Dynamic Earth

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Dynamic Earth

• Class 10
• 9 February 2006

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VideoContinental Drift Legacy of Fire
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Any Questions?
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Homework, Chapter 3

• What is the association between tectonic plate
  boundaries and volcanism? Can the eruptive style
  and the chemical composition of the volcanic
  deposits or the rock type be correlated with the
  type of plate boundaries (convergent versus
  divergent)?

More than 90 of the world's volcanic activity
occurs at plate boundaries, and the type of
activity can be related to the type of boundary.
Free-flowing basaltic lavas appear at divergent
boundaries andesitic and rhyolitic lavas
generally erupt violently at convergent
boundaries.
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Homework, Chapter 3

• What kinds of forces result in earthquake-producin
  g faults occur at the three types of plate
  boundaries?

Earthquakes that occur along mid-oceanic ridges
result from tension those along transform
boundaries are produced by shearing and those
along convergent boundaries are caused by
compression.
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Homework, Chapter 3

• At what depth do earthquakes form at the three
  types of plate boundaries?

Most major earthquakes occur along plate
boundaries. Shallow-focus earthquakes tend to
occur at divergent and transform boundaries.
Deep-focus earthquakes occur at convergent
boundaries. Several shallow-focus earthquakes
occur each year at locations that are not
associated with plate boundaries.
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Homework, Chapter 3

• At a place along a boundary fault between the
  Nazca Plate and the South American Plate, the
  relative plate motion is 8 cm/year. The last
  great earthquake, in 1880, showed a fault slip or
  displacement of 12 m. When should local
  residence begin to worry about another great
  earthquake?

With a relative motion of 80 mm per year, the
total motion will be about 8 m in 100 years (or
80 m in 1000 years). This is not that close to
the 12 m of slip that occurred in 1880 and more
than 100 years have passed since that time. They
might be more concerned in 2025 when the total
motion may be nearer 12 m.
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Exam Review

• First Exam Tuesday February 14th
• Exam will be fill in the blank and short answer
  questions
• Questions will come from Lectures (including
  videos), reading and homework

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Uniformitarianism
The present is the key to the past James
Hutton

• Geologic processes that we see in operation today
  have worked much the same way over geologic time
  however, rates and intensity of processes may
  have changed

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Relative vs Absolute Age

• Relative Rock A is older (or younger) than
  rock B
• Absolute The exact age of a rock (in years)
• Usually geologists first establish relative ages
  then try to get absolute age dates
• Know the difference between absolute and relative
  age and be able to give an example

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Radiometric Dating

• Use of radioactive decay to determine the age of
  a rock
• First proposed 1896-1902

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Radiometric Dating

• Key principle Half Life time required for 1/2
  of the nuclei in a sample to decay

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Radioactivity and Absolute Time
Decay of parent atoms
Growth of daughter atoms
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Radiometric Dating

• Shows that the earth is much older than people
  had previously suspected
• Earth formed about 4.6 billion years ago
• But, oldest rocks on earth are only about 4
  billion years old

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Relative vs Absolute Age

• Usually geologists first establish relative ages
  then try to get absolute age dates
• Determining relative age relies on a number of
  geologic principles that were developed during
  the 17th to early 19th centuries

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Principle of Superposition
Sedimentary rocks are deposited in a layer-cake
fashion
Layer 4
Layer 3
Layer 2
Layer 1
Each layer is older than the one above and
younger than the one below
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Principle of Original Horizontality

• Layers of sediment are deposited in a nearly
  horizontal position

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Principle of Cross-cutting Relationships

• Something (such as a dike or fault) that cuts
  across a layer must be younger than the layer

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Magnetostratigraphy

• An alternate method for absolute age dating that
  works well with volcanic rocks

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Earths Magnetic Field
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Magnetization of Magnetite
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The Geologic time scale

• Divisions in the worldwide stratigraphic column
  based on variations in preserved fossils
• Built using a combination of stratigraphic
  relationships, cross-cutting relationships, and
  absolute (isotopic) ages

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The Geologic Column and Time Scale
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The first 4 billion years of Earths history was
nearly devoid of life this time is known as the
Precambrian. Age of Earth 4.6 by Age of oldest
rocks 4.0 by
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After the Precambrian, life began to develop very
rapidly. The Paleozoic lasted for 300
my. Dinosaurs dominated the Mesozoic, which
lasted for almost 200 my. After the dinosaurs
died out, mammals began to dominate in the
Cenozoic. These are the main subdivisions that
you should know.
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Plate Tectonics

• Fundamental Concept and Unifying Theory in Earth
  Science
• Idea is gt 100 yrs old
• Acceptance only within the past 30 yrs

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Plate Tectonics

• How and why did it come about?
• During the 1800s geologists recognized many
  strange things that seemed to imply that the
  continents had once been together.

