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Chapter 2
Plate Tectonics and the
Ocean Floor
How Old is The Earth
The age of Earth has been subject to debate. Scientists
now use an age of 4.6 billion years.
Catastrophism is the thought that Earth is very young,
and events described in the Bible are responsible for the
appearance of Earth’s features.
The principle of uniformitarianism was introduced in
1788. This principle states that the forces which shaped
the Earth are identical to the forces which are working
today.
Where did oceans come from?
Theory:
H2O came from big comets during period of
heavy bombardment
H2O locked up in minerals released from
differentiation and heating
Origin of the Oceans
Beginning
- 4.5 Billion years ago
Differentiation
- heaviest materials sank to the middle (basalt-oceanic crust)
- lightest materials rose to the surface (granite-continental)
- created high and low spots made up of granite and basalt
- Earth began to cool and the materials began to harden
Origin of the Oceans
Degassing
3.8 Billion years ago
Earth filled with volcanoes (erupt & release
large gas clouds)
Filled with water vapor
Atmosphere could not hold it all
Rain, evaporated, rain, evaporate (Earth is
still VERY hot)
Origin of the Oceans
Deluge
Slow rain (Earth is cooling)
low spots on the earth began to fill with water
Salinity
Weathering of rocks on land
Silicate, calcium, sodium
Volcanic gases
chlorine
Alfred Wegener
1880 – 1930
In 1910, Wegener noticed the
matching coastlines of the Atlantic
continents -- they looked on maps
like they had once been fit together.
He first spoke on the topic in
January of 1912, where he put forth
the idea of "continental
displacement" or what later was
called continental drift.
Evidence for continental drift
Matching coastlines on different continents
Figure 2-2
Evidence for continental drift
Matching mountain ranges across oceans
Today
300 million years ago
Figure 2-4
Evidence for continental drift
Glacial ages and climate evidence
Evidence for continental drift
Distribution of fossils such as Mesosaurus
Figure 2-6
Objections to the continental
drift model
Wegener envisioned continents plowing through
ocean basins
Wegener did not provide a plausible mechanism to
explain how the continents could have drifted
apart
Most Earth scientists rejected continental drift
because it was
Too far-fetched
Contrary to the laws of physics
The theory of plate tectonics
Continental drift was reexamined in the
1960s when new information became
available
Sea floor features became better known
A technique was developed that enabled
scientists to determine the original positions of
rocks on Earth (paleomagnetism)
Harry Hess
1906-1969
Harry Hess proved Wegener's basic idea correct and
clarified the mechanism that broke the once-joined
continents into the seven continents which we are now
familiar with.
The continents are attached to the plates and do not
move independently of them. But the plates themselves
shift and change shape, carrying the continents along.
Evidence for plate tectonics
Earth’s magnetic field affects all magnetic
objects on Earth
Figure 2-7
Evidence for plate tectonics
When rocks
cool at Earth’s
surface, they
record Earth’s
magnetic field
(normal or
reversed
polarity)
Figure 2-9
Evidence for plate tectonics
Paleomagnetic studies
indicate alternating
stripes of normal and
reverse polarity at the
mid-ocean ridge
Pattern was created by
sea floor spreading
Figure 2-11
Evidence for plate tectonics
Harry Hess
envisioned
new sea floor
being created
at the midocean ridge
and destroyed
in deep ocean
trenches
Figure 2-10
Evidence for plate tectonics
Age of the sea floor
matches pattern
predicted by sea
floor spreading
Youngest sea floor
is at mid-ocean ridge
Sea floor is older
with increasing
distance from midocean ridge
Figure 2-12
Evidence for plate tectonics
Pattern of worldwide earthquakes (left)
matches plate boundaries (right)
Figure 2-13
Earth structure
Chemical composition
Crust
Mantle
Core
Physical properties
Lithosphere
Asthenosphere
Mesosphere
Outer core
Inner core
Figure 2-14
Principles of plate tectonics
Lithosphere - The outermost
portion of Earth is composed of a
mosaic of thin rigid plates that
move horizontally with respect to
one another
Asthenosphere – middle of the
mantle
lithosphere ‘floats on top’
zone where magma is formed
easily deformed, can be pushed
down by overlying lithosphere –
“plastic” – tar or asphalt
Principles of plate tectonics
Plates interact with each other along their
edges (called plate boundaries)
Plate boundaries have a high degree of
tectonic activity (mountain building,
earthquakes, active volcanoes)
The 3 types of plate boundaries
1. Divergent
2. Convergent
3. Transform /Lateral
Slipping
Divergent plate boundaries
2 plates moving
away from each
other
Formation of
new crust on
ocean basin by
rifting and sea
floor spreading
http://www.youtube.com/watch?v=GyMLlLxbfa4&feature=r
elated
Divergent plate boundaries
The MidAtlantic Ridge
is a divergent
plate boundary
where sea floor
spreading
occurs
Divergent plate boundaries
Iceland sits atop
a divergent
plate boundary
where
continental
rifting occurs
Figure 2-19
Divergent plate boundaries
Crustal plates get older as you move from the spreading
center
New crust is created at spreading centers at a rate of
approximately 1-10cm per year
Old crust is destroyed at the same rate at subduction zones
Spreading Centers
Magnetic anomalies are found in
ocean crust and we can find them
by at spreading centers
paleomagnetism
every so often Earth’s magnetic field
flips (every 300K-500K years
Magnetic signal recorded in crust at
spreading center as it’s formed, forms
bands of crust with either a weak or
strong magnetic signal
The record of the magnetic polarity is
forever “recorded” in the iron deposits
at the time they are formed. The iron
deposits become fixed upon the
magnetic pull.
