ch – 15 plate tectonics. earth ’ s layers by physical properties crust and upper mantle:...

Ch – 15 Plate Tectonics

Earth’s layers by physical propertiesCrust 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 solidCore

– 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 well-developed 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, low-elevation 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 ocean-floor 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