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Home , Environmental geology, Geology, Plate tectonics, Slab suction, Tectonics

Environmental Chapter 2 - Plate and ’s Internal Structure

• Earth’s internal structure - Earth’s layers are defined in two ways.

1. Layers defined by composition and o -Less dense rocks, similar to o -More dense rocks, similar to peridotite o Core-Very dense-mostly iron & nickel 2. Layers defined by physical properties (solid or liquid / weak or strong) o – (solid crust & upper rigid mantle) o – “gooey”&hot - o Mesosphere-solid & hotter-flows slowly over millions of o Outer Core – a hot liquid-circulating o Inner Core – a solid-hottest of all-under great pressure • There are 2 types of crust ü Continental – typically thicker and less dense (about 2.8 g/cm3) ü Oceanic – typically thinner and denser (about 2.9 g/cm3)

The Moho is a discontinuity that separates lighter crustal rocks from denser mantle below

• How do we know the Earth is layered? That knowledge comes primarily through the study of : Study of and seismic waves. We look at the paths and speeds of seismic waves. Earth’s interior boundaries are defined by sudden changes in the speed of seismic waves. And, certain types of waves will not go through liquids (e.g. outer core).

• The face of Earth - What we see (Observations) Earth’s surface consists of and , including belts and “stable” interiors of continents. Beneath the , there are continental shelfs & slopes, deep basins, , deep and high mountain . We also know that Earth is dynamic and earthquakes and volcanoes are concentrated in certain zones.

• How can we explain what we see? -An idea before its . Alfred first proposed his continental drift hypothesis in 1915. He named a called (Pan-jee- ah) that began breaking apart about 200 million years ago & the continents then drifted into their current positions. Evidence used in support of continental drift hypothesis • Fit of the continents – like puzzle pieces • evidence – match up across separate continents • type and structural similarities – (e.g. Appalachian and Caledonian Mtns • Paleoclimatic evidence

• There were objections to the continental drift hypothesis. There was a Lack of a mechanism for moving continents. Wegener incorrectly suggested that continents broke through the ocean crust, much like ice breakers through ice. Wegener’s hypothesis was correct in principle, but contained incorrect details and there was strong opposition to the hypothesis from all areas of the scientific community – although some thought his ideas were plausible.

During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor and the hypothesis was proposed by Harry Hess in the early 1960s. He hypothesized that the mid- oceanic ridges were spreading centers where new crust is formed.

• Evidence for Seafloor spreading • Age of seafloor rocks: Progressively younger toward the mid-oceanic – evidence from drilling ocean floor • Thickness of seafloor : Progressively thinner toward the ridge – evidence from drilling ocean floor • Paleomagnetic reversals – the most convincing evidence • Earth has a dipolar magnetic • Earth’s magnetic polarity reverses periodically • Geomagnetic reversals are recorded in the iron bearing rocks igneous rocks of the ocean crust as symmetrical magnetic anomalies (stripes). • In 1963 Vine and Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading

• - A unified : Study the dynamic creation, movement, and destruction processes of plates. There are 7 major plates associated with Earth's strong, rigid outer layer (the lithosphere- uppermost mantle and overlying crust) The lithosphere overlies a weaker in the mantle called the asthenosphere. The plates move relative to each other at a very slow but continuous rate and the rates are different on each plate - Average 5 centimeters (2 inches) per (approx. growth rate of fingernails). The rates and direction of movement changes over time.

• Dynamic actions are concentrated along plate boundaries: Defined by the areas of concentrated seismic and volcanic activities, , faults, and mountain ridges.

• There are 3 major types of plate boundaries • Divergent: plates moving apart and new lithosphere is produced at mid-oceanic ridge. Divergent boundaries are also associated with continental rifting as two split apart due to extensional forces acting on the plates (e.g. East African ).

• Convergent: plates collide, (Cooler, denser slabs of oceanic lithosphere descend into the mantle) and mountain building. Old crust is “crumpled” or recycled back into the mantle. There are 3 types of convergent boundaries. • Oceanic-Oceanic: Formed when two oceanic slabs converge and one descends beneath the other. Volcanoes are often formed on the ocean floor. If the volcanoes emerge as , a volcanic arc is formed (, , Tonga islands) • Oceanic-Continental: Formed when ocean crust is subducted beneath . Melting adds material to the continental crust and volcanic arcs are formed on the (e.g. Cascades of the Pacific Northwest U.S.) • Continental-continental: Continued subduction can bring two continents together and the less dense, buoyant continental lithosphere does not subduct. The resulting collision between two continental blocks produces (Examples: , , Appalachians)

• Transform: two plates slide one another and no new lithosphere is created or destroyed. Most join two segments of a mid-ocean ridge along breaks in the known as zones. A few (the San Andreas and the of ) cut through continental crust.

• Hot Spots are caused by rising plumes of mantle material (). The source may be near the core-mantle boundary. Volcanoes can form over them (Hawaiian Island chain). They are generally stationary and can be located beneath continents (Yellowstone) and oceans (). The age of volcanoes/islands (e.g. Hawaii) can be used to determine the direction and rate of plate movement. The Hawaii-Emperor formed over a hot spot. The bend of this seamount chain represents a change in direction of plate .

• in rocks on the ocean floor provides a method for determining plate , and bBoth the direction and rate of seafloor spreading can be established • Plate motions can also be measured from by establishing exact locations on opposite sides of a plate boundary and measuring relative motions.

• What drives plate motions? Researchers agree that convective flow in the mantle is the basic driving force of plate tectonics. Forces that drive plate motions include -pull – probably the most significant force, Ridge-push and .

• The – shows the cyclic of Plate Tectonics

Tectonics builds mountains. What happens to mountains when they erode? and isostacy are interrelated. Because of isostacy, if ice is floating in water and some is removed above the waterline the ice rises so that 10% of the ice is always above the waterline. The erosion of mountains from continents result in a similar but slower “bobbing up” of the continents to “isostatic equilibrium”& with more material bobbing up, they take a long time to erode down.

• Plate Tectonics & –The Significance of tectonic cycle – Global zones of resources (oil, gas, and )-processes and Earth’s structures help create and localize resources in certain areas) – Global belts of earthquakes and volcanic activities & safety issues (e.g. U.S. East -quiet vs. West Coast-active) – Geologic knowledge on plate tectonics: for urban development and hazard mitigation – Impacts on the and global (e.g. mountainous vs. low/flat and change over time due to drift)

WSE 8/2012