Plate Tectonics 2 The Workings of a Natural Machine

Our Earth is made up of a lithosphere… The lithosphere makes up the uppermost mantle and the crust. Both the upper mantle and crust are rigid. There are two different types of crust—ocean crust and continent crust. Ocean crust is thin and dense--basalt; continent crust is thick and lighter--granite. The lithosphere overrides the asthenosphere, which is soft and slush- like. It acts like a lubricant for the rigid lithosphere to ride on top of.

The earth’s lithosphere is fragile and has been known to break apart… Millions of years ago, the continents were all in one mass called Pangaea. However, Pangaea did not stay that way. At one point, Pangaea started to break up. It broke into two new continents. Scientists called these continents Laurasia and Gondwanaland.

The lithosphere continued to divide up into separate plates and move about on top of the aestenosphere. The plates are still in motion today. And their movements are what causes many dynamic phenomena to occur on the earth. Space technologies have helped scientists to study plate motions. Satellite Lasar Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Global Positioning System (GPS) allow scientists to understand plate movements.

Most dynamic events occur along the boundaries of plates… There are 7 large plates and many small ones. Most of the world’s volcanoes and earthquakes are known to occur mainly along each of the boundaries of the plates.

Earthquake and Volcano Locations Earthquakes are plotted on a world map. The locations coincide with plate boundaries. Volcanic activity is also prominent along plate boundaries. A belt of volcanoes and earthquakes along the Pacific Plate has been given the title, “Ring of Fire”.

There are three types of boundaries… Divergent Boundaries Convergent Boundaries Transform Boundaries

Divergent Boundaries --“Sea Floor Spreading”-- Features: 1) Forms new crustForms new crust --Ridge Push 2) Mid-Ocean Ridges 3) Rift Valleys 4) Earthquakes 5) Volcanic activity Examples: 1) Mid-Atlantic Ridge 2) Great African Rift 3) East Pacific Rise

Convergent Boundaries --Subduction Zones-- Denser plate “subducts” beneathe the lighter plate --Slab pull Features: 1) Destroys crust 2) Mountain building 3) Deep sea Trenches 4) Volcano arcs -Island formation 5) Earthquakes Web

Three Types of Convergent Boundaries 1) Ocean-ContinentOcean-Continent -ocean and continental crust collide. -the denser ocean crust subducts -creates volcanic mountain chain on continent -Example: Washington State (west coast) 2) Ocean-Ocean -two ocean plates collide. -The denser of the 2 ocean crusts subducts. -creates volcanic island arc -Example: Aleutian Islands Japan / Indonesia 3) Continental-Continental -Two continents collide. -The denser continent subducts. -Creates mountains -Example: India / Himilayans

Cascade Mountain Range on West Coast Andes Mountain Range in Western South America

Aleutian Island Arc Japanese Islands

Himalayan Mountains

Transform Boundary --Plate Sliding-- Feature: 1) earthquake activity Examples: 1) San Andreas Fault -California Web 2) Fractures along divergent boundaries Web

Mechanism for Plate Movement --Mantle Convection-- Hotter, less dense magma comes up at the mid-ocean ridge. As the current moves away from the ridge, it drags the lithospheric plate with it. The cooler magma sinks back down into the mantle. **Scientists are still not convinced this is what is moving the plates.

Continental Growth --The Core-- Continental Growth --The Core-- Craton—an old, stable part of a continent. It has survived the merging and splitting of the continents.

Continental Growth --Additional Material-- 1) Deep Sea Sediments --gathered up at Subduction Zones 2) Igneous Rock 3) River Sediments 4) Terranes—smaller amounts of continental crust or small islands that crash into a large continent and attaches to it. Web

Test Your Knowledge 1)Millions of years ago, the continents were in one mass called a)Laurasia b) Gondwanaland c) Pangea d) Wegener 2)What geological events occur along plate boundaries a)Volcanoes b) Earthquakes c) Plate Spreading d) both a & b 3)A belt of volcanoes and earthquakes along the continent edges in the Pacific Ocean is called a)A divergent boundary b) the Ring of Fire c) the Aleutian Islands c) the San Andreas Faultline 4)The boundary where ocean crust and continental crust collide is called a)a convergent boundary b) a divergent boundary c) a transform boundary d) a craton 5)The mechanism that is hypothesized to be driving the movement of the plates is a) Plate subduction b) Mantle Convection c) Sea-floor spreading d) Continental drifting

End of Chapter 8