Theory of Plate Tectonics Chapter 10.3 pg SPI

Learning Objectives Compare and contrast different kinds of plate boundaries. Explain how heat inside Earth causes plate tectonics. Recognize features caused by plate tectonics.

Plate Tectonics Seafloor spreading showed that more than just the continents were moving as Wegener thought Scientists now believe that sections of the seafloor and the continents move in relation to one another Wegener’s theory eventually lead to the discovery of continents moving as well New theory developed in 1960s that combined continental drift & seafloor spreading Plate tectonics: Earth’s crust and part of the upper mantle are broken into plates that move on the mantle

Composition of Earth’s Plates Plates made of crust and part of the upper mantle (lithosphere) Plates float on the (asthenosphere)

Plate Boundaries When tectonic plates move, they interact in several ways: – Move toward each other & converge/collide – Pull Apart – Slide against each other Results of this movement is seen at plate boundaries 1.Convergent boundary 2.Divergent boundary 3.Transform boundary Movement at one plate boundary means that changes must happen at other boundaries, as well.

Moving Apart Divergent boundary: boundary between two plates that are moving apart Seafloor spreading is an example In the Atlantic Ocean the North American plate is moving away from the Eurasian & African plates (Mid-Atlantic Ridge) Great Rift Valley in Africa may become divergent boundary – Valley has formed where plates are pulling apart

Divergent Boundary

Plates Moving Together As new crust is added in one place, it disappears below the surface at another Can occur when seafloor cools, becomes denser, and sinks (occurs at convergent boundaries) When an oceanic plate converges with a less dense continental plate, the denser oceanic plate sinks under the continental plate Area where the oceanic plate sinks into the mantle is the subduction zone

Plates Moving Together Some volcanoes form above subduction zones Deep-sea trench forms where one plate bends & sinks below the other High temps cause rock to melt around slab sinking beneath as it passes underneath the other plate Newly formed magma forced upward along these boundaries & forms volcanoes – Andes Mountain range contains many volcanoes that were formed at the convergent boundary of the Nazca & South American plates

Convergent Boundary

Where Oceanic Plates Collide Subduction zones also form where two oceanic plates converge Colder, older, denser oceanic plate bends and sinks down to mantle Example: Mariana Islands in Western Pacific are a chain of volcanic islands formed where two oceanic plates collide

Where Continental Plates Collide Usually no subduction occurs when two continental plates collide Plates are less dense than material in asthenosphere so material crumples up forming mountain ranges Earthquakes are common at these convergent boundaries Volcanoes are not common because there is little to no subduction – Himalaya Mountains forming as Indo-Australian Plate and Eurasian plates are colliding

Plates Sliding Past Each Other 3 rd type of boundary is called a transform boundary Forms when plates slide past each other Plates can move in opposite directions or in the same direction at different rates One plate slips past another suddenly = earthquake San Andreas Fault : forms where the Pacific Plate is sliding past the North American Plate

Transform Boundary

Causes of Plate Tectonics Convection currents: cycle of heating, rising, cooling, and sinking Version of this process occurs in mantle and is thought to cause tectonic movement It is thought that differences in density cause hot, plastic-like rock to be forced upward toward the surface

Convection Currents

Moving Mantle Material Wegener never came up with an explanation for why plates move There have been several possible explanations for the movement of the plates Each one uses convection as an explanation for tectonic movement The transfer of heat inside Earth that provides the energy – Figure 12 pg 313

Features Caused by Plate Tectonics Interactions of plates produce forces that build mountains, create ocean basins, cause volcanoes, cause earthquakes, etc… Forces that pull Earth apart (tension forces) stretch Earth’s crust Blocks of crust break off & fall or tilt down broken surfaces of the crust When these rocks break & move along surfaces, fault lines form

Fault Lines and Rift Valleys Faults interrupt rock layers by moving them out of place Entire mountain ranges can form in the process (called fault block mountains) Faults formed on pull-apart forces are normal faults Rock layers above the fault move down when compared with rock layers below the fault Rift valleys & mid-ocean ridges can form where Earth’s crust separates

Mountains & Volcanoes Compression forces squeeze objects together and create several effects Continental plates collide and cause massive folding and faulting of rock layers into mountain ranges (Himalaya & Appalachian Mountains) Produces reverse faulting: rock layers above the surface move up relative to rock layers below the fault

Oceanic Plates Converge Curved chains of volcanic islands called island arcs form above a sinking oceanic plate If an oceanic plate converges with a continental plate, the oceanic plate slides UNDER the continental plate. Folding & faulting at continental plate can thicken the crust to produce mountain ranges Volcanoes are typically formed at this type of convergent boundary

Island Arcs

Strike-Slip Faults At transform boundaries two plates slide past one another without converging or diverging.transform boundaries Plates stick then slide, mostly in a horizontal direction Rocks on opposite sides of the fault move in opposite directions, or in the same direction just at different rates San Andreas Fault in California Vibrations generated inside Earth are felt as earthquakes on the surface

Evidence for Plate Tectonics Earthquakes, volcanoes, and mountain ranges are evidence of plate motion Plate tectonics explains how activity within Earth can affect Earth’s crust differently in different locations Until recently, the only tests scientists could use to check for plate movement were indirect – Study magnetic characteristics on seafloor – Study volcanoes & earthquakes – These ideas provide EVIDENCE but not PROOF

Testing for Plate Tectonics Methods had to be discovered to provide concrete evidence and measure the small movements on Earth’s plates One method uses lasers & satellites Scientists can measure exact movements as little as 1cm per year

SLRS Examples Satellite Laser Ranging System data show that Hawaii is moving toward Japan at a rate of 8.3cm/yr Maryland is moving away from England at a rate of 1.7 cm/yr Using these methods, its been shown that continental plates move at a rate ranging from 1cm-12cm per year

Review Questions Describe what occurs at plate boundaries that are associated with seafloor spreading. What are the 3 types of boundaries where volcanic eruptions can occur? Explain how convection currents are related to plate tectonics.