Download Volcanoes and Igneous Activity Earth

Earth’s major plates
• Seven major lithospheric plates
• Plates are in motion and change in shape
and size
• Largest plate is the Pacific plate
• Several plates include an entire continent
plus a large area of seafloor
Earth’s major plates
• Plates move relative to each other at a
very slow but continuous rate
– Average about 5 centimeters (2 inches) per
year
– Cooler, denser slabs of oceanic lithosphere
descend into the mantle
– Motion defined by rotation around a pole
Plate boundaries
• Interactions among individual plates
occur along their boundaries
• Types of plate boundaries
– Divergent plate boundaries
– Convergent plate boundaries
– Transform fault boundaries
Types of Plate Margins
Plate boundaries
• Each plate bounded by combination of all
three boundary types
• New plate boundaries created in response
to changes in forces acting on rigid slabs
Divergent plate boundaries
Two plates move apart, resulting in upwelling
of material from the mantle to create new
seafloor.
Most are located along the crests of oceanic
ridges and can be thought of as constructive
plate margins
Oceanic ridges and seafloor spreading
• seafloor is elevated forming oceanic ridges
Oceanic ridges and seafloor spreading
• Seafloor spreading occurs along the oceanic ridge
system
Spreading rates and ridge topography
• Ridge systems exhibit topographic differences
• Topographic differences are controlled by
spreading rates (see map of age of oceanic crust
for width of ridges relative to their age)
Divergent boundaries are located
mainly along oceanic ridges
Spreading rates and ridge topography
• Topographic differences are controlled by
spreading rates
– Slow rates (1-5 cm/year), rift valley develops
on ridge crest (30 to 50 km wide, 1500-3000 m
deep)
– Intermediate spreading rates (5-9 cm/year),
rift valleys are shallow with subdued
topography
– At rates > 9 cm/year no rift valley develops or
are narrow and extensively faulted
– Rift valleys are deep-faulted structure
Divergent boundaries in Continents
Continental rifts
• Spreading centers can also develop within
a continent
• Splits landmasses into two or more
smaller segments
Divergent boundaries
Continental rifts
• Example includes East African rifts
• Produced by extensional forces acting on
the lithospheric plates
• Not all rift valleys develop into spreading
centers
• Otherwise Nevada would be an ocean!
The East African Rift
Development of Continental Rift into Ocean Basin
Convergent plate boundaries
Two plates move together, resulting in oceanic
lithosphere being thrust beneath an overriding plate,
eventually to be reabsorbed into the mantle
Convergence can also result in the collision of two
continental plates to create a mountain system
Old portions of oceanic plates are returned to the mantle
• Surface expression of descending plate is an ocean
trench called subduction zones
• Average angle at which oceanic lithosphere
descends into the mantle is about 45
All have same basic characteristics, but can have
highly variable features
Types of convergent boundaries
• Oceanic-continental convergence
– Denser oceanic slab sinks into the asthenosphere
– As plate descends, partial melting of mantle
rock makes basaltic or andesitic magmas
– Volcanic mountains associated with subduction
of oceanic lithosphere are called continental
volcanic arcs (Andes and Cascades)
Types of Arcs
Types of convergent boundaries
• Oceanic-oceanic convergence
– When two oceanic slabs converge, one
descends beneath the other
– Often forms volcanoes on the ocean floor
– If the volcanoes emerge as islands, a volcanic
island arc is formed (Japan, Aleutian islands,
Tonga islands)
Types of Arcs
Types of convergent boundaries
• Continental-continental convergence
– Continued subduction brings continents together
– Less dense, buoyant continental lithosphere does
not subduct
– Result is a collision between two continental
blocks
– Process produces mountains (Himalayas, Alps,
Appalachians)
The collision of India and Asia
produced the Himalayas
Transform fault boundaries
Third type of plate boundary
Plates slide past one another and no new
lithosphere is created or destroyed
Transform faults
• Most join two segments of a mid-ocean ridge as
parts of linear breaks in the oceanic crust known
as fracture zones
• Accommodate simultaneous movement of offset
ridges
Transform faults accommodate
movement on offset ridge segments
Testing the plate tectonics model
Plate tectonics and earthquakes
• Plate tectonics model accounts for the global
distribution of earthquakes
– Absence of deep-focus earthquakes along the
oceanic ridge is consistent with tectonic theory
– Deep-focus earthquakes associated with
subduction zones
– Deep-focus earthquakes occur along convergent
boundaries
– The pattern of earthquakes along a trench
provides method to track plate's descent
Deep-focus earthquakes occur
along convergent boundaries
Importance of plate tectonics
Provides a unified explanation of Earth’s major
surface processes, especially oceans
Within framework of plate tectonics, we find
explanations for the distribution of
earthquakes, volcanoes, and mountains
Plate tectonics provides explanations for
distribution/evolution of plants and animals
and climate record