Ch – 15 Plate Tectonics

Fig. 6.10, p.139

Plate tectonics map showing Somali Plate

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

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?

What happens at a convergent plate boundary Subduction Oceanic-continental convergence Denser oceanic slab sinks into the asthenosphere Pockets of magma develop and rise Continental volcanic arcs form (e.g. Andes, Cascades)

Figure 15.14a

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

Fig. 6.10, p.139

Ring of Fire

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 Symmetry of magnetic polarity across mid-ocean ridges Distribution and depths of earthquakes and volcanoes Relatively young age of the oceanic crust (less than 180 million years) Lack of deep-ocean sediment

Paleomagnetism: study of ancient magnetic fields The inner and outer core of the Earth cause the earth to act like a magnet, with north and south poles Iron minerals orient themselves towards the north pole as lava solidifies on the earth’s surface and become fixed in that direction. Ancient lavas tell us the strength and direction of the earth’s magnetic field during geologic history.

Magnetic polarity reversals At irregular time intervals, the “magnet turns around”. Lava that solidified during these reversals allows us to determine the date of these reversals. For example, volcanic rocks dated to 760,000 years ago in several locations, including California, show evidence of reversed magnetic polarity.

Magnetic polarity reversals on the ocean floor Development of ocean- going magnetometers allowed remote mapping of the magnetic field of the ocean crust. Symmetrical pattern at mid-ocean ridges could best be explained by the sea-floor spreading hypothesis, which was still being debated at the time.

Paleomagnetic reversals recorded by basalt flows at mid-ocean ridges

Age of the Sea Floor  Evidence from ocean drilling Age of deepest sediments indicates ocean crust much younger than continental crust, which supports both subduction and sea- floor spreading hypotheses. Lack of sediments at at mid ocean ridges supports seafloor spreading. Age distribution of ocean crust indicates location of divergent and convergent boundaries

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

Earthquake depths and distributions Shallow earthquake (red) occur along the oceanic ridge systems. Deep earthquakes (green-blue) occur on the “Ring of Fire”. Earthquake depth increases in direction of subducting plate.

What drives plate motion?

Convective flow in the mantle is the underlying driving force for plate movement. Mantle convection and plate tectonics are part of the same system. Warm, buoyant rock rises and “cold” dense rock, e.g. subducting plates, sink. Plate tectonic movements are ultimately due to unequal heat distribution in the earth’s interior.

Slab-pull, ridge-push, slab suction Descending “cold”oceanic crust pulls the plate in direction of subduction. This is probably the main mechanism of plate motion Elevated ridge system pushes the plate away from ridge Descending plate causes suction which pulls the two sides closer.

Plate-mantle convection: Descending plates, along with rising mantle plumes cause convection within the mantle