Transcript Plate Tectonics - My Teacher Pages

Plate Tectonics
Plate Tectonics and the Ocean
Floor
tectonics – Theory that the
outermost portion of the Earth is
composed of plates that move on a liquid
molten layer (similar to icebergs floating
on water)
 Alfred Wegener a German meteorologist
developed the theory of continental drift in
1912
 Continental drift- Continents slowly drift
across the globe
 Plate
Evidence of Continental
Drift
– (pan= all; gaea= Earth)
A large landmass that the continents
fit together to form
 Evidence
 Pangaea

Matching rocks on different continents
• Similar rocks form Appalachian mountains match rocks in
British Isles

Distribution of organisms
• Finding fossils of the same species on different continents

Glacial Activity in now tropical areas
The continents fit together
Fossils of same species found on
different continents.
More on Continental Drift
 As
the seafloor spreads the lithosphere
(continental and oceanic crust) moves
in sections on a viscous liquid layerasthenosphere
 On average these tectonic plates move 15 inches/year
More on Continental drift
If new crust is being formed all the time why isn’t
the earth covered in crust?
 Because as new crust is formed old crust is
destroyed
 Let’s start by looking at the 2 types of crust



Both types of crust make up the outer layer of earth,
called the lithosphere
Continental Crust
• Granite
• Lower density than continental

Oceanic Crust
• Igneous rock + basalt
• Higher density than continental
Continental drift
crust sits on top of a viscous layer –
the asthenophere
 The

A Viscous substance is a thick liquid
• What are some examples of a viscous substance
that you are familiar with?
 The
asthenosphere is part of the mantle
called the upper mantle
Convection
 Convection
is the heat transfer in a gas or
liquid by the circulation of currents from
one region to another
 Convection cell is a circular-moving loop of
matter (gas or liquid) involved in
convection movement
Convection
 Convection
causes movement in the liquid
mantle layer which the crust sits upon
 As a result the crust slowly moves on this
layer

Kind of like an iceberg floating on water
 As
this occurs the tectonic plates move
slowly over the asthenosphere
Tectonic plates
Forming Crust

Mid-Ocean ridge- Underwater mountain range
winds through all oceans
 Seafloor spreading occurs at the ridge



This is where molten rock reaches the surface, cools,
and forms new crust
As this new crust forms the magnetic minerals in the
crust align in relation to Earth’s current magnetic field
Scientists can look at the sea floor to get a history of
Earth’s magnetic reversals (we will talk about how this
works in a minute)
Seafloor Spreading
 Harry
Hess (1906-1969)- left his depth
recorder on while traveling as sea in the
Navy. He noticed mountain ridges at the
center of the ocean basins and deep,
narrow trenches at the edges

Published his idea of sea floor spreading
which suggested that new ocean crust is
formed at ridges, split apart and moves away
from ridges
Seafloor Spreading
 It
was confirmed that sea floor spreading
does occur at mid-ocean ridge
 Crust is also being destroyed at ocean
trenches (we will talk about this in a
minute)

This is called subduction
New crust is made at the mid-ocean ridge
Magnetic Field Reversals
 Fredrick
Vine and Drummond Matthews
(1963)

Combined the pattern of magnetic sea floor
stripes with sea floor spreading. Can see
fluctuations in rock from normal polarity to
reverse polarity.
Magnetic field reversalsThis is the piece of
Evidence that made
The continental drift
Theory widely accepted
Among scientists.
As the sea floor spreads
(at mid-atlantic ridge) the
Magnetism of the rocks
Formed reverses. These
Magnetic shifts can be
Observed in crust.
Being able to identify these magnetic pole reversals in the rock
Confirmed the theory of continental drift.
Plate Boundaries- 3 types
 Divergent
Boundaries
 Convergent Boundaries



Ocean to Ocean
Ocean to Continental
Continental to Continental
 Transform
Boundaries
Divergent boundaries
 Plates

are moving apart
Ocean crust – Ocean crust
• Sea floor created
• Mid-Atlantic Ridge

Continental crust – Continental crust
• Continental rifting
• Sea floor created
• Continents are splitting apart
Red Sea, Gulf of California
This is how the Mid-Atlantic ridge started!
Convergent Boundaries

Plates are moving together
 Ocean –Continent
• Old sea floor destroyed
• Subduction zone (continental arcs)
 Subduction- A process in which one plate
descends below another (volcanoes)

Ocean –Ocean
• Old sea floor destroyed
• Subduction zone (island arcs) (volcanoes)
Continent –Continent
• Collision Zone
• Mountains form
Why does subduction occur? Hint: composition of plates

Examples of Convergent Plate
boundaries
Continental/Oceanic
Transform Boundaries
 Plates


are moving past each other
Oceanic
Continental
• Both of these neither create nor form sea floor
• Causes Earthquakes
What do we know about all of the locations of these active
volcanoes?