IV. Modern Plate Tectonic Theory
Major plates and plate margins
A.


B.
C.
D.
E.
F.
Names and types of major plates
Symbols for plate margins
Divergent (constructive) margins
Convergent margins
Transform or Strike Slip margins
Volcanic Hotspots
What drives it all?
IV. Modern Plate Tectonic Theory
Major plates and plate margins
A.

Names and types of major plates
Image from USGS plate tectonics web site
IV. Modern Plate Tectonic Theory
Major plates and plate margins
A.

Symbols for plate margins
Convergent margin
Teeth on upper plate side
Transform margin
Divergent margin
IV. Modern Plate Tectonic Theory
A.
Major plates and plate margin
Major tectonic plates, plate boundaries, plate motion vectors
Arrow are plate vectors, numbers are rates of plate motion in centimeters per year
IV. Modern Plate Tectonic Theory
A.
Major plates and plate margin
A slightly different
representation from
This Dynamic Earth
USGS poster
Shading (red) shows
diffuse zones of crustal
deformation, most of
these are associated
with (adjacent to ) plate
boundaries and are areas
of high earthquake
activity
Red dots are locations
of “hot spot” volcanism,
many of these are
within plates
IV. Modern Plate Tectonics
Major plates and plate margins
Divergent (constructive) margins
A.
B.


mid-ocean ridges (=spreading centers)
Continental rift zones (incipient divergent margins)
Divergent plate margins are also referred to a constructive
plate margins; this is because igneous activity at these
plate margins generates new lithospheric plates
including new oceanic crust
IV. Modern Plate Tectonic Theory
B. Divergent (constructive) margins
•
mid-ocean ridges (=spreading centers)
IV. Modern Plate Tectonic Theory
B. Divergent (constructive) margins
•
mid-ocean ridges (=spreading centers)
elevated ridge but with an axial rift (faulted) valley
along the ridge axis
active magmatism including sea floor volcanism
highest rate of heat loss
(surface heat flow) on Earth
magmatism generates new
oceanic crust and lithospheric
plates
Mid Atlantic
Ridge is a slow
spreading ridge
Water depth or
depth below sealevel
IV. Modern Plate Tectonic Theory
B. Divergent (constructive) margins
•
mid-ocean ridges (=spreading centers)
East Pacific Rise
is a fast spreading
ridge
highest rate of heat
loss (surface heat
flow) on Earth
magmatism generates
new oceanic crust and
lithospheric plates
IV. Modern Plate Tectonic Theory
B. Divergent (constructive) margins
•
mid-ocean ridges (=spreading centers)
East Pacific Rise is a fast
spreading ridge
Another look at an axial rift
valley
Major Tectonic Plates & Plate Boundaries
•
Divergent (constructive) margins
•
•
•
Continental rift zones (incipient divergent margins)
Magmatism and volcanic activity accompanies continental rifting (ex. East Africa
Rift)
Such rifting can lead to development of a new plate margin and ultimately a new
ocean basin (ex. Red Sea)
Major Tectonic Plates & Plate Boundaries
•
Divergent (constructive) margins
•
•
•
Continental rift zones (incipient
divergent margins)
East African Rift Zone
Red Sea is a new ocean basin
Active
volcanoes
Image from USGS plate tectonics web site
IV. Modern Plate Tectonic Theory
Major plates and plate margins
Divergent (constructive) margins
Convergent margins (two main types)
A.
B.
C.

Subduction zones (two sub-types)
○
○

D.
E.
Ocean-ocean
Ocean-continent
Continent-Continent collision zones
Transform or Strike Slip margins
What drives it all?
IV. Modern Plate Tectonic Theory
•
Convergent margins
•
Subduction zones
•
ocean-ocean
•
ocean-continent
Deep-sea trench is site where
oceanic lithosphere starts to descend
into mantle
Ocean-ocean
Ocean-continent
IV. Modern Plate Tectonic Theory
•
Convergent margins
•
•
•
Subduction zones are sites where an oceanic tectonic plate descends into
the mantle
Overriding plate may also be an oceanic tectonic plate (ocean-ocean) or
may be a continental tectonic plate (ocean-continent)
Chain of volcanoes are found on the overriding plate
•
•
Island arcs (ocean-ocean)
Continental volcanic arcs (ocean-continent)
Ocean-ocean
Ocean-continent
IV. Modern Plate Tectonic Theory
Convergent margins
Subduction zones are sites of deepest earthquakes on earth (to 700 km)
Array of earthquakes marks the subducting lithosphere = Benioff Zone
Earthquake array starts at trench (shallow) and gets progressively deeper beneath the over
riding plate
Japan
IV. Modern Plate Tectonic Theory
Convergent margins
Subduction zones are sites of deepest earthquakes on earth (to 700 km)
Array of earthquakes marks the subducting lithosphere = Benioff Zone
Earthquake array starts at trench (shallow) and gets progressively deeper beneath the over
riding plate
Benioff Zone =
earthquakes within the
cold subducting
lithospheric plate
IV. Modern Plate Tectonic Theory
Major plates and plate margins
Divergent (constructive) margins
Convergent margins (two main types)
A.
B.
C.

