Warm Up 11/5Warm Up 11/51. Why are subduction zones not commonly found at

convergent continental-continental boundaries?a. Oceanic lithosphere is too buoyant to be forced down into the mantle.b. Continental lithosphere is too dense to be forced down into the mantle.c. Continental lithosphere is too buoyant to be forced down into the mantle.d. Subduction zones are never found at convergent boundaries.

2. Which of the following is a geographic example of a transform fault boundary?a. the Andes Mountains c. the San Andreas Faultb. the East African Rift valley d. the Mid-Atlantic Ridge

3. The Red Sea is theorized to be the site of a recently formed ____.a. convergent boundary c. divergent boundaryb. hot spot d. transform fault boundary

Answers: 1) c. 2) c. 3) c.

Testing Plate Tectonics & Testing Plate Tectonics & Mechanisms of Plate MotionMechanisms of Plate Motion

Chapter 9, Sections 4 & 5Chapter 9, Sections 4 & 5

PaleomagnetismPaleomagnetism

Paleomagnetism – Paleomagnetism – the study of changes the study of changes in Earth’s magnetic field, as shown by in Earth’s magnetic field, as shown by patterns of magnetism in rocks that have patterns of magnetism in rocks that have formed over timeformed over time

Some rocks contain iron-rich minerals that Some rocks contain iron-rich minerals that will align with Earth’s magnetic field at the will align with Earth’s magnetic field at the time of their formation, creating a time time of their formation, creating a time capsule of the magnetic field at that time capsule of the magnetic field at that time in timein time

When the rock moves, or the magnetic When the rock moves, or the magnetic pole changes position, it will still retain its pole changes position, it will still retain its original alignmentoriginal alignment

Apparent Polar WanderApparent Polar Wander

Paleomagnetism Cont.Paleomagnetism Cont. Normal Polarity –Normal Polarity – the same direction of the same direction of

magnetism as the present daymagnetism as the present day Reverse Polarity –Reverse Polarity – the reverse direction of the the reverse direction of the

magnetic field todaymagnetic field today The discovery of strips of rocks of alternating The discovery of strips of rocks of alternating

polarity, which lie as mirror images across the polarity, which lie as mirror images across the ocean ridges, provided strong evidence of ocean ridges, provided strong evidence of seafloor spreadingseafloor spreading

Ships towed instruments called magnetometers Ships towed instruments called magnetometers across the sea floor, revealing alternating high across the sea floor, revealing alternating high and low magnetism running in parallel bands to and low magnetism running in parallel bands to the oceanic ridgesthe oceanic ridges

As new basalt is added to the ocean floor, it gains As new basalt is added to the ocean floor, it gains the magnetism of the current magnetic field, and the magnetism of the current magnetic field, and when the field changes over time, the basalt when the field changes over time, the basalt records the changesrecords the changes

PaleomagnetismPaleomagnetism

Time Scale of Earth’s PolarityTime Scale of Earth’s Polarity

Magnetic Reversals in Seafloor Magnetic Reversals in Seafloor SpreadingSpreading

Earthquake PatternsEarthquake Patterns Scientists found a close link between Scientists found a close link between

deep-focus earthquakes and ocean deep-focus earthquakes and ocean trenches.trenches.

Also, the absence of deep-focus Also, the absence of deep-focus earthquakes along the oceanic ridge earthquakes along the oceanic ridge system was shown to be consistent with system was shown to be consistent with the new theorythe new theory

Scientists have found that intermediate Scientists have found that intermediate and deep focus earthquakes occur within and deep focus earthquakes occur within the subducting plate as it goes into the the subducting plate as it goes into the mantlemantle

Shallow-focus earthquakes are produced Shallow-focus earthquakes are produced as the descending slab reacts with the as the descending slab reacts with the lithosphere around itlithosphere around it

Earthquake DistributionEarthquake Distribution

Distribution of Earthquake Foci in Distribution of Earthquake Foci in Japan TrenchJapan Trench

Ocean DrillingOcean Drilling

Some of the best evidence has come from Some of the best evidence has come from deep-sea drilling into the sediments on the deep-sea drilling into the sediments on the ocean floorocean floor

The data on the ages of seafloor sediment The data on the ages of seafloor sediment confirmed what the seafloor spreading confirmed what the seafloor spreading hypothesis predictedhypothesis predicted

