Summary of Important Concepts
Plate tectonics refers to the concept that the earth’s outer rigid rock
shell, called the lithosphere, is divided up into a number of separate
pieces, called tectonic plates, that are shoved around by the slow
convection of hot rock in the underlying asthenosphere.
The movement of the tectonic plates causes many of the major geologic
features of the earth’s surface, including earthquakes, volcanoes,
mountain ranges, and major features of the ocean floor like mid-ocean
ridges and oceanic trenches. The movements of these plates causes the
continents to move (drift) over time, opens up new ocean basins, and
closes old ones.
Alfred Wegener, in the early 1900’s, was the first person to gather
evidence that the continents had moved - a process called continental
drift. His evidence suggested that the continents on both sides of the
Atlantic Ocean had once been joined into a single large continent he
called Pangaea, that gradually split apart over geologic time. In spite of
the evidence, his theory was not widely accepted at the time.
Research in the 1960’s revived Wegener’s idea with new data from the
ocean floor. Harry Hess proposed that new oceanic crust is created by
sea floor spreading at mid-ocean ridges. This sea floor moves along
horizontally until it sinks into the mantle at oceanic trenches and is
destroyed: a process called subduction.
The concepts of continental drift, sea floor spreading, and subduction
are all tied together into the theory of PLATE TECTONICS, in which the
lithosphere is split up into a number of moving plates. These plates
move apart at divergent boundaries, creating new sea floor by
volcanic eruptions at mid-ocean ridges (sea floor spreading);
come together at convergent boundaries, destroying old sea
floor by subduction at oceanic trenches, creating volcanoes and
mountain ranges in the process;
slide side-by-side at transform boundaries.
PLATE TECTONICS - A POWERFUL UNIFYING THEORY
• Plate tectonics is a relatively new scientific concept, introduced some 30
years ago, but it has revolutionized our understanding of the dynamic
planet upon which we live.
PLATE TECTONICS INTRODUCED
Earth’s lithosphere, which consists of the earth’s crust and upper mantle, is
cut up into roughly 20 plates that move relative to one another atop of the
Plates Interact: They converge, diverge or slide horizontally past one
Alfred Wegener in the early 1900’s proposed that the continents were once
joined together in a single large land mass he called Pangea (meaning “all
land” in Greek). He proposed that Pangea had split apart around 200-250
million years ago moving gradually to their present positions - a process that
became known as continental drift.
Wegner’s Evidence for Continental Drift
Continents fit together like a puzzle….e.g. the Atlantic coastlines of
Africa and South America.
Fossils of plants and animals of the same species found on different
The distribution of climate sensitive sedimentary rocks on the different
Glacial evidence that shows that the land masses were all joined and
partially covered by a single large ice cap over the ancient south pole!
o Glacial sediment deposits found in places where glaciers do not
o Glacial Scratches (scratches on rock caused by glacial
movement) line up like a jigsaw puzzle when continents are
Although today we know that Alfred Wegener was correct about
continental drift, at the time his theory was not widely accepted.
Wegener never supplied a viable mechanism to explain how continental
movement could be accomplished.
Confirmation of continental drift would have to wait until the 1960’s,
when a better understanding of the ocean floor lead to the concept of
sea floor spreading.
Sea floor spreading would eventually vindicate Wegener and lead to the
most important unifying concept in geology: the theory of plate
The Revival of Continental Drift 1940’s & 1950’s
Work in the 1940’s and 50’s set the stage for the revival of Wegner’s work.
During the 1950’s, intense oceanographic research and technological
advancements provided maps of the sea floor showing mid-ocean ridges and
deep sea trenches.
Harry Hess and Sea Floor Spreading 1960’s
Hess suggested that the continents may be moving along with the sea
floor, not plowing through it as Wegener suggested.
Harry Hess concluded that new sea floor was being created at mid-ocean
ridges (MOR) by volcanic activity. But the earth is not getting larger.
Therefore he concluded that an equal amount of oceanic crust is probably
being lost at trenches.
The driving force is convection. Locations of spreading ridges
(upwelling) and trenches (downwelling) are determined by the
Evidence of Sea Floor Spreading
Throughout earth’s time the magnetic north and south have switched
RANDOMLY and sporadically every 1000-10,000 years. When magnetism
switches, its called a REVERSAL.
Rocks record the direction of the earth’s magnetic field at the time the
rocks form. Small magnetite (Fe) crystals in cooling magma act like
compass needles that record the direction of the earth’s magnetic field
when the magma solidifies.
