Transcript Chapter 14 lesson 3

Chapter 14 lesson 3
THE THEORY OF PLATE TECTONICS
The Plate Tectonic Theory
 So we learned that new crust forms at the mid ocean
ridges, but what happens to the older oceanic crust?
 Plate tectonics states that Earth’s surfaces is made of
a series of rigid slabs of rock or plates that move with
respect to each other
Tectonic plates
 17 total plates
 8 major plates and 9 smaller
 Largest plate- the Pacific plate
Lithosphere
 Outermost layer of Earth
 Consists of the crust and the solid, upper mantle
Asthenosphere
 Directly below the lithosphere
 Very hot part of the mantle
 Solid but behaves like a liquid plastic material that
flows
Plate boundaries
 3 types
 Divergent
 Convergent
 Transform
Divergent
 Two plates are separating
 Can occur at a mid ocean ridge
 Oceanic and Oceanic plate
 Can also occur in the middle of a continent
 Continental and Continental plate
 Rift Valley like the Eastern African Rift Valley
Convergent
 When two plates come together or collide
 The denser plate sinks below the less dense in a process
called subduction.
 Continental and Continental


Neither plate sinks below the other because the plates are of the
same density
Mountains are formed like the Himalayas
 Oceanic and Continental



Oceanic plate is dense compared to a continental plate
Oceanic plate subducts or sinks below the continental plate
Deep ocean trench and line of volcanoes occur
Transform
 Two plates slide past each other
 As they move, it is common for the plates to get
“stuck”
 Stress builds up and the rocks break
 This leads to earthquakes and fault lines
What causes the plates to move
 Convection Currents- the circulation of material
caused by differences in temperature and density.
 Convection in the mantle- hot mantle rises toward
the crust, it cools, and sinks back down only to be
heated again.
Forces causing plate motion
 Basal Drag- convection currents in the mantle act like a
conveyor belt in the lithosphere
 Ridge Push- Rising mantle material at mid-ocean ridges
creates the potential for the plates to move away from the
ridge.
 Slab pull- the denser plate is called a slab when it collides
with a less dense plate.
 As the slab sinks it pulls on the rest of the plate with a
force called a slab pull.
Theory in Progress
 Still many unanswered questions
 Why is Earth only planet in solar system with plate
tectonic activity?
 Why do earthquakes and volcanoes happen away
from plate boundaries?
 What forces dominate plate motion because we can’t
measure convection currents?