Plate Tectonics and the
Geography of the Earth
Plate Tectonic Theory
A good theory successfully explains
new data.
World Wide Seismographic Monitoring Network
shallow
intermediate
deep
Global Distribution of Earthquakes
Global Earthquake Patterns
• 1963, establishment of Worldwide Standardized
Seismograph Network (WWSSN) to monitor nuclear
test ban.
• Global pattern of earthquakes shows most activity at
mid-ocean ridges and trenches (plate boundaries).
• Earthquakes at trenches show increasing depth along
an incline - Wadati-Benioff Zone.
• Earthquakes at mid-ocean ridges are shallow.
Wadati-Benioff Zone - region of earthquake activity at a subduction zone
shallow
intermediate
deep
Plate Tectonic Theory
A good theory makes testable predictions.
What is the predicted pattern in the age of the
ocean floor from the mid-ocean ridge to the trench?
Age of the Ocean Floor
• Ocean floor age mapped based on magnetic reversal “bar
code” preserved in oceanic crust.
• 1968 - establishment of the Deep Sea Drilling Project
(DSDP).
• Specially outfitted ship for drilling into the ocean floor
and retrieving rock samples.
Deep Sea Drilling Project ship Glomar Challenger
Age of the Ocean Floor
• Sediment layers and igneous rock dated to test the age of
the ocean floor in different places.
• Ocean floor is youngest at mid-ocean ridges, oldest at
trenches.
• Oldest ocean
floor found is
180 ma.
• Oldest
continental rock
is 3800 ma.
Acceptance of Sea Floor Spreading
(Plate Tectonics)
• 1970’s - rapid confirmation of remaining predictions
and acceptance of the basic theory by most
geologists. Plate tectonics applied to mountain
building.
• 1980’s - Plate Tectonic Theory becomes the
dominant paradigm in geology.
• 1990’s - Development of a detailed plate tectonic
history of the last 700 million years.
• 2000’s - Detailed studies of plate motion using GPS,
investigation of the dynamics of the mantle, forces
driving plate motion.
Types of plate boundaries
transform
divergent
convergent
Divergent Plate Boundaries
continental rift
incipient
ocean basin
mid-ocean
ridge
Divergent Plate Boundary
Central rift valley
Shallow earthquakes
Volcanoes
African continental rift
and incipient ocean
basins
African Volcanoes
Kilimanjaro
African rift valley - Lake Turkana
Seafloor Age - Mid-Atlantic Ridge
Mid-ocean ridge is visible where it crosses Iceland
Rifting and volcanism
in Iceland
Central Rift Valley - Iceland
20
Convergent Boundary: Ocean - Continent
Convergent Plate Boundary
Ocean - Continent
Shallow - intermediate - deep
earthquakes
Wet mantle volcanism - stratovolcanoes
Continental compression - high mountain
range with volcanoes
Seismicity - South America
Volcán Tungurahua, Ecuador
Cascades Volcanic Range
Mt. Rainier
Mt. Adams
Mt. St. Helens
Mt. Hood
Mt. Rainier, Seattle, Washington
Convergent Boundary: Ocean - Ocean
Convergent Plate Boundary
Ocean - Ocean
Shallow - intermediate - deep
earthquakes
Wet mantle volcanism - stratovolcanoes
Volcanic island arc
Mt. Redoubt - Aleutian Islands, Alaska
Volcanism - Central America / Caribbean
Seismicity
Volcanoes of the Lesser Antilles
Soufriere Hills, Montserrat
36
Seismicity - Southern Asia
Convergent Plate Boundary
Continent - Continent
Shallow earthquakes
No volcanism
High mountain range and plateau due to
crustal thickening
Convergent Boundary
Continent - Continent
Subduction and
volcanism stop.
Crust is compressed and
thickened.
High mountains form
with deep crustal roots.
Alps
Zagros Mtns.
Western Himalaya
Seismicity - Europe
Tibetan Plateau
Himalayan Mtns.
Mt. Everest
Seismicity - Southern Asia
Himalaya Mountains
Makalu
Everest
Mt. Everest
Transform Plate Boundary
Tectonic plates sideswipe each other
Shallow earthquakes
horizontal displacement of the crust
over great distances
Transform boundary
displaced stream
San Andreas Fault
Transform boundary
Hot Spots
• Localized regions of high heat flow in the
mantle.
• Generates melting in the overlying crust.
• Remain fixed for between 20 million and 100
million years while plate rides over.
• Many aseismic ridges produced by hot spots.
Hot Spots
Hawai’ian Ridge and
Emperor Seamount
hotspot trail
Emperor Seamounts
Plate motion
Hawai’ian Ridge
Hawai’ian Islands
Hot Spot
Causes of Plate Movement
• Ridge Push
– plates are moving down slope due to gravity, from
high elevation at ridges to low elevation in
trenches.
• Slab Pull
– Subducting slab sinking into mantle pulls the rest
of the plate behind it.
• Both!