Warm Up 11/5 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. Which of the following is a geographic example of a transform fault boundary? a. the Andes Mountains c. the San Andreas Fault b. the East African Rift valley d. the Mid-Atlantic Ridge The Red Sea is theorized to be the site of a recently formed ____. a. convergent boundary c. divergent boundary b. hot spot d. transform fault boundary Answers: 1) c. 2) c. 3) c.

Testing Plate Tectonics & Mechanisms of Plate Motion Chapter 9, Sections 4 & 5

Paleomagnetism Paleomagnetism – the study of changes in Earth’s magnetic field, as shown by patterns of magnetism in rocks that have formed over time Some rocks contain iron-rich minerals that will align with Earth’s magnetic field at the time of their formation, creating a time capsule of the magnetic field at that time in time When the rock moves, or the magnetic pole changes position, it will still retain its original alignment

Apparent Polar Wander

Paleomagnetism Cont. Normal Polarity – the same direction of magnetism as the present day Reverse Polarity – the reverse direction of the magnetic field today The discovery of strips of rocks of alternating polarity, which lie as mirror images across the ocean ridges, provided strong evidence of seafloor spreading Ships towed instruments called magnetometers across the sea floor, revealing alternating high and low magnetism running in parallel bands to the oceanic ridges As new basalt is added to the ocean floor, it gains the magnetism of the current magnetic field, and when the field changes over time, the basalt records the changes

Paleomagnetism

Time Scale of Earth’s Polarity

Magnetic Reversals in Seafloor Spreading

Earthquake Patterns Scientists found a close link between deep-focus earthquakes and ocean trenches. Also, the absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory Scientists have found that intermediate and deep focus earthquakes occur within the subducting plate as it goes into the mantle Shallow-focus earthquakes are produced as the descending slab reacts with the lithosphere around it

Earthquake Distribution

Distribution of Earthquake Foci in Japan Trench

Ocean Drilling Some of the best evidence has come from deep-sea drilling into the sediments on the ocean floor The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted The youngest oceanic crust is at the ridge crest and the oldest oceanic crust is at the continental margins No sediment older than 180 million years has been found in the ocean basins

Hot Spots Mapping revealed that there was a chain of volcanic structures in the middle of the Pacific Ocean ranging from the Hawaiian Islands to Midway Island and then north to the Aleutian trench Hot Spot – a rising plume of mantle material that melts as it nears the surface, creating a volcanic area As the Pacific plate moves over this spot, the island chain gets longer The age of each volcanic island indicates when that part of the Pacific plate was over the hot spot Hot spot evidence supports the idea that the plates move over Earth’s surface

Hot Spots Trace Plate Motion

Directions and Rates of Plates

Causes of Plate Motion Scientists generally agree that convection occurring in the mantle is the basic driving force for plate tectonics Convective Flow – the motion of matter resulting from convection The slow movements of the plates and mantle are driven by the unequal distribution of Earth’s heat from the radioactive decay elements

Convective Flow

Slab-Pull and Ridge-Push Slab-Pull – old oceanic crust, which is relatively cool and dense, sinks into the asthenosphere and “pulls” the trailing lithosphere along Slab-pull is thought to be the primary downward arm of convective flow in the mantle Ridge-Push – causes oceanic lithosphere to slide down the sides of the oceanic ridge as a result of gravity

Mantle Convection Mantle Plumes – hot plumes of rock that are attributed to the upward flowing arms of mantle convection Mantle plumes sometimes show themselves on the surface as hot spots and volcanoes Whole-mantle convection is when slabs of cold oceanic lithosphere descend into the lower mantle, at the same time, hot mantle plumes originating near the mantle-core boundary move heat toward the surface Another model is called the deep-layer model, and can be likened to a lava lamp, heat from Earth’s interior causes the layers to mix and combine in swirling patterns The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion

Whole-Mantle Convection

Deep-Layer Model

Assignment Read Chapter 9, Sections 4 & 5 (pg. 265-270) Do Chapter 9 Assessment #1-30 (pg. 275-276) For Section 4: Do #’s 9, 13, 26, & 30 For Section 5: Do # 24