The youngest oceanic crust is at the ridge The youngest oceanic crust is at the ridge crest and the oldest oceanic crust is at the crest and the oldest oceanic crust is at the continental marginscontinental margins

No sediment older than 180 million years No sediment older than 180 million years has been found in the ocean basinshas been found in the ocean basins

Hot SpotsHot Spots Mapping revealed that there was a chain of Mapping revealed that there was a chain of

volcanic structures in the middle of the Pacific volcanic structures in the middle of the Pacific Ocean ranging from the Hawaiian Islands to Ocean ranging from the Hawaiian Islands to Midway Island and then north to the Aleutian Midway Island and then north to the Aleutian trenchtrench

Hot Spot – Hot Spot – a rising plume of mantle material a rising plume of mantle material that melts as it nears the surface, creating a that melts as it nears the surface, creating a volcanic areavolcanic area

As the Pacific plate moves over this spot, the As the Pacific plate moves over this spot, the island chain gets longerisland chain gets longer

The age of each volcanic island indicates when The age of each volcanic island indicates when that part of the Pacific plate was over the hot that part of the Pacific plate was over the hot spotspot

Hot spot evidence supports the idea that the Hot spot evidence supports the idea that the plates move over Earth’s surfaceplates move over Earth’s surface

Hot Spots Trace Plate MotionHot Spots Trace Plate Motion

Directions and Rates of PlatesDirections and Rates of Plates

Causes of Plate MotionCauses of Plate Motion

Scientists generally agree that Scientists generally agree that convection occurring in the mantle is convection occurring in the mantle is the basic driving force for plate the basic driving force for plate tectonicstectonics

Convective Flow –Convective Flow – the motion of the motion of matter resulting from convectionmatter resulting from convection

The slow movements of the plates The slow movements of the plates and mantle are driven by the and mantle are driven by the unequal distribution of Earth’s heat unequal distribution of Earth’s heat from the radioactive decay elementsfrom the radioactive decay elements

Convective FlowConvective Flow

Slab-Pull and Ridge-PushSlab-Pull and Ridge-Push

Slab-Pull – Slab-Pull – old oceanic crust, which is old oceanic crust, which is relatively cool and dense, sinks into the relatively cool and dense, sinks into the asthenosphere and “pulls” the trailing asthenosphere and “pulls” the trailing lithosphere alonglithosphere along

Slab-pull is thought to be the primary Slab-pull is thought to be the primary downward arm of convective flow in the downward arm of convective flow in the mantlemantle

Ridge-Push – Ridge-Push – causes oceanic lithosphere causes oceanic lithosphere to slide down the sides of the oceanic to slide down the sides of the oceanic ridge as a result of gravityridge as a result of gravity

Mantle ConvectionMantle Convection Mantle Plumes – Mantle Plumes – hot plumes of rock that are hot plumes of rock that are

attributed to the upward flowing arms of mantle attributed to the upward flowing arms of mantle convectionconvection

Mantle plumes sometimes show themselves on Mantle plumes sometimes show themselves on the surface as hot spots and volcanoesthe surface as hot spots and volcanoes

Whole-mantle convection is when slabs of cold Whole-mantle convection is when slabs of cold oceanic lithosphere descend into the lower oceanic lithosphere descend into the lower mantle, at the same time, hot mantle plumes mantle, at the same time, hot mantle plumes originating near the mantle-core boundary originating near the mantle-core boundary move heat toward the surfacemove heat toward the surface

Another model is called the deep-layer model, Another model is called the deep-layer model, and can be likened to a lava lamp, heat from and can be likened to a lava lamp, heat from Earth’s interior causes the layers to mix and Earth’s interior causes the layers to mix and combine in swirling patternscombine in swirling patterns

The unequal distribution of heat within Earth The unequal distribution of heat within Earth causes the thermal convection in the mantle causes the thermal convection in the mantle that ultimately drives plate motionthat ultimately drives plate motion

Whole-Mantle ConvectionWhole-Mantle Convection

Deep-Layer ModelDeep-Layer Model

AssignmentAssignment

Read Chapter 9, Sections 4 & 5 (pg. 265-Read Chapter 9, Sections 4 & 5 (pg. 265-270)270)

Do Chapter 9 Assessment #1-30 (pg. Do Chapter 9 Assessment #1-30 (pg. 275-276)275-276)

For Section 4: Do #’s 9, 13, 26, & 30For Section 4: Do #’s 9, 13, 26, & 30 For Section 5: Do # 24For Section 5: Do # 24