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Early geologists sawsimilarities between the
coasts of Africa and South America.
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This map from 1858 shows that Africa and South
America can be fit together very well.
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If the northern continents are fit together, rock
units match very well.
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Again, a nice match if the continents are fit
together
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Why was Continental Drift not accepted?

• Because it was difficult to under-stand how
  continents could move.
• What did we learn to make us more willing to
  accept the idea that the continents have moved?

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Mid-ocean Ridges
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Drilling into the seafloor provided more evidence
supporting sea-floor spreading

• Micropaleontology of sediments
• Dating of the underlying lavas

Drilling ship Glomar Challenger
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Age of Seafloor Crust
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Formation of Magnetic Anomalies
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Convection
Water boiling in a pan on your stove is an
example of convection
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Convection within the Earth
Transports or drives plate motion Ridge push and
slab pull Gravity pushes plates away from
uplifted MOR and pulls the plates down into
Earths interior at subduction zones
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Mosaic of Earths Plates
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Rates of plate motion

• Mostly obtained from magnetic anomalies on
  seafloor
• Fast spreading 10 cm/year
• Slow spreading 3 cm/year

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What do we find right at the spreading axis?
Black smokerHydrogen sulfide
Giant tube worms and clams live onthe Black
smokers
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How does it work?

• Crabs live on dead worms, bacterial mats, and
  snow
• A complete complex food chain is established

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Were submarine hot springs the origin of life on
Earth?
Maybe?
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Seismology

• Study of the propagation of mechanical energy
  released by earthquakes.
• When energy is released in this fashion, waves of
  motion (like the effect of a pebble tossed into a
  pond) are set up in the Earth.

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Earthquakes

• earthquake movement of rock bodies past other
• fault locus of the earthquake movement
• faults come at all scales, mm to separation of
  lithospheric plates (e.g., San Andreas).

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Earthquake terms

• focus site of initial rupture
• epicenter point on surface above the focus

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Seismic Waves Radiate from the Focus of an
Earthquake
Know what is needed to locate an epicenter
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Elastic Rebound Theory
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Elastic Rebound Theory
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Elastic Rebound Theory
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Elastic Rebound Theory
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What Causes Earthquakes?

• Sudden release of energy accumulated in deformed
  rocks
• At shallow depths, rocks are brittle and deform
  elastically
• When subjected to sufficient stress, they
  fracture forming a fault
• When the fault is locked, stress builds
• When the friction along the two sides of a fault
  is overcome, stress is released and an earthquake
  results

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Distribution of earthquakes

• Not random
• Focused in linear zones

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Most earthquakes occur at plate boundaries
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Plates

• Rigid Lithosphere with definite boundaries
• Can have both oceanic and continental crust or
  just one kind.

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Note that crust under continentsis thicker (45
km)than under oceans (8 km).
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Types of plate boundaries

• Divergent mid-ocean ridges
• Convergent collision zones
  volcanic arcs
• Strike-slip San Andreas Fault
  (California) Anatolian Fault
  (Turkey)

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Divergent (Spreading)
Convergent (Subduction Zone)
Transform
Three Types of Plate Boundaries
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Earthquakes Associated with Divergent and
Transform Margins
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Earth-quakes in subduction zones
Benioff- Wadati Zone
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Convection within the Earth
Lithosphere is created at spreading centers and
destroyed at Trenches (Subduction Zones) Know
mechanisms of plate transport
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Subduction zones
Ocean-continent convergence
Ocean-ocean convergence
Continent-continent collision
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Growth of Continents

• Addition of volcanic arc material to continent
  (e.g. Andes, as on the video).
• Collisions Continents
• Collisions Exotic terranes

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Elastic Rebound Theory
Works for 1964 Alaska and 2004 Sumatra earthquakes
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Tsunami
Series of very long-wavelength waves on the
ocean tidal wave Has nothing to do with tides
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Generation of a Tsunami
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Tsunami waves

• Very small out in the open ocean
• Amplitude of only 1 meter
• Very long wavelengths (up to 100 km)
• Travel very fast (as much as 500 mph)

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Tsunami waves

• When waves reach shallow water, they "feel" the
  shallow bottom, just like ordinary waves, and
  they slow down (to 20-30 mph)
• Because of the massive energy, this slowdown
  causes them to build up very high (up to 50-100
  m)

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Runup

• Maximum height above sea level reached by a
  tsunami when it reaches shore

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Inundation

• Horizontal distance from the normal water's edge
  reached by a tsunami

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Big Island Field Trip

• Need deposit of 50 on or before February 17th