Download Testing the plate tectonics model Evidence for the plate tectonics

Ch – 15 Plate Tectonics
Earth’s layers by physical properties
Crust and upper mantle:
– Lithosphere – rigid solid which make up the tectonic plates,
includes both crust and upper mantle
– Asthenosphere – partially molten “weak” layer
Lower mantle (mesosphere) mostly solid
Core
– outer core (molten)
– inner core (solid)
Ocean and Continental Crust
• Oceanic Crust
– primarily basalt
– 4-7 km thickness (thin
relative to continental crust)
– denser (heavier) than
continental crust
• Continental Crust
– primarily granite
– 20-70 km thickness
– less dense (will not
undergo subduction)
Age of Sea Floor Rocks
(red-young, blue-old)
Fig. 6.10, p.139
Plate Tectonics: the new paradigm
From left to right:
• Transform boundary (conservative)
• Convergent boundary (destructive)
• Divergent boundary (constructive)
What happens at a divergent plate
boundary
Sea Floor Spreading
• Two plates move apart
• Mantle material upwells to create new
seafloor
• Mid-Oceanic ridges (underwater
mountain range) develop along welldeveloped divergent boundaries
• Mid-Atlantic Ridge
• East Pacific Rise
Geologic features found at divergent
boundaries
• volcanic activity (often underwater)
• mid-ocean ridge (underwater mountain
chain)
• very young volcanic rock
• “linear” seas (e.g. Red Sea, Sea of Cortez)
• rift valley – long narrow valley, such as
found in East Africa
Figure 15.10
Sea Floor Spreading on Land
• Sea floor spreading
adds thin, lowelevation ocean crust
to landmass.
Eventually water fills
in
• Arabian peninsula
split from African
continent
• Process continues in
East Africa rift valleys
(note lakes filling in
low lying ocean crust)
• Somali Plate?
Geologic features found at divergent
boundaries
• volcanic activity (often underwater)
• mid-ocean ridge (underwater mountain
chain)
• very young volcanic rock
• “linear” seas (e.g. Red Sea, Sea of Cortez)
• rift valley – long narrow valley, such as
found in East Africa
• shallow focus earthquakes
What happens at a convergent plate
boundary
1. Oceanic-continental subduction
– Denser oceanic lithosphere sinks into the
asthenosphere under more buoyant
continental lithosphere
– Pockets of magma develop and rise
– Continental volcanic arcs – chain of volcanoes
a short distance from plate boundary (e.g.
Andes, Cascades)
– Deep focus earthquakes
Figure 15.14a
What happens at a convergent plate
boundary
2. Oceanic-oceanic subduction
• Two oceanic plates converge and the older,
denser one descends beneath the younger,
more buoyant one.
• Pockets of magma develop and rise
• Volcanic Island Arcs forms as volcanoes
emerge from the sea
• Examples include Japan, Philippines, and
the Aleutian Island,
• Deep focus earthquakes
What happens at a
convergent plate boundary
• Subduction (Cont’d)
• Oceanic-oceanic convergence
• Two oceanic slabs converge and the
older, denser one descends beneath
the younger, more buoyant one.
• Forms volcanoes on the ocean floor
• Volcanic Island Arcs forms as
volcanoes emerge from the sea
• Examples include the Aleutian,
Mariana, and Tonga islands
Figure 15.14b
What happens at a convergent
boundary
Continental Collision (no
subduction)
• Continental-continental convergence
• When subducting plates contain
continental material, two continents
collide
• Can produce non-volcanic mountain
ranges such as the Himalayas
Figure 15.14c
What happens at Transform Fault
Boundaries
Conservative boundary (no loss or gain
of lithosphere)
Plates slide past one another
• Most transform faults join two segments
of sea-floor spreading
• Significant non-oceanic tranform fault
boundaries include
• San Andreas Fault,
• Alpine Fault
• Anatolian Fault (Turkey)
Figure 15.16
Figure 15.17
Modern discoveries supporting
Plate Tectonic Theory
• Mid-ocean ridges – underwater mountain
chains that circle the globe and often
mimic the shape of the coastline
• Distribution and depths of earthquakes
and volcanoes
• Relatively young age of the oceanic crust
(less than 180 million years)
• Lack of deep-ocean sediment
Testing the plate
tectonics model
Evidence for the plate tectonics
model
• Paleomagnetism
• Probably the most persuasive evidence for
sea floor spreading
• Ancient magnetism preserved in rocks
• Paleomagnetic records show Earth's
magnetic field reversals recorded in rocks
as they form at oceanic ridges
Figure 15.19
Paleomagnetic reversals recorded by basalt
flows at mid-ocean ridges
Figure 15.24
Testing the plate
tectonics model
Evidence from ocean drilling
• Some of the most convincing evidence
confirming seafloor spreading has
come from drilling directly into oceanfloor sediment
• Age of deepest sediments
• Thickness of ocean-floor sediments verifies
seafloor spreading
Testing the plate
tectonics model
Hot spots and mantle plumes
• Caused by rising plumes of mantle
material
• Volcanoes can form over them
(Hawaiian Island chain)
• Originate at great depth, perhaps at the
mantle-core boundary
Figure 15.18
Testing the Plate Tectonics
Model
• Earthquake depths
• Definite patterns exist
– Shallow focus occur along the oceanic
ridge system
– Almost all deep-focus earthquakes occur
in the circum-Pacific belt, particularly in
regions situated landward of deep-ocean
trenches
What drives plate motion
Driving mechanism of plate tectonics
• No one model explains all facets of
plate tectonics
• Earth's heat is the driving force
• Several models have been proposed
Fig. 6-13, p.136
What drives plate motion
Slab-pull and slab-push model
• Descending oceanic crust pulls the plate
• Elevated ridge system pushes the plate
• Plate-mantle convection
• Mantle plumes extend from mantle-core
boundary and cause convection within the
mantle
Several mechanisms
contribute to plate motion
Figure 15.26
Whole-mantle convection
Figure 15.27 B