Plate Tectonics Chapter 17

 All of the phenomena that we will discuss in the upcoming weeks are all a result of plate tectonics.  Plate Tectonics is the idea that the Earth is broken up into a number of plates. These plates have been and are currently moving around. Some are pulling away, some are pushing together, and others are sliding by one another.  The majority of volcanic eruptions and earthquakes take place at these plate boundaries.

Alfred Wegener  Was the first to come up with a theory of continents moving  Called Continental Drift  He used data other than the puzzle-like fit of the continents to support his theory  He believed that a super- continent existed when all of the continents were together

Evidence Used  Similar Mountain Ranges found on different continents separated by sea

 Glacial Deposits

 Fossil evidence

Seafloor Spreading  Wegener believed only the continents were moving around the globe while plowing through the ocean floor  How could they move through something that was solid?  Technology allowed scientists to map the sea floor and they found that the sea was not completely flat  There were deep canyons, huge mountains, etc.  How did they form?

Seafloor Spreading  Harry Hess proposed that sea floor was being formed at ridges and by intruding magma  This pushes older ocean floor further and further apart  Trenches were the sites where ocean crust was destroyed

Evidence for Sea Floor Spreading  Age of the rocks  As you go away from the ridge the rocks get progressively older

Evidence for Seafloor Spreading  The pattern of normal and reverse magnetism  Mirror images on either side of the ridge

Seafloor Spreading Animation  Subduction Subduction

Plate Movement  The Earth is divided into many layers  Crust, Mantle, Core  However there are other layers  Lithosphere – solid, crust and upper mantle  Asthenosphere – partially molten and will flow, part of the mantle

Convection  Heat from the core of the earth rises, cools and sinks  This puts the asthenosphere in motion  The overlying plates are then put into motion as well

Convection Currents  Convection #2 Convection #2

Types of Plate Boundaries  Convergent – Plates moving towards one another  Divergent – Plates moving away from one another  Transform – Plates move side by side

Convergent Plate Boundaries  Can form mountain ranges/volcanoes  Types  Continent-Continent collisions  Ocean-Continent collisions  Ocean-Ocean collisions

Continent-Continent collisions  Rocks have the same densities on either plate so upon collision the rocks go up.  Example: Himalaya Mountain Range  Indo-Australian Plate and the Eurasian Plate  Example: Atlas/Alps  African and Eurasian  No subduction once continents collide  Lots of folded mountains

Continent-Ocean Collisions  The 2 plates of different densities so the plate with higher density goes underneath the plate with the lower density  This is called subduction  The oceanic plate goes deeper and deeper until it melts and eventually gets erupted from volcanoes  A trench is formed where the subduction occurs

Examples of continent-ocean collisions  Nazca plate – South American plate  Results in the Andes Mountains

 Juan de Fuca plate and the North American plate  Creates the Cascade Range

Ocean-Ocean collisions  Subduction still occurs because the older of the ocean plates will have a higher density  Creates volcanic islands  Example: Pacific Plate and the Eurasian Plate  Creates the Japanese Volcanic Island Arc  Example: Pacific /N. American plate  Example: Aleutian Islands

Divergent Plate Boundaries  Forms a rift valley where the plates are splitting apart  Examples:  Mid-Atlantic Ridge  East African Rift Valley

Transform Boundaries  Plates are sliding past  Can create mountains  Example: Pacific plate and the North American plate  The boundary between these two is known as the San Andreas Fault