The Earth
The Theory of Plate Tectonics
• The theory that Earth’s lithosphere is divided
into tectonic plates that drift on the
athenosphere.
• Divided into seven to eight main plates and
several smaller ones.
• The plates move by convection currents that
either push the plates apart, together or make
them slide against one another.
Divergent Plate Boundaries
• Oceanic: Where two oceanic plates move
apart
– Magma from the mantle flows up into the space
between the plates and the magma hardens to
form new oceanic crust.
– This process is called sea-floor spreading.
– Causes the oceans to widen!!
– Long, continuous chains of underwater volcanoes
form along the boundaries and are called midocean ridges.
*What happens to the ocean during a divergent boundary?
*What forms on each sides of the divergent boundary?
*What new is being formed where the magma hardens?
Note: The Mid-Atlantic Ridge is created by this and moves
North America and Europe further apart.
Divergent Plate Boundaries
• Continental: Where two continental plates
move apart
– Since continental crust is MUCH thicker, it doesn’t
split fully causing a big ridge.
– The ridge can allow rivers to form in it and as it
continues to split can allow ocean waters to flow
in as well.
– The Red Sea was formed when two continental
plates diverged.
*What would fill in the rift valley above?
*Why didn’t the plates split all the way?
*The faulted valleys will cause what
phenomenon?
Convergent Plate Boundaries
• Oceanic = 2 plates made of oceanic crust collide
– One plate is pushed under the other
– Subduction zone – region where one plate slides
under another plate
– A deep canyon (trench) forms under the ocean where
the plates meet
– The plate sinks into the hot mantle and melts to form
magma
– Creates a chain of undersea volcanoes
– As they keep erupting and forming new land, they will
rise above the ocean floor and form volcanic islands
– The Aleutian Islands formed this way.
• What happens to the plate that slides
underneath?
• What is eventually formed after years of
underground volcanic eruptions?
Convergent Plate Boundaries
• Continental-Oceanic Boundaries = 1 oceanic
plate collides with 1 continental plate
– The oceanic plate slides under the continental
plate due to the increased density
– This also forms a trench and volcanic mountains
form next to the trench on the continental plate
– The western coast of South America have many
volcanoes formed this way.
• Which plate slides under the other?
• What is formed on the continental plate?
Convergent Plate Boundaries
• Continental = 2 continental plates collide
– Creates the highest mountains in the world!
– Folded mountains – the rock layers are squeezed
together causing folds in the crust
– The plates buckle and thicken.
– Created: The Himalays, the Alps, The Ural
Mountains and the Great Smoky Mountains
• What are these types of continental mountain
ranges called?
• What makes these mountain ranges so special?
Transform Boundaries
• Two plates slide against each other
• Cause the most earthquakes!
• When they slide, pressure builds up and some
rock breaks or slips and releases energy in the
form of seismic waves.
• Seismic wave – a wave of energy that travels
away from the center of an earthquake in all
directions
• Fault – The break or crack along Earth’s surface
where movement occurs.
• Transform boundaries causes the most…?
• What is a fault?
Hot Spots
• Hot Spot – an area of volcanic activity (usually
in the middle of the plate)
– Stays in the same place even though the plate
moves
– Results in a chain of volcanoes some of which are
no longer active
– Created the Hawaiian Islands
– Due to a hot spot in the Mantle
Ring of Fire
• Ring of Fire – zone of frequent volcanic
eruptions and earthquakes encircling the
Pacific Ocean basin.
– Contains more than 75% of all volcanoes on Earth
– Located along the boundary of the Pacific Plate
Mostly caused by
subduction of
plates resulting in
melting magma.
Question 1
1. True or False: Earthquakes can occur at any
type of plate boundary.
Question 1 Answer
• True! Faults can occur anywhere there is a
crack and pressure is built up releasing energy
and causing seismic waves.
Question 2
2. At which location are earthquakes mostq
likely to occur?
A. At a boundary where plates push
together
B. At a boundary where plates pull apart
C. At a boundary where plates slide by
each other
D. In the middle of the plate
Question 2 Answer
• Transform plate boundaries (where two plates
slide against one another) cause the most
earthquakes because they cause the most
faults.
Question 3
3.
A chain of mountains is located near the western coast of a
continent next to a plate boundary. Which sentences describes
how the mountains most likely formed?
A. As a continental plate and an oceanic plate slide past each
other, a fault formed, and mountains were exposed by
earthquake movement.
B. As a continental plate and an oceanic plate collided, part of
the oceanic plate subducted, and volcanoes erupted,
forming the mountains.
C. As two oceanic plates collided, one of the oceanic plates
subducted, and the other plate rose to the surface,
forming mountains.
D. As two oceanic plates moved apart, new seafloor was
produced, forming a ridge.
Question 3 Answer
• B! When a continental and oceanic plate
collides, the oceanic plate subducts below the
continental and volcanic mountains are
created on the continental plate.
Question 4
4. What is the region where one plate slides
under another plate called?
A. An earthquake zone
B. A hot spot
C. A volcano
D. A subduction zone
Question 4 Answer
D. When one plate slides under another, it is
called a subduction zone.
Question 5
5. Which landforms is most likely to form
where two oceanic plates push together?
A. Folded mountain
B. Mid-ocean ridge
C. River valley
D. Volcanic island
Question 5 Answer
D. One oceanic plate subducts under the other
and melts. This produces magma and many
undersea volcanoes. When these break the
surface of the water, volcanic islands are
formed.
Question 6
6. Where does sea-floor spreading occur?
A. Where two oceanic plates collide
B. Where an oceanic plate and a
continental plate collide
C. Where two oceanic plates pull apart
D. Where two continental plates pull apart
Question 6 Answer
C. As two oceanic plates pull apart, magma is
exposed creating new sea floor and the ocean
expands.
Question 7
7. A hot spot can produce:
A.
B.
C.
D.
A chain of islands
A mid-ocean ridge
A trench
A tectonic plate
Question 7 Answer
A. A hot spot will produce a chain of islands. As
a plate moves over a hot spot, it will form
volcanoes, keep moving and then form
another.