Plate tectonics
GEOL 115
#taftisheavy
Why does the Earth have
mountains and basins?
• Basic question asked by many: erosion is
evident everywhere, so why haven’t all
mountains eroded and filled in all basins?
• Alfred Wegener proposed continental drift
in 1912.
Evidence for continental drift
• Continental “fit” – South America and
Africa separated by the Atlantic
• Similar rocks separated by the Atlantic
• Similar climate markers (glacial striations)
separated by the Atlantic
• Similar fossils separated by the Atlantic
Unifying theory of geology
• Extension of Wegener’s continental drift
theory from the early 20th century
• Needed not only information about rocks
but also breakthroughs in geochronology
and geophysics
• Continental drift failed to provide a
sufficient mechanism (tides and wind were
not enough); plate tectonics does
Mechanism(s) of tectonics
• First, and foremost, the second law of
thermodynamics says that heat is
transferred from hot bodies to cold
bodies (e.g., Earth’s core to space)
Mechanism(s) of tectonics
• Heat transfers in three ways: radiation,
conduction and convection. It is the last of
these that occurs within the mantle.
• Convection (and internal heating) seems
to keep the mantle hot and plastic enough
for plates to sink into it -- may not be
enough to move plates
Mechanism(s) of tectonics
• Slab pull seems to be the major “drag”,
ridge push and plate sliding are more
minor forces
Summary of plate tectonic theory
• Plates exist
• Plates move
• Plates interact and generate geologic
phenomena
• Interiors of plates are geologically quiet*
Synthesis
• All three types
Nearby!
Tenets of plate tectonics
• Plates exist
Plate materials
•
•
•
•
Remember: lithosphere, not crust
Continental plates are made of granite
Oceanic plates are made of basalt
Granite and basalt are types of igneous
rocks
Tenets of plate tectonics
• Plates move
Hotspots
• Focused convection plumes from midmantle
Samoa
Hotspots are areas of magma upwelling from the mantle
They can occur in the middle of a tectonic plate (e.g.,
Hawaii) or at a plate boundary (e.g., Iceland)
As shown below for Hawaii, the plate onto which the hotspot
magma erupts continues to move, leaving a “trail” of extinct
volcanoes
Hotspots remain more or less stationary during their
lifetime, though there is some controversy about this.
Hotspot islands show plate movement
Haleakala NP (1961)
is considered active
Pacific Plate has moved
northwest as the
hotspot remained
relatively stationary;
older volcanoes are to
the northwest
Loihi
Haleakala’s last major mountain-building flows occurred perhaps
10,000 years ago, which means that streams have dissected the crater
walls, leaving large gaps.
Haleakala (= East
Maui Volcano) is
considered active
and capable of an
eruption that
produces a
significant amount of
lava – note the
volcano hazard
zones
Cinder cones are one-off volcanoes that dot the
floor of Haleakala crater; they are piles of ash
(tephra) that may be made of any kind of
volcanic rock
Haleakala –
Hawai’ian
“House of the
Sun”
Tall volcanoes in the tropics produce climates that are found nowhere
else – and thus species that are found nowhere else – silversword, for
instance
Aleutian Trench
Emperor
Seamount
Chain
~43 Ma
Midway
Hawaii
Topography of the Pacific Basin and Hawaiian Hotspot Track
Clear most of the year, with much less atmosphere above it, makes
Red Hill (Pu’u’ula’ula, the high point on Haleakala) excellent for
professional astronomy
Hawaii Volcanoes
National Park
National Park of
American Samoa has
three units, two of
which are accessible
only by boat. Coastal
erosion is a significant
problem compounded
by sea level rise.
Within the park are other
NPS-designated areas; for
instance, Val’ava Strait
between Pola Island and
the north shore of the
main Tutuila Island is a
National Natural
Landmark.