Spreading Centers – Hydrothermal Vents
Concentrated areas of hot rising water
As new crust is created cold dense sea water circulates
through magma chambers close to the surface; the water
becomes heated and is released through cracks (rifts) in the
newly created sea floor and create vents
Vents are chimney shaped (65ft-200ft)
Types of Hydrothermal Vents
White smoker – Milky
discharge
Temp. – 2000C-3000C
(3920F-6260F)
Black Smoker – Black
Discharge
Temp: 4000C (7000F)
contains zinc, lead, cobalt,
and silver
Hydrothermal Vent Community
Rich animal life supported by
the vent
Water is rich in sulfide
compounds which are utilized
by the chemosynthetic bacteria
that are the producer organisms
for an ecosystem that has no
dependence on sunlight
This bacteria has a mutualistic
relationship with giant tube
worms that are found living
along the vents.
Tube worms can grow from
6inches to 10ft in length
(Depending on species)
Other Vent Life
Clams
Mussels
Crabs
Fish
Hydrothermal Vent - Ecosystem
Ecosystem =
Energy Transfer =
Trophic Levels =
Food Web Arrows =
Convergent plate boundaries
a. Ocean-continent
Convergent
plate
boundaries
vary
depending
on the type
of crust
c. Continent-continent
b. Ocean-ocean
Convergent plate boundaries
An Ocean-Continent convergent plate boundary
produces the Cascadia subduction zone and
Cascade Mountains & Mt. St. Helens
RING
OF
FIRE
Subduction zones, like the so-called Ring of Fire that
surrounds the basin of the Pacific Ocean, are among the
most violent on Earth.
RING OF FIRE
The scraping of one plate on another generates powerful
earthquakes; the heating of the plate within the depths of the
mantle releases fluids which melt the rock over it,
producing molten rock (magma) that surface as volcanoes.
Convergent plate boundaries
An Ocean-Ocean
convergent plate boundary
older denser plate pushed
under
causing magma to rise and
form island arcs and
trenches like Marianna
Island Arc system and
most famous trench
Challenger Deep
Trenches – Undersea Valley
Trenches are formed when two plates push toward each
other and there is a valley in between the plates.
Life in a Trench
Water temperature – 0C-2C
Poor Oxygen
Extreme pressure - At more than 8 tons per
square inch, it’s the equivalent of one person trying
to hold 50 jumbo jets!
No sunlight
Marine Life – sea cucumbers, mollusks, aneomes,
crustaceans, and jellies
Convergent plate boundaries
A continentcontinent
convergent plate
boundary produces
the Himalaya
Mountains
Transform plate boundaries
Two plates sliding
past each other
Transform plate
boundaries occur
between segments of
the mid-ocean ridge
Can also occur on
land (ex: San
Andreas Fault)
Hotspots and plate tectonics
Hotspots are stationary
and have abundant
volcanic activity
The lithospheric plate
moves over the hotspot
Creates a row of
volcanoes progressively
older toward one end
Creates a row of volcanoes progressively
older toward one end… Like Hawaii
Loihi
(New
Hawaiian
Island)
The world as it may look 50
million years in the future
Figure 2-35