to south; south to north) seems
random
– Reversals probably caused by
changes in the flow of iron-rich
liquid in the Earth’s outer core
Figure 2.15
Magnetization Patterns on the Seafloors
• Atlantic Ocean floor is striped by parallel bands of
magnetized rock with alternating polarities
• Stripes are parallel to mid-ocean ridges, and pattern of
stripes is symmetrical across mid-ocean ridges (pattern
on one side of ridge has mirror opposite on other side)
• Pattern of alternating polarity stripes is same as pattern of
length of time between successive reversals of Earth’s
magnetic field
Figure 2.16
Magnetization Patterns on the Seafloors
• Magma is injected into the ocean ridges to cool and
form new rock imprinted with the Earth’s magnetic
field
• Seafloor is then pulled away from ocean ridge like
two large conveyor belts going in opposite
directions – seafloor spreading
Figure 2.17
Other Evidence of Plate Tectonics
Earthquake epicenters outline plate boundaries
• Map of earthquake epicenters around the world shows not a
random pattern, but lines of earthquake activity that define
edges of tectonic plates
Figure 2.18
Other Evidence of Plate Tectonics
Deep earthquakes
• Most earthquakes occur at depths less than 25 km
• Next to deep-ocean trenches, earthquakes occur along
inclined planes to depths up to 700 km
• These earthquakes are occurring in subducting plates
Figure 2.19
Other Evidence of Plate Tectonics
Ages from ocean basins
• Oldest rocks on ocean floor are ~200
million years old (<5% Earth’s age)
• Ocean basins are young features –
continually being formed (at mid ocean
ridges) and destroyed (at subduction zones)
• Hot spots in the mantle (plumes) cause
volcanoes on overlying plate, which form
in a line as plate moves over hot spot,
getting older in direction of plate
movement
• Sediment on the seafloor is very thin at mid
ocean ridges (where seafloor is very
young) and thicker near trenches (where
seafloor is oldest)
Figure 2.21
Other Evidence of Plate Tectonics
Oceanic mountain ranges and deep trenches
• Ocean bottom is mostly about 3.7 km deep, with two areas of
exception:
• Continuous mountain ranges extend more than 65,000 km along
ocean floors
– Volcanic mountains that form at spreading centers, where plates
pull apart and magma rises to fill gaps
• Narrow trenches extend to depths of more than 11 km
– Tops of subducting plates turn downward to enter mantle
Other Evidence of Plate Tectonics
Systematic increases in seafloor depth
• Ocean floor depths increase systematically with seafloor age,
moving away from mid-ocean ridges
• As oceanic crust gets older, it cools and becomes denser,
therefore sinking a little lower into mantle
• Weight of sediments on plate also cause it to sink a little into
mantle
Figure 2.22
Other Evidence of Plate Tectonics
The Fit of the Continents
• If continents on either side of the Atlantic Ocean used to be
adjacent, their outlines should match up
• Outlines of continents at the 1,800 m water depth line match
up very well
– 1,800 m water depth line marks boundary between lowerdensity continental rocks and higher-density oceanic
rocks
– Continental masses cover 40% of Earth’s surface, ocean
basins cover other 60%
Other Evidence of Plate Tectonics
Changing Positions of the Continents
• 220 million years ago, supercontinent Pangaea covered 40%
of Earth (60% was Panthalassa, massive ocean)
Figure 2.23
Other Evidence of Plate Tectonics
Changing Positions of the Continents
• 180 million years ago: Pangaea had broken up into Laurasia and
Gondwanaland
Figure 2.24a
Other Evidence of Plate Tectonics
Changing Positions of the Continents
• 135 million years ago: north Atlantic Ocean was opening; India was
moving toward Asia
Figure 2.24b
Other Evidence of Plate Tectonics
Changing Positions of the Continents
• 65 million years ago: south Atlantic Ocean was opening; Africa and
Europe had collided
Figure 2.24c
Other Evidence of Plate Tectonics
Changing Positions of the Continents
• Present: India has collided with Asia; Eurasia and North America
are separate; Australia and Antarctica are far apart
Figure 2.24d
The Grand Unifying Theory
Tectonic cycle:
– Rising hot rock in mantle melts to liquid magma
– Buildup of magma causes overlying lithosphere to uplift
and fracture; fractured lithosphere is then pulled outward
and downward by gravity, aided by convection in mantle
– Asthenosphere melts and rises to fill fractures, creating
new oceanic lithosphere
– New oceanic lithosphere becomes colder and denser as it
gets older and farther from the ridge where it formed
– Eventually oceanic lithosphere collides with another
plate; whichever is colder and denser will be forced
underneath and pulled back down into the mantle
– Lateral spreading may be aided by convection cells of
mantle heat
The Grand Unifying Theory
Figure 2.25
The Grand Unifying Theory
Tectonic cycle:
• When two plates collide, denser (colder, older) plate goes beneath
less-dense (warmer, younger) plate in subduction
– Oceanic plate beneath oceanic plate:
• Volcanic island arc next to trench (Aleutian Islands of
Alaska)
– Oceanic plate beneath continental plate:
• Volcanic arc on continent edge next to trench (Cascade
Range)
• Plate tectonics requires time perspective of millions and billions
of years
– Plate movement may be 1 cm/year  75 cm in human lifetime
• Uniformitarianism: small events add up to big results
How We Understand the Earth
• Must think in terms of geologic time rather than human
time – thousands, millions and billions of years
• In 1788, Hutton introduced concept of geologic time:
– “No vestige of a beginning, no prospect of an end.”
– Everyday changes over millions of years add up to major
results
• Uniformitarianism: natural laws are uniform through
time and space; present is the key to the past
• Contrast to previously believed catastrophism
• Currently modified actualism: rates of Earth processes
can vary
End of Chapter 2