Frederick Vine and Drummond Matthews found that alternating normal and
reversely polarized rock form a symmetrical stripe-like pattern parallel to
the ridge crest.
At MOR new sea floor is added and spreads laterally from the axis. As the
magma cools and the iron bearing minerals crystallize they align
themselves parallel to the lines of force of the earth’s magnetic field.
Therefore, the sea floor is a ticker tape recording of the earth’s magnetism
through geologic time. (Only for about ~200million years….why?)
Evidence of Sea Floor Spreading
Ocean Floor Ages
The rocks of the sea floor, are youngest close to the MOR and become
progressively older the farther away they are from the ridges on either
side. The age pattern is symmetrical across the ridge.
Sediments deposited on the sea floor and radiometric dating of basalt
have ages no older than ~200 million years. Anything older has been
recycled during subduction…so there is no sea floor older than the last
Pangaea (~200-250 mya).
Evidence of Sea Floor Spreading
Hot Spot Volcanic Islands
A hot spot is a persistent volcanic center located directly above a rising
plume of hot mantle rock.
Hot spot mantle plumes remain stationary while the lithosphere moves
over it. This process forms a chain of volcanic islands. The chain of
islands formed indicates the direction of plate movement over the hot
Evidence of Sea Floor Spreading
Interactions at Plate Boundaries
The majority of earthquakes and volcanic eruptions are concentrated in
belts or linear chains at the boundaries of the lithospheric plates.
DIVERGENT PLATE BOUNDARIES
Also called spreading centers and rifts; occurs where two plates move apart
horizontally and new oceanic lithosphere is created.
Continent-Continent Divergent Boundaries
Continental rifting results from upwelling mantle beneath the
continent. The continent thins out and is eventually torn apart
producing earthquakes and volcanic eruption of basaltic magma. The
upward rise of basaltic magma forms new oceanic crust between the
two diverging continents.
Examples include: The break-up of Pangea,The East African
Rift Valley, Basin and Range of Nevada and Utah.
Ocean-Ocean Divergent Boundaries
Mid-ocean ridges where rising basaltic magma convects upward
forming new ocean floor.
Examples include: Mid-Atlantic Ridge spreading ~1 cm/yr; East
Pacific Rise spreading ~6 cm/yr.
CONVERGENT PLATE BOUNDARIES
Develop where two plates are moving horizontally toward each other and
therefore are colliding. Can result in orogenic events (mountain building) or
volcanism and deep ocean trenches. Depends on the plates involved.
Continent-Ocean Convergent Boundaries
Oceanic crust is denser (more Fe and Mg) than continental crust.
When they collide, the denser oceanic plate will SUBDUCT beneath
that of the lower density continental plate.
Volcanism. Melting of the subducting plate generates a (mafic)
magma. The magma, being less dense than the surrounding solid
mantle, rises up through the continental (felsic) crust. The end
result is a volcanic arc on the continent paralleling the oceanic
Examples include the Andes Mountains in South America, the
Cascade Mountains in Western US
Continent-Continent Convergent Plate Boundaries
When two plates carrying continental crust converge (after all the
oceanic crust separating them is consumed by subduction) neither
plate will subduct because of low densities.
Orogeny. The result of collision is the construction of large scale
high pointy mountain chains.
Examples include the Appalachian Mountains formed during the
formation of Pangaea, the Himalayas from the collision of India
Ocean-Ocean Convergent Plate Boundaries
Collision of two oceanic slabs will result in the descent of one below
the other initiating volcanic activity in a similar manner to ocean-
continent collision. In the case of two oceanic plates colliding, the
older (colder, denser) oceanic crust subducts.
Volcanism. Forms volcanic island arc.
Examples include Japan, Aleutian islands, Caribbean islands,
TRANSFORM PLATE BOUNDARIES
Occurs when two lithospheric plates slide past one another horizontally.
Not associated with volcanism or mountain building.
Lots of shallow earthquakes.
Ocean-Ocean Transform Boundaries or Ridge-ridge transform
Major offsets of mid-ocean ridge axis.
Continent-Continent Transform Boundaries.
Example:San Andreas Fault
Driving force is not just convection?
Ridge Push and Slab Pull
Ridge Push: Gravity pulls the plates away from the mid-ocean ridge
Slab Pull: Gravity pulls the plates into the mantle. Plates tied to
subducting limbs spread faster. Examples include: Mid-Atlantic Ridge
spreading ~1 cm/yr; East Pacific Rise spreading ~6 cm/yr.