The Two Types of Crust. Plate Tectonics. Physical Layers of the Earth. The Asthenosphere. The Lithosphere. Chemical Layers of the Earth

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1 Plate Tectonics A Reminder of Earth s Structure Two physical units of Earth that are important too! How the Earth Works Plate Tectonics Plate Boundaries Chemical Layers of the Earth Crust - low density rock Mantle - high density rock Core - metal (iron & nickel) The Two Types of Crust Divisions based on chemical composition Continental: granitic, low density, km thick. Oceanic: basaltic, slightly higher density, 5-12 km thick. Physical Layers of the Earth Outer layers divided based on how easily the rock flows. The Lithosphere Relatively rigid, nonflowable layer that lies between the surface and depth of 100 km. The Asthenosphere Relatively weak, flowable layer that lies below a depth of 100 km.

2 How The Earth Works: The Basic Premise of Plate Tectonics The Earth s lithosphere is divided into plates that move relative to one another and the underlying asthenosphere. Identifying Plate Boundaries The Major Plates As a plate moves, its interior area remains largely intact and rigid, but rock along the plate s boundaries undergoes deformation (cracking, sliding, bending, stretching, and squashing) as it grinds against its neighbor causing earthquakes, volcanism, and mountain building events. As plates move, so do the continents that are part of the plates, resulting in continental drift. Because of plate tectonics, the map of Earth s surface constantly changes over time. Most plates contain both continents and oceans. Types of Plate Boundaries Based simply on relative motion of the plates on either side of boