Download File

What is the evidence for Plate
Tectonics?
•
•
•
•
Sea-floor spreading & Paleomagnetism
Earthquake patterns
Ocean drilling
Hot spots
How does the sea-floor spread?
1. Melted rock (molten material) rises and
erupts at the mid-ocean ridge (less
dense).
2. The lava spreads out, pushing older rock
to both sides of the ridge.
3. It forms a strip of solid rock in the center
of the ridge.
4. Then more lava erupts continuing the
sea-floor spreading!
YouTube:
Divergent
Boundary
This is called sea-floor spreading!!!! New material is continually added to the
ocean floor.
9.4 Testing Plate Tectonics
What is paleomagnetism?
Magnetic
Field Reversal
 Paleomagnetism is using a rock’s magnetic
properties to determine the location of the poles at
the time the rock formed.
 The Earth’s magnetic field periodically reverses
polarity (north pole becomes south pole and vice
versa)
 When minerals in rocks solidify, iron-rich mineral
grains will be permanently magnetized in the
direction of the Earth’s magnetic field
Paleomagnetism Preserved
in Lava Flows
• Normal polarity—
when rocks show the
same magnetism as
the present
magnetism field
• Reverse polarity—
when rocks show the
opposite magnetism
as the present
magnetism field
9.4 Testing Plate Tectonics
How does paleomagnetism show
evidence for Sea-floor spreading?

The rocks that make up the ocean floor lie in patterns of
magnetized “stripes”.

This pattern (N-S; N-S; N-S) is the same on both sides of
the ridge, this means that at one time, those stripes used
to be together in the middle of the ridge.
Polarity of the Ocean Crust
Link to
Simulation
9.4 Testing Plate Tectonics
Evidence for Plate Tectonics
 Earthquake Patterns
• Scientists found a close link between
earthquakes and plate boundaries.
• When the depths of earthquakes and their depth
within a trench are plotted, a pattern emerges.
•Deep earthquakes are found in trenches
(evidence for sinking plates).
•Shallow earthquakes are found in oceanic
ridges. (evidence for spreading plates).
9.4 Testing Plate Tectonics
Evidence for Plate Tectonics
 Ocean Drilling
• Scientists drilled and took
samples from several sites in
the ocean-floor crust.
• The youngest oceanic crust is at
the ridge crest, and the oldest
oceanic crust is at the deep
ocean trenches.
old
rock
new
rock
new
rock
old
rock
9.4 Testing Plate Tectonics
Evidence for Plate Tectonics
 What is a Hot Spot?
• A hot spot is a concentration of heat in the
mantle capable of producing magma, which rises
to Earth’s surface.
How do hot spots provide evidence
for Plate Tectonics?
• The Hawaiian Islands in the Pacific are
formed by a hot spot.
• Dates of volcanoes in this chain increase
with age with increasing distance from the
east most island of Hawaii.
• The age of each volcano shows the time
when it was located over the hot spot.
• This shows that the pacific plate is moving
north-west.
Animation
9.5 Mechanisms of Plate Motion
Causes of Plate Motion
 The plates of the lithosphere float
on top of the asthenosphere.
 Convection currents within the
asthenosphere push the plates of
the lithosphere.
Link to
Convection
Simulation
9.5 Mechanisms of Plate Motion
Causes of Plate Motion
 GRAVITY
• Slab-pull takes place at TRENCHES.
•cool, dense oceanic crust sinks into the
mantle gravity “pulls” the lithosphere
downward with the convective flow.
• Ridge-push takes place at RIDGES.
•the lithosphere slides down the sides of
the oceanic ridge under the pull of gravity.
How are Sea-floor Spreading and
Subduction affecting our oceans?
• The Atlantic Ocean has very few trenches. As a
result, the Atlantic Ocean is getting wider. As it
spreads, it pushes the continents further away.
How are Sea-floor Spreading and
Subduction affecting our oceans?
• The Atlantic Ocean has very few trenches. As a
result, the Atlantic Ocean is getting wider. As it
spreads, it pushes the continents further away.
• The Pacific Ocean covers almost 1/3 of the
planet, yet it’s shrinking! The Pacific is
surrounded by many trenches. The deep-ocean
trenches swallow more oceanic crust than the
mid-ocean ridge can create.
How long does it take for new rock
to be recycled?
• New rock that is formed at the
mid-ocean ridge takes about
200 million years to move
across the ocean, and sink
into a trench.