Earth Science 9.3 Theory Tectonic Plates
Theory Tectonic Plates
Earth’s Moving Plates
During the 1960s, scientistsrealized that the theory ofsea-floor spreading explainedthe idea of continental drift.It explained how oceanbasins could open and closeover time.
Canadian geologist J. Wilsoncame up with a theory thatled to a revolution in geology.
Wilson suggested that thelithosphere was broken intoseveral huge pieces, calledplates. Deep faults, likecracks in an eggshell,separate the differentplates.
Earth’s Moving Plates
In the theory of platetectonics, Earth’s lithosphericplates move slowly relative toone another, driven byconvection currents in themantle.
Previously scientists had failedto explain how the lithospherecould move. The theory ofplate tectonics identified aforce that could set Earth’souter shell in motion.
According to Wilson,convection currents withinEarth drive plate motion.
Hot material deep within themantle moves upward byconvection. At the same time,cooler, denser slabs of oceaniclithosphere sink into themantle
Earth’s Moving Plates
Effects of plate motion:
Plate motion averages about5 centimeters per year;about as fast as yourfingernails grow.
The results of plate motioninclude earthquakes,volcanoes, and mountainbuilding.
Earth’s Moving Plates
Effects of plate motion:
Interactions among different plates happen along plate boundaries.
Three types of plate boundaries exist
Convergent boundaries
Divergent boundaries
Transform boundaries
Earth’s Moving Plates
Divergent Boundaries:
Divergent boundaries are found when two of Earth’s plates moveapart. Oceanic lithosphere is created where divergentboundaries occur and sea-floor spreading happens.
Earth’s Moving Plates
Convergent Boundaries:
Convergent boundaries happen when two plates move togethertowards each other. Lithosphere can be destroyed at convergentboundaries when oceanic lithosphere sinks into the mantle duringsubduction
Earth’s Moving Plates
Transform Boundaries:
Transform boundaries occur when two plates grind past each other.
Along transform boundaries, lithosphere is neither created nor is itlost.
Earth’s Moving Plates
Plates may shrink or grow depending on the locations of theconvergent and divergent boundaries.
Slowly over time some plates grow over others, pushing themunder through subduction when borders converge. Some platesslowly expand while others shrink due to this.
Earth’s Moving Plates
Divergent Boundaries:
Along divergent boundaries, platesmove apart. Because they are theareas where sea-floor spreadingbegins, divergent boundaries arecalled spreading centers.
Most divergent boundaries occuralong the crests of the mid-oceanridges. Some spreading-centersoccur on land on the continents.
We think of these plate boundariesas constructive plate marginsbecause this is where new oceaniclithosphere is produced.
Divergent Boundaries
When a spreading center formsalong land, the process can literallysplit a continent apart.
The process begins when forces ofplate motion begin to stretch thelithosphere. At the same time,plumes of hot rock rise from themantle.
The rising plumes bend the crustupward, weakening and fracturingit.
The fractures allow magma toreach the surface. The result is anew floor of a rift valley
Divergent Boundaries
Examples of rift valleys include theRhine Valley in Europe and theGreat Rift valley in East Africa
The Great rift valley in East Africamay represent the first step in theprocess of the breakup of Africa.This process may take millions ofyears
If the sides of the rift valleycontinue to move apart, the valleycould become a narrow sea like theRed Sea shown at right.
Convergent Boundaries
At convergent boundaries,plates collide and interact,producing featuresincluding trenches,volcanoes and mountainranges
Along convergentboundaries, older portionsof oceanic plates return tothe mantle. As a result,Earth’s total surfaceremains the same, eventhough new lithosphere isconstantly being added atmid-ocean ridges.
Convergent Boundaries
Because lithosphere isdestroyed at convergentboundaries, they are alsocalled “destructive platemargins”.
As two plates slowlyconverge, the leading edgeof one plate is bentdownwards, allowing it toslide beneath the otherplate. We call this slidingunder the other platesubduction.
At destructive platemargins, oceanic crust issubducted into the mantleof the Earth.
Convergent Boundaries
The type of lithosphereinvolved and the forcesacting upon it determinewhat happens atconvergent boundaries.
Convergent boundaries canform between
two pieces of oceaniclithosphere,
between oceaniclithosphere andcontinental lithosphere,
and between two piecesof continentallithosphere.
Oceanic-Continental
When the leading edge ofcontinental lithosphereconverges with oceaniclithosphere, the less densecontinental lithosphereremains floating. Thedenser oceanic slab sinksinto the asthenosphere.
When a descending platereaches about 100 to 150kilometers below the Earth,some of the asthenosphereabove the descending platemelts.
The newly formed magma,being less dense than therock mantle, rises.Eventually some of itreaches the surface andbecomes volcanic activity.
Oceanic-Continental
A continental volcanic arcis a range of volcanicmountains produced inpart by the subduction ofoceanic lithosphere.
The volcanoes of theAndes in South Americaare the product of magmaformed during subductionof the Nazca Plate
Everywhere around theworld, volcanic activityoccurs where thesesubduction zones occurcreating a “ring of fire”along these plateboundaries
Oceanic-Oceanic
When two oceanic slabsconverge, one descendsbeneath the other. Thiscauses volcanic activitysimilar to what happens inoceanic-continental.
The volcanoes form on theocean floor instead of onland, however. If thisactivity continues, it willbuild a chain of volcanicstructures that becomeislands.
This newly formed land wecall a volcanic island arc.
Continental-Continental
When oceanic lithosphere issubducted beneathcontinental lithosphere, acontinental volcanic arcdevelops along the marginof the continent.
However, if the subductionplate also containscontinental lithosphere, thesubduction eventually bringstwo continents together.
The result is a collisionbetween the two continentalplates. Since neither sinksbelow the other, collisionresults and mountains form.
Continental-Continental
Before continents collidethey are separated by anocean basin. As thecontinents move towardeach other, the sea-floorbetween them issubducted beneath one ofthe plates.
When the continentscollide, the collision foldsand deforms thesediments along themargin as if they wereplaced in a giant vice.
A new mountain rangeforms that is composed ofdeformed andmetamorphosizedsedimentary rocks.
Continental-Continental
This kind of collisionoccurred when Indiarammed into Asia andproduced the Himalayas.
Mountain systems such asthe Alps, theAppalachians, and theUrals were formed by sucha process.
Transform Fault Boundaries
The third type of plateboundary system is thetransform fault boundary.Pieces of lithosphere move pasteach other horizontally along atransform fault boundary.
At a transform fault boundary,plates grind against each otherwithout destroying or creatinglithosphere.
Most transform faults join twosections of mid-ocean ridge.These faults occur about every100 kilometers along the ridgeaxis.
Transform Fault Boundaries
Active transfer faults liebetween the two offset ridgesegments.
The seafloor produced at oneridge axis moves in a directionopposite to that of the seafloorproduced at the next ridgesegment.
Between the ridge segments,these slabs of oceanic crust aresliding past each other along atransfer fault.
Transform Fault Boundaries
Although most transform faultsare located within the oceanbasins, a few cut throughcontinental lithosphere.
One example is the San AndreasFault in California, where thePacific Plate is moving past theNorth American Plate.
If this movement continues, thepart of California west of thefault zone will become an islandsomeday.
The more immediate concern isthe earthquake activity thatresults from the tension createdby this fault.