Subduction zones (two sub-types)
○
○

D.
E.
Ocean-ocean
Ocean-continent
Continent-Continent collision zones
Transform or Strike Slip margins
What drives it all?
IV. Modern Plate
Tectonic Theory
•
Convergent margins
•
Continent-Continent collision zones
•
•
•
Himalayas (India & Tibet)
Zargos (Middle East)
Alps (Southern Europe)
Image from USGS plate tectonics web site
IV. Modern Plate Tectonic Theory
•
Convergent margins
•
Continent-Continent collision zones
Clay block
model
Rigid indenter
Escape tectonics
IV. Modern Plate Tectonic Theory
•
Convergent margins
•
Continent-Continent collision zones
Escape tectonics
IV. Modern Plate Tectonic Theory
Major plates and plate margins
Divergent (constructive) margins
Convergent margins
Transform or Strike Slip plate margins
A.
B.
C.
D.


At mid-ocean ridges
San Andreas fault zone
IV. Modern Plate Tectonic Theory
Transform or Strike Slip Plate Margins
San Andreas fault zone
At mid-ocean ridges
Image from USGS plate tectonics web site
IV. Modern Plate Tectonic Theory
A.
B.
C.
D.
E.
Major plates and plate margins
Divergent (constructive) margins
Convergent margins
Transform or Strike Slip margins
Volcanic Hotspots
IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A volcanic hotspot is
mantle plume that is
stationary with respect
to overriding tectonic
plate.
Examples include:
Hawaii
(oceanic hotspot)
Yellowstone
(continental hotspot)
IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A hotspot is mantle plume that is stationary with respect to overriding tectonic plate.
IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A hotspot is mantle plume that is stationary with respect to overriding tectonic plate.
Emperor-Hawaii Seamount Chain
Hotspot (mantle plume stationary with
respect to overriding tectonic plate)
IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A hotspot is mantle plume that is stationary with respect to overriding tectonic plate.
Pacific plate has moved over the Hawaii
hotspot for over 60 million years (Ma)
leaving a hotspot track of now extinct
volcanoes, all similar to Hawaii
The Hawaii-Emperor Volcanic Chain
IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A hotspot is mantle plume that is stationary with respect to overriding tectonic plate.
Yellowstone-Snake River Plain
Hot Spot (mantle plume stationary
with respect to overriding tectonic
plate)
IV. Modern Plate Tectonic Theory
A.
B.
C.
D.
E.
F.
Major plates and plate margins
Divergent (constructive) margins
Convergent margins
Transform or Strike Slip margins
Volcanic Hot Spots
What drives it all?
1. Earth’s internal heat engine
2. Density “currents” in plastic mantle
Volcanoes
IV. Modern Plate Tectonic Theory
F. What drives it all?
1. Earth’s internal (endogenic) heat engine

Geothermal gradients (evidence of hot interior)
○
Hot springs
Temperature increases with depth in solid
earth, most obvious surface expressions of
geothermal gradients


Volcanic eruptions, geysers and hot
springs, fumaroles
Radioactive heat production and residual heat of
planet formation
Radioactive heat
production declining
with time
Only during earth’s
formation
Heat produced by impact
during planet formation
Rocks are poor heat
conductors
Earth is still cooling
Heat production
with time
IV. Modern Plate Tectonic Theory
F. What drives it all?
2. Density “currents” in plastic mantle


Ridge push (rising mantle)
Slab pull (sinking plate)
Slab pull
New (young) oceanic
lithospheric plates are hot,
but they cool over 10’s of
millions of years as they
move away from the oceanic
spreading ridge.
As the oceanic plates cool
they become more dense,
eventually becoming dense
enough to sink into the
mantle.
Mid-ocean spreading
ridges have the highest
surface heat flow on
earth
Subduction zone
trenches have the
lowest surface
heat flow on
earth
Age of the Crust
Shield areas on the continents are Precambrian in age (0.54 – 4.2 billion years)
Remainder of continental crust is Phanerozoic in age (Recent to 542 million years)
Oceanic crust is Mesozoic and Cenozoic in age (170 million years [Jurassic] to Recent)
Age of the Crust
Oceanic crust is generated by igneous activity at mid-ocean ridges and eventually is recycled
back into the mantle at subduction zone (due to its higher density)
Age of the Crust
Igneous activity above subduction zones (island arcs and continental volcanic arcs) generates
lower density continental crust
This lower density crust is too buoyant to be subducted
The inability to subduct explains why much of the continental crust is so old (billions of years)