Matafao Volcano is the broad shield volcano
which is the base of Tutuila; on the island itself
are numerous cinder cones
CONTINENTAL HOTSPOT
Parks and Plates
©2005 Robert J. Lillie
Yellowstone Hotspot Track
North American Plate moves at ~3.5 cm/year
Yellowstone National
Park
Problems with using hotspot islands to
calculate plate motion rates
• Hotspots may not be fixed in position and
they are certainly not permanent
• GPS does a better job of measuring
annual rates, though over the short-term
only
Tenets of plate tectonics
• Plate boundaries are areas of geological
phenomena such as earthquakes,
volcanoes and mountain building
(orogenies)
Parks and Plates
©2005 Robert J. Lillie
Plate interactions
• Convergent
Types of convergent boundaries
• Continental plate - continental plate
convergence results in non-volcanic
mountain ranges
The Himalayas and the
Appalachians are
examples of
continent-continent
collisions
Shenandoah National Park, VA
Appalachians Mountains
Formed ~260-300 Ma
Types of convergent boundaries
• Oceanic plate - oceanic plate
convergence results in a volcanic oceanic
island arc
Subduction occurs of the
older (further from the
divergent boundary, thus
colder, thus denser) plate
under the younger plate,
even though they are made
of the same rock (basalt)
Oceanic-Oceanic Convergent Boundary
Island Arcs
Aleutian Islands, Alaska
Kenai Fjords National Park
Aleutian Volcanoes, Alaska
Cleveland Volcano
Cleveland Volcano, Alaska
South Pacific Ocean has many islands
National Park of American Samoa (1988) – these islands arise
from the hotspot and so only the easternmost one is active
Note that these
islands are
volcanoes that
are due to the
subduction
zone, and are
all considered
active
Types of convergent boundaries
• Continental plate - oceanic plate
convergence results in a volcanic
continental arc
Accretionary
Wedge
Volcanic
Arc
Convergent Plate Boundary
Subduction Zone
Accretionary
Wedge
Volcanic
Arc
Olympic National Park
Olympic National Park
Accretionary
Wedge
Volcanic
Arc
Mount Shuksan, North Cascades National Park
Mount Rainier, Rainier National Park
Plate interactions
• Divergent
Types of divergent boundaries
• Between two oceanic plates
Divergent Boundary
Mid-Ocean Ridges
What does a slice of oceanic lithosphere look like?
Asthenosphere
Asthenosphere
Mantle
Sea floor spreading
• Oceanic divergent boundaries provided
evidence for plate tectonics.
Paleomagnetism (the measurement of
“stripes” of different polarity minerals in
ocean floor rock of different ages) allowed
Hess and others (1950s) to postulate the
movement of the sea floor itself.
Divergent Boundary
Mid-Ocean Ridges
200 Myr
80-100
0 Myr
Sixty percent of volcanic rocks are produced at spreading ridges.
Types of divergent boundaries
• Between two continental plates
The Rio Grande Rift is an
example of a midcontinent rift; El Malpais
NM (1987) is an example
of a volcanic field that has
erupted lots of basalt onto
a continent
Plate interactions
• Transform
Pinnacles NM (1908)
shows many features
consistent with shearing
from the San Andreas
Fault
Tenets of plate tectonics
• Plate interiors are geologically quiet,
except for hotspots
Earthquake epicenters in the United States over
the past decade
Not very many earthquakes on the passive margin East Coast!
PLATE BOUNDARIES
• Divergent:
– Plates move away from
one another
– Oceanic lithosphere
created
• Convergent:
– Plates move toward one another
– Lithosphere destroyed
• Transform:
– Plates slide past one another
– Lithosphere neither created nor
destroyed
• (Hotspot):
– Plate rides over plume of hot
mantle
– Important, but NOT a plate
boundary
Parks and Plates ©2005 Robert J. Lillie
Where does volcanism occur and why?
• How does rock melt?
Mt St Helens, WA
Kilauea, Hawaii
How does the mantle melt?
• The melting temperature of most solids increases
with increasing pressure.
solidus
How does the mantle melt?
Lower the Pressure (Decompression Melting)
•
Lowering the pressure lowers
the melting temperature
– How would one decompress
parts of the mantle?
solidus
How does the mantle melt?
Add Water
• Water significantly lowers the melting temperature of
the mantle
• Where does this occur?
How does the mantle melt?
Add Water
• Water significantly lowers the melting temperature of
the mantle
How does the mantle melt?
• Are there any other mechanisms?
How does the mantle melt?
Hotspots, which are plumes of very hot molten material
rising through the asthenosphere
Mantle Convection and Loss of Heat
Cold crust sinks to the core
Hot plumes rise towards the surface