Transcript Lecture#3 part1: Dynamic Earth

GEOG 3251 summer ‘10
Adina Racoviteanu
Objectives
• Understand how Earth is dynamic and how moving
plates form ocean basins, mountain ranges, islands,
volcanoes, and earthquakes
• Identify the three general categories of plate boundaries
recognized by scientists: convergent, divergent, and
transform
• Understand how the theory of plate tectonics was
developed and supported
Why is the Earth so restless?
• ground shakes violently
• volcanoes erupt with explosive force
• mountain ranges rise to incredible heights
Catastrophism world view
Until 1700s: common belief:
• Biblical Flood shaped Earth's surface
• All earthly changes were sudden and
caused by a series of catastrophes.
Uniformitarian principle
• 1785 : James Hutton, a Scottish geologist
proposes new world view:
The present is the key to the past.
• geologic forces and processes acting on
the Earth today are the same as those that
have acted in the geologic past.
EARTH’S LAYERS
• Three layers:
– the core,
– the mantle and
– the crust
• Chemical &
mechanical
differences
Like boiled egg
THE CORE
• Composition: mostly iron and nickel
• Temperature: very hot, even after 4.5
billion years of cooling.
• divided into two layers:
– a solid inner core
– liquid outer core.
CORE GENERATES CURRENTS
• Core is so hot  it
radiates natural heat
to the upper layers.
• convection currents
generated.
•The convection currents
cause the movement
of the tectonic plates.
MANTLE
• Composition:
mafic minerals - rich in the elements iron (Fe) magnesium (Mg),
silicon, and oxygen
• Dense, hot layer of semi-solid rock
• Upper part of the mantle is cooler and more rigid than the deep
mantle
CRUST
• Composition: oxygen and silicon with lesser
amounts of aluminum, iron, magnesium,
calcium, potassium, and sodium.
• rigid and very thin
LITHOSPHERE
• Greek (lithos = stone)
rigid outermost layer
made of crust and
uppermost mantle
• broken up into the
moving plates that
contain the world's
continents and
oceans
ASTHENOSPHERE
• Greek (asthenos = weak)
= part of the mantle that
flows, a characteristic
called plastic behavior.
• The flow of the
asthenosphere is part of
mantle convection, which
plays an important role in
moving lithospheric
plates.
CRUST/MANTLE AGAIN
• lithosphere
–
–
–
–
hard
~100 km thick
crust floats on top
continental crust
20 to 70 km thick
– oceanic crust
~ 8 km thick
• asthenosphere
– soft
– ~3000 km thick
– “fluid-like”
Crust: 2 types
• Oceanic crust is made of relatively dense
rock called basalt
• Continental crust is made of lower density
rocks, such as andesite and granite.
Oceanic vs.continental crust
Oceanic
Continental
basaltic rocks
granitic rocks
"mafic” Mg, Fe
“felsic” (Fe-Si, Al)
denser
less dense
thinner
thicker
EARTH STRUCTURE SUMMARY
•
•
The Earth is sphere with a diameter of about 12,700Kilometres.
Temperature and pressure rise with depth.
•
CORE:
–
–
–
–
temperature is believed to be an incredible 5000-6000°c.
Inner core: solid FE and NI
Outer core: liquid Fe and Ni
Complex convection currents give rise to a dynamo effect which is responsible for the
Earth's magnetic field.
• MANTLE:
– It's made of solid rock and behaves like an extremely viscous liquid - (This is the tricky
part... the mantle is a solid which flows????)
• CRUST:
– very thin (average 20Km)
– OCEANIC CRUST: thinner (10 kilometres)
– CONTINENTAL CRUST: thicker (35 kilometres on average).
• The continental crust in the Himalayas is some 75 kilometres deep.
THEORIES OF MOUNTAIN
ORIGIN
• CONTINENTAL DRIFT
– Alfred Wegener (1880 – 1930)
• PLATE TECTONICS
– Harry Hess and G. Mason (1962)
VIDEO
• PLATE TECTONICS INTRO
CONTINENTAL DRIFT
FOSSIL EVIDENCE
• Fossils of the same
species found on several
different continents.
• Wegener’s idea:
species dispersed when
the continents were
connected and later
carried to their present
positions as the
continents drifted.
For example, Glossopteris, a fern, was found
on the continents of South America,
Africa, India, and Australia.
TORN NEWSPAPER IDEA
• Rock sequences in
South America, Africa,
India, Antarctica, and
Australia show
remarkable similarities.
• Wegener’s idea:
the same three layers
occur at each of these
localities.
SHIFTING CLIMATE
SHIFTING CLIMATE ZONES
• Wegener’s interpretation:
• climate zones remained stationary and the
continents drifted to different locations.
• The drift of the continents caused the
apparent movement of the climate zones.
ARGUMENTS AGAINST
• What mechanism
moves the
continents??
Wegener’s idea:
Earth's spin caused the continents to move,
plowing through the oceanic plate and
producing mountains on their leading edges.
Continental Drift Evidence
Summary
•
•
•
•
Puzzle idea: continents fit together
Torn newspaper: matching geology
Fossils: same fossil on different continent
Paleoclimate
SEA-FLOOR SPREADING
In the late 1950's, scientists mapped the present-day magnetic field
generated by rocks on the floor of the Pacific Ocean.
SEA FLOOR SPREADING HO
When mapped, the anomalies produce a zebra-striped pattern of parallel positive
and negative bands. The pattern was centered along, and symmetrical to,
the mid-ocean ridge
Computer-generated detailed topographic map
of a segment of the Mid-Oceanic Ridge.
"Warm" colors
(yellow to red)
indicate the ridge
rising above the seafloor,
and the "cool" colors
(green to blue)
represent lower elevations.
New discoveries: Sea-floor
spreading
• 1962: Hess (1962):
New ocean floor is formed at the rift of
mid-ocean ridges.
VIDEO
SEA FLOOR SPREADING
SEA FLOOR SPREADING HO
SEA FLOOR SPREADING
• So, continents are no longer thought to
plow through oceanic crust
• Continents are part of plates that move on
the soft, plastic asthenosphere.
• driving force: convection currents
If new oceanic lithosphere is created at
mid-ocean ridges, where does it go?
Convection cells in the
mantle help carry the
lithosphere away from the
ridge. The lithosphere
arrives at the edge of a
continent, where it is
subducted or sinks into the
asthenosphere.
PLATE TECTONICS SUMMARY
• Plate = large, rigid slab of solid rock
• Tectonics (Greek) = to build
• PLATE TECTONICS: states that the
Earth's outermost layer is fragmented into
a dozen or more large and small plates
that are moving relative to one another as
they ride atop hotter, more mobile
material.