Download Theory of Plate Tectonics

Theory of Plate Tectonics
Earth’s Plates
• Are large slabs of rock composed of crust and solid part of
upper mantle collectively called the lithosphere
• Lithosphere floats on liquid like molten rock of upper mantle
called asthenosphere
• Lithosphere is broken up into different sections called plates
• 12 major plates and many smaller ones exist on Earth’s
surface
• All these plates fit together like pieces of jigsaw puzzle
Earth’s Tectonic Plates
Theory of Plate Tectonics
• Explains many of the features and events that take place
at and below Earth’s surface
• It explains the following geological processes:
a) how and why the continents move
b) how and why sea floor spreading occurs
c) how and why and where earthquakes, volcanoes, and
mountain formation occur
Type of Tectonic Plates
• Some are composed solely of oceanic crust
• Other plastes are composed of only
continental crust
• Most plates are mixed oceanic and
continental crust
Movement of Tectonic Plates
• Earth’s Tectonic Plates float and move slowly over
the asthenosphere because plates are less dense
than underlying asthenosphere
• Convection currents in asthenosphere can cause
fractures of Earth’s plates and are also
responsible for their movement
Divergent Plate Boundaries
• Are regions where 2 plates separate and
create new oceanic crust
• This process occurs where the sea floor
spreads along a mid-ocean ridge
• Can also occur in middle of continent
called “continental rifting”
Divergent Plate Boundaries cont…
Convergent Plate Boundaries
• Are regions where 2 plate boundaries collide
• Typically the heavier and more dense
oceanic plate slides under the less dense
and lighter continental plate
• This process is called subduction and region
where this occurs is called the “subduction
zone”
Convergent Plate Boundaries cont….
• Subduction causes deep sea trenches to
form
• Volcanoes and volcanic mountains such as
Garibaldi and Mount Baker, are some
examples of volcanic mountains formed at
these regions
• Earthquakes are also common at these
regions
Convergent Plate Boundaries cont….
Transform Plate Boundaries
• Are regions where 2 or more plates slide past each
other
• Typically occurs when both plates are of the same
type and therefore same density and weight
• Identified by Canadian Geologist Tuzo Wilson
• Earthquakes are very common at these boundaries
Transform Plates cont…..
Movement of Earth’s Tectonic Plates
• Earth’s tectonic plates are constantly moving
• Using sensors embedded in Earth’s plates and
using satellites we know that they move at a rate
of about 1-15 cm per year depending on the plate
• The following map shows in which direction the
different plates are moving
Direction of Movement of Earth’s Plates
Movement of Earth’s Plates
•Now we will see the different
processes responsible for making
the Earth’s plates move
Mantle Convection
• Part of the Earth’s mantle is made of molten (i.e.
liquid) material
• Energy from radioactive decay of some elements
as well as heat from the Earth’s core provides
enough heat to melt some of the mantle resulting
in aesthenosphere
Earth’s Aesthenosphere
Convection Currents in Aesthenosphere
• Warmer, less dense material rises and cooler, denser material sinks
with any liquid
• The asthenosphere is no exception
• Warmer, less dense asthenosphere (closest to the Earth’s core)
rises and cooler, more dense asthenosphere (closest to the
lithosphere) sinks downward resulting in convection currents in
the asthenosphere
• These currents drag the tectonic plates with them through friction
Aesthenosphere Convection Currents
Two Other Processes That Aid In Plate
Movement
• There are two other processes (other than
convection currents in the aesthenosphere) that
also aid in movement of the Earth’s Plates.
These processes are referred to as:
a) Slab Pull
b) Ridge Push
Ridge Push
• Because mid-ocean ridges lie at a higher
elevation than the rest of the ocean floor, gravity
causes the ridge to push on the lithosphere that
lies farther from the ridge.
• Weight of elevated ridge pushes the oceanic plate
towards the subduction zone
Ridge Push
Slab Pull
• As the leading edge of the subducting
plate sinks, it pulls the rest of the plate
with it at convergent plate boundaries
• Gravity and convection assist in this
movement
Slab Pull
In Class/Homework
• Complete questions 1, 2, 3, 5, 6, 7 on page 295 in your textbooks
• Complete the Interactions of Earth’s Tectonic Plates Worksheet