4. the face of earth

Global TectonicsGlobal TectonicsOur Dynamic Planet Our Dynamic Planet

CVE 3205CVE 3205Engineering GeologyEngineering Geology

Wong Jee KhaiWong Jee Khai

IntroductionIntroduction• Each rocky body, whether planet or moon,

started with a hot interior.• Each has been kept warm over time by

energy released by the decay of radioactive isotopes.

• Despite radioactive heating, rocky bodies have cooled considerably since their formation, so that their outer layers have stiffened into lithospheres.

IntroductionIntroduction• The interior of Earth and Venus remain hot

and geologically active.• The mantles of Earth and Venus lose

internal heat by convection, the slow flow of solid rock.

• Hot rock rises upward to near the surface.• Earth’s stiff lithosphere is broken into a

collection of near-rigid plates.

IntroductionIntroduction• Most large-scale geologic events, like

earthquakes or volcanic eruptions, originate within Earth’s interior.

• Many other processes in the Earth system, such as the hydrologic and biogeochemical cycles, are profoundly affected by plate tectonics.

The face of the earthThe face of the earthTwo principal divisions of earth’s surface:

1.Continents2.Ocean basins

The face of the earthThe face of the earthRelative level:• Continents have average elevation of

about 0.8 km above sea level.• Ocean basins have average depth of 3.8

km below sea level.

The face of the earthThe face of the earthDensities and thickness:• Continents are average 35-40 km thick

and composed of granitic rocks with about 2.7 g/cm3 density.

• Basaltic rocks that comprise the oceanic crust average only 7 km thick and have an average density of about 3.0 g/cm3.

From Hypothesis to TheoryFrom Hypothesis to Theory• Plate tectonics is a scientific theory that

explains two centuries of often puzzling observations and hypotheses about our planet Earth.

• The continents are drifting very slowly across the face of our planet.– Continental drift is a concept with a long

history.

From Hypothesis to TheoryFrom Hypothesis to Theory• A century ago geologists puzzled over the fit

of the shorelines of Africa and South America.– They noted that fossils of extinct land-bound

plants and animals, glacial deposits, and ancient lava flows could be matched together along coastlines that today are thousands of kilometers apart.

– Coal was found in Antarctica.• Coal forms in tropical climates, implying that Antarctica

has moved in the past.

From Hypothesis to TheoryFrom Hypothesis to Theory• Faced with puzzling data, scientist

developed hypotheses to explain them.• Alfred Wegener proposed the most

comprehensive early hypothesis for Continental Drift in 1912.

From Hypothesis to TheoryFrom Hypothesis to Theory• His theory was widely rejected because:

– Ocean floor was too strong to be plowed aside.

– Wegener had not proposed a plausible force that could induce the continents to drift.

– Attempts to test Wegener’s hypothesis with observations had mixed success.

From Hypothesis to TheoryFrom Hypothesis to Theory• In 1960, the Theory of Plate Tectonics was

born.– Plate tectonics is the process by which

Earth’s hot interior loses heat. – We can measure the slow drift of plates

worldwide using satellite navigation systems.– The basic premises of plate theory are secure

because they can be tested against a wide variety of observations.

What Earth’s Internal What Earth’s Internal Phenomena Tell UsPhenomena Tell Us

• Rocks are poor conductors of heat, so Earth moves its internal heat by moving the rock itself.– The circulation of hot rock is maintained by

mantle convection.

Mantle ConvectionMantle Convection• Conduction is the process by which heat

moves through solid rock.• Earth’s heat can move in a second

process called convection.– Convection can happen in gases, in liquids,

or, given enough time, in ductile solids.– A prerequisite condition for mantle convection

is the thermal expansion of hot rock.

Fig. 2.7

Mantle ConvectionMantle Convection– Rock expands as its temperature increases.– Its density thereby decreases slightly.– The hot rock is buoyant relative to cooler rock

in its immediate neighborhood.– A 1 percent expansion requires an increase of

300-400oC and leads to a 1 percent decrease in density.

– Viscosity is the propensity of rock to ductile flow.

Mantle ConvectionMantle Convection– Rock does not need to melt before it can flow.– The presence of H2O encourages flow in solid

rock.– Convection currents bring hot rocks upward

from Earth’s interior.– The rock in the lithosphere is too cool for

convection to continue.– Heat moves through the lithosphere primarily

by conduction.

Earth’s Convection: Driven Earth’s Convection: Driven From the TopFrom the Top

• Below the lithosphere, rock masses in the deeper mantle rise and fall according to differences in temperature and buoyancy.

• The densest lithosphere is most likely to sink back into the asthenosphere and the deeper mantle.

• Ocean floor and the continents are slowly moving (up to 12 cm/yr).

Plates and Mantle Plates and Mantle ConvectionConvection

• When continents split apart, a new ocean basin forms. – The Red Sea was formed this way 30 million

years ago.• Subduction: the old lithosphere sinks

beneath the edge of an adjacent plate.

Plate MarginsPlate Margins• The lithosphere currently consists of 12

large plates.• The seven largest plates are:

– North American Plate.– South American Plate.– African Plate. – Pacific Plate.– Eurasian Plate.

Plate MarginsPlate Margins– Australian-Indian Plate.– Antarctic Plate.

• All are moving at speeds ranging from 1 to 12 cm a year.

Fig. 2.9 c

Plates BoundariesPlates BoundariesPlates have four kinds of boundaries or

margins:1.Divergent margin (also called a spreading

center): magma rises to form new oceanic crust between the two pieces of the original plate.

2.Convergent margin/subduction zone: two plates move toward each other and one sinks beneath the other.

Plates BoundariesPlates Boundaries3. Convergent margin/collision zone: two

colliding continental plates create a mountain range.

4. Transform fault margin: two plates slide past each other, grinding and abrading their edges.

Seismology and Plate Seismology and Plate MarginMargin

• Earthquakes occur in portions of the lithosphere that are stiff and brittle.

• Earthquakes usually occur on pre-existing fracture surfaces, or faults.

• There are distinctive types of earthquakes that correlate nicely with motion at plate boundaries.

Divergent MarginDivergent Margin• Where two plates spread apart at a

divergent boundary, hot asthenosphere rises to fill the gap.

• As it ascends, the rock experiences a decrease in pressure and partially melts.

• The molten rock is called magma.

Divergent MarginDivergent Margin• Midocean ridges occur in oceanic crust.

– Found in every ocean.– Form a continuous chain that circles the

globe.

• Oceanic crust is about 8 km thick.

Characteristics of Spreading Characteristics of Spreading CentersCenters

• Earthquakes at midocean ridges occur only in the first 10 km beneath the seafloor and tend to be small.

• Normal faults form parallel lines along the rifted margin.

• Volcanic activity occurs at midocean ridges and continental rifts (along narrow parallel fissures).

• The midocean ridges rise 2 km or more above surrounding seafloor.


• The principle of isostasy applies: lower-density rock rises to form a higher elevation.

• The seafloor around midocean ridges for nearly all oceanic lithosphere is younger than 70 million years.


• If the spreading rate is fast: – A larger amount of young warm oceanic

lithosphere is produced.– The ridge will be wider.

• A slow-spreading ridge will be narrower.• The Atlantic Ocean spreads slowly, growing

wider at 2-4 cm/yr.• The Pacific spreading center is fast by

comparison: 6-20 cm/yr.

Role of Seawater at Role of Seawater at Spreading CentersSpreading Centers

• Seawater circulates through cracks beneath the ocean floor.

• Cold water percolates through these cracks, warms in contact with subsurface rock, and rises convectively to form undersea hot springs.

Role of Seawater at Role of Seawater at Spreading CentersSpreading Centers

• Seawater reacts chemically with lithospheric rock, leaching many metallic elements from it.

• A small fraction of the seawater remains in the rock, chemically bound within hydrous (water-bearing) minerals like serpentine and clays.

Convergent Convergent Margin/Subduction ZoneMargin/Subduction Zone

• Over 70 million years, oceanic lithosphere can drift 1500 to 3000 km from the spreading center.

• As the plate cools, it grows denser.• The principle of isostasy demands that the

plate subsides as it grows denser.• The process by which lithosphere sinks into

the asthenosphere is called subduction.


• The margins along which plates are subducted are called subduction zones.– These are active continental margins.

• The sinking slab warms, softens, and exchanges material with the surrounding mantle.


• Under elevated temperature and pressure, the crust expels a number of chemical compounds.– Water (H2O).– Carbon dioxide (CO2).– Sulfur compounds.

• A small addition of these volatile substances can lower the melting point of rock by several hundred degrees Celsius.


• The hot mantle rock immediately above the sinking slab starts to melt.– Magma rises to the surface to form

volcanoes.– Subduction zones are marked by an arc of

volcanoes parallel to the edge of the plate.

Earthquakes in Subduction Earthquakes in Subduction ZonesZones

• The largest and the deepest earthquakes occur in subduction zones.

• The location of most earthquakes define the top surface of a slab as it slides into the mantle (the surface to as deep as 670 km).

• Quakes deeper than 100 km are more likely associated with faults caused by stresses within the slab.

Convergent Margin/Collision Convergent Margin/Collision ZoneZone

• Continental crust is not recycled into the mantle.

• Continental crust is lighter (less dense) and thicker than oceanic crust.

• When two fragments of continental lithosphere converge, the surface rocks crumple together to form a collision zone.

Convergent Margin/Collision Convergent Margin/Collision ZoneZone

• Collision zones that mark the closure of a former ocean form spectacular mountain ranges.– The Alps.– The Himalayas.– The Appalachians.

Transform Fault MarginTransform Fault Margin• Along a transform fault margin, two plates

grind past each other in horizontal motion.• These margins involve strike-slip faults in

the shallow lithosphere and often a broader shear zone deeper in the lithosphere.

• Most transform fault occur underwater between oceanic plates.

Transform Fault MarginTransform Fault Margin• Two of Earth’s most notorious and

dangerous transform faults are on land.– The North Anatolian Fault in Turkey.– The San Andreas Fault in California.

Fig. 2.12

Fig. 2.18

Topography of the Ocean Topography of the Ocean Floor Floor

• Two main features:– Midocean ridges.

• Some 64,000 km in length.• The spreading centers separate plates.• The oceanic ridge with its central rift reaches sea

level and forms volcanic islands.– Iceland.

– The oceanic trenches.

What Causes Plate What Causes Plate Tectonics?Tectonics?

• There is more agreement on how plate tectonics works than on why it works.

• Mantle convection occurs in a variety of patterns, large-scale and small-scale.

• Hotter rock is less viscous than cooler rock (critical to convective circulation).


• Hot, buoyant, low-viscosity material rises in narrow columns that resemble hot spot plumes.

• Cooler, stiffer material from the surface sinks into the mantle in sheets (similar to subducting slabs).


• Three forces seem likely to have a part in moving the lithosphere:– Ridge push: the young lithosphere sits atop a

topographic high, where gravity causes it to slide down the gentle slopes of the ridge.

– Slab pull: at a subduction zone, as the cold, dense slab is free to sink into the mantle, it pulls the rest of the lithosphere into the oceanic trench behind it.


– Friction: • Slab friction drags the top, the bottom, and the

leading edge of descending lithosphere in the subduction zone.

• Plate friction drags elsewhere at the base of the plate.

Why Does Plate Tectonics Why Does Plate Tectonics Work?Work?

• The theory of plate tectonics does not explain why the plates exist.

• At the present time, a number of scientific clues point to water as the missing ingredient in the plate tectonics.– Water molecules can diffuse slowly through

solid rock.– Water can weaken rock in several ways.

4. the face of earth

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