PLATE TECTONICS The Earth’s Crust is in Motion
4 Main Layers of the Earth • Crust – Made up of two types: Oceanic and Continental. Contains mostly Silicon, Oxygen, and Aluminum • Mantle – Divided into upper and lower mantle; mostly Iron and magnesium • Outer Core – Liquid, mostly Iron and Nickel • Inner Core – Solid, Mostly Iron and Nickel
Oceanic Continental Thinner and more dense Made up mostly of mafic (basaltic) igneous rocks basaltand gabbro Thicker and lessdense Made up mostly of the felsic(granitic) igneous rocks granite and rhyolite Comparison of Earth’s Two Types of Crust
Continental Drift Proposed by Alfred Wegener Says that the continents have movedhorizontally to their present location Theories Prior to Plate Tectonics
Continents fit together Fossils Continents fit like a puzzle Wegener called this large landmass Pangaea Fossils of similar animals such as mesosaurusand plants like glossopteris were found on different continents. (See fig. 2, p.273) Evidence for Continental Drift
Climate Rock Type and Structure Warm-weather fossils found in the Arctic Evidence of glaciers found in present-day warm areas. Same rock types and structures on different continents (example: Eastern U.S. and Western Europe Evidence for Continental Drift (continued)
Why was Wegener’s Original Proposal of Continental Drift not Accepted?
Seafloor Spreading Proposed by Harry Hess . The theory states that hot, less dense material in the mantle is forced upward to the surface at the mid -oceanridges. Theories Prior to Plate Tectonics (continued)
Seafloor Spreading (continued) • As magma is forced upward at mid-ocean ridges, it flows in oppositedirections away from the ridge. • As it moves away from the ridge, the magma cools, solidifies, gets thinner and becomes ___more___dense. • Becoming more dense than the asthenospherebelow it, the oceanic crust sinks downward at convergent boundaries, forming trenches
Age of the Seafloor Oceanic crust is youngerthan continental crust Oceanic crust is 160-180million years old while continental crust is almost 4 billion years old. Rocks are younger at the mid-oceanridgesand get increasingly olderaway from the ridge on both sides Evidence for Seafloor Spreading
Magnetic Clues Ironminerals in basaltic rocks align with the Earth’s magneticfield. The magnetic field has reversedseveral times. The reversal creates an alternatingpattern of normal and reversed polarity that is thesameon both sides of the ridge (see fig.7, p.278). Evidence for Seafloor Spreading (continued)
Theory of Plate Tectonics • (Unifying theory that explains many complex Earth systems); • This theory helps explain continental _______, seafloor ________, AND provides a cause (mechanism) for these movements of the crust. • The cause was what was lacking in the previous theories.
Theory of Plate Tectonics (cont.) • The theory of plate tectonics states that the Earth’s continental and oceanic crust is broken up into sections. These sections are called tectonic or lithospheric plates, and they move over the surface the Earth. • The larger plates and their boundaries can be seen in figure 9 on page 281.
Lithosphere Asthenosphere Consists of the crustand rigid upper mantle Broken into large pieces called tectonicplates. Plastic-like part of the upper mantle. Is beneath the lithosphere Layers Related to Plate Tectonics
3 Types of Boundaries Divergent – plates move away from each other Convergent – plates move toward each other Transform – plates slide past each other horizontally Tectonic Plate Boundaries As the tectonic (lithospheric) plates move along and above the asthenosphere, the plates meet at three types of boundaries:
Divergent Boundaries • The tectonic plates are moving awayfrom each other • Characteristics of a divergent boundary are: - Shallow-focus Earthquakes - Volcanoessince molten material of the mantle is so close to the surface. • Examples of divergent boundaries are the Mid-Atlantic _______ and the East-African rift valley.
A plume of hot magma rises from deep within the mantle pushing up the crust and causing pressure forcing the continent to break and separate. Lava flows and earthquakes would be seen.
As the rift valley expands two continental plates have been constructed from the original one. The molten rock continues to push the crust apart creating new crust as it does.
Oceanic-to-Continental Oceanic-to-Oceanic Continental-to-Continental Convergent Boundaries • Plates move towardseach other • There are three types of convergent boundaries:
Oceanic-to-Continental Convergent Boundaries • The more dense __oceanic_____crust subducts under the less dense ____continental_______crust. • Characteristics include: - Trenchat subduction zone - Volcanoes - Shallow- and deep-focus earthquakes • An example is the __Andes____ mountains on the west coast of South America
Oceanic-to-Oceanic Convergent Boundaries • An older, cooler, and _____ dense oceanic plate subductsunder another oceanic plate • Characteristics include: - Volcanic islandarcs - Earthquakes and volcanoes - Ocean or sea on bothsides of the boundary • Examples of this type of boundary are Japan and the AleutianIslandsin Alaska