Download Plate Tectonics

Key Content Summary ­ Tectonics SC.7.E.6.5 (These summaries are works in progress and are subject to change at anytime. They are intended to help provide core information and basic understanding allowing students to spend more time investigating areas of interest at greater depth.) Earth’s Internal Structure: The Earth is made up of several layers: ● a thin hard rocky outer crust​
, ● a large (about 80% of Earth’s volume) ​
molten mantle​
, ● a ​
liquid outer core​
of iron and nickel and ● a ​
solid inner core ​
of iron and nickel. The layers of the Earth are arranged by density. Density is a measure of how tightly packed or compressed a material is (density equals mass divided by volume). Early in the Earth’s formation, when it was still molten, the more dense materials sank to the bottom (middle of the Earth) and the less dense materials rose. The least dense, gases, formed an atmosphere. The most dense materials, iron and nickel, sank towards the core. The crust is a continuous layer of rock covering all the Earth. Oceans, soil and plants all rest on top of the crust. There are 2 types of crust: continental and oceanic crust. ● Continental crust is primarily ​
granite​
and is thicker (thickest at mountains) and less dense than oceanic crust. ● Oceanic crust is primarily ​
basalt ​
and is thinner and more dense than continental crust. A layer referred to as the lithosphere is comprised of the Earth’s crust and the rigid upper portion of the mantle. The lithosphere is broken into pieces called plates. The ​
Earth gets hotter​
as you get closer to the center. Some of the heat is left over from when the Earth was first formed, while additional heat comes from radioactive decay and some from the friction of convecting materials. It is the heat from inside the Earth that drives the movement of plates, causes earthquakes, builds mountains and results in erupting volcanoes. The mantle is comprised of thick melted rock called magma. This magma flows very slowly due to changes in density caused by uneven heating from the core (some places are hotter than others). This movement is called ​
convection​
(think of the lava lamp) and causes the movement of tectonic plates. The Earth’s outer core is liquid, melted iron and nickel (metals), which flows due to convection and the rotation of the Earth. This flowing iron and nickel generates a magnetic field around the Earth. This is a complicated process and we have discovered Earth’s ​
magnetic field sometimes reverses​
so at times in the past the magnetic pole was not north but instead south. The inner core, while still made out of iron and nickel, surprisingly is not liquid, even though the inner core is hotter. While it would make sense for this layer to be liquid because temperatures exceed its melting point, the pressure from all the layers above forces the molecules to remain so close together it’s a solid. The Theory of Plate Tectonics: Theories about Earth’s surface have changed over time. In the early 1900’s Alfred Wegener proposed the ​
theory of “Continental Drift” ​
( all continents had once joined to formed in a single landmass called Pangea and since drifted to their current positions). This was based on observations of similarities in the outlines of continents especially South America and Africa, along with fossil and climate evidence, which seemed to suggest the continents were once connected. This was a radical idea as it was long thought that continents and oceans formed as earth cooled and always remain in the position they occupy today. One of the key problems with Wegener’s theory was that it did not provide a mechanism for how the continents moved which prevented it from being widely accepted. It wasn’t until the mid to late 1900’s when advances in technology allowed for new discoveries of seafloor spreading, paleomagnetism (or magnetic striping reversals), seismic recordings showing the outlines plates, and the detection of convection currents within the mantle, that the ​
theory of Plate Tectonics​
was fully developed. Now we know the lithosphere is divided into segments called plates that move and continually change shape and size. Their movement is caused by the unequal distribution of heat within the Earth which sets up convection currents in the mantle’s slowly moving material. In some places magma is being pushed up between plates forming new crust, while in other places crust is being destroyed as it is pulled back down into the mantle. Interaction at Plate Boundaries: There are 3 main types of plate boundary interactions: convergent, divergent and transform fault. The interaction of plates often result in earthquakes and the formation of specific types of surface features such as rift valleys, mountains and volcanoes. Convergent plate boundaries​
are where adjoining plates are moving towards each other and oceanic crust is destroyed. If one plate is comprised of continental crust and the other oceanic crust, then the oceanic crust, which is denser, will subduct (go beneath). Noted features will be a deep ocean trench caused by the subduction and continental volcanoes. If they are both oceanic crust then one will subduct and volcanic islands and a deep ocean trench will form. If both are continental crust then neither will subduct and mountains such as the Himalayas will form. Divergent plates boundaries ​
are where adjoining plates are moving away from each other. In this situation new ocean crust is formed. Noted features will be rift valleys, ocean ridges (underwater “mountains”) and volcanoes. Divergent boundaries happen mostly along mid­ocean ridges and result in seafloor spreading. The newest crust is formed along the ridge and gets older as you move away on each side of the ridge. Transform fault boundaries​
are where adjoining plates grind past each other; crust is neither formed nor destroyed. An example is the San Andreas fault. Earthquakes: Earthquakes​
frequently occur along plate boundaries and much of what we know about the inside of the Earth was learned from monitoring earthquake waves. As the plates move strain is built up until an earthquake occurs rapidly relieving the stress (this is called elastic rebound). Earthquakes result in the formation of various ​
types of waves​
which travel at different speeds in different layers and are reflected and refracted by the boundaries between the layers. ● P or primary waves​
are compression waves and travel the fastest; ● S or secondary waves ​
are transverse waves and travel slower, these waves do not travel through the liquid outer core of the earth; ● surface waves​
which travel along the surface and do the most damage. Earthquakes occur at faults, a fracture or crack in the Earth’s crust usually along plate boundaries. Major earthquakes are often preceded and followed by smaller earthquakes called foreshocks and aftershocks respectively. A seismograph measures the amount of movement of the earth resulting from an earthquake. The location of the epicenter (the spot on Earth’s surface directly above the focus of an earthquake) can be determined using measurements from 3 seismographs and the time between the arrival of p and s waves. Volcanoes: Volcanoes form most often along convergent and divergent plate boundaries or over hotspots. A high concentration of volcanoes surround the Pacific Ocean in what is frequently referred to as the “Ring of Fire”​
. Hotspots are areas where hot plumes of magma rise up through the crust often in the middle of plates and form volcanoes. The Hawaiian Islands and Yellowstone are examples. As the magma reaches the surface, it loses some of its gases and turns into lava. Volcanoes are created by the release and build­up of lava and other materials. There are 3 major types of volcanoes​
: Composite volcanoes (stratovolcanoes)​
develop from repeated explosive and nonexplosive eruptions of tephra (airborne lava fragments that can range in size from tiny particles of ash to house­size boulders) and lava that build up layer by layer. These volcanoes are the largest and form symmetrical cones with steep sides, such as Mount Hood and Mount St. Helens in the Cascade Range along the northwestern coast of the United States. Shield volcanoes​
form from "gentle" or nonexplosive eruptions of flowing lava. The lava spreads out and builds up volcanoes with broad, gently sloping sides. The low­profile shape resembles a warrior's shield. Currently active volcanoes of this type are found in the Hawaiian islands. Cinder cones​
build from lava that is blown violently into the air and breaks into fragments. As the lava pieces fall back to the ground, they cool and harden into cinders (lava fragments about ½ inch in diameter) that pile up around the volcano's vent. Cinder cones are the smallest volcanoes and are cone­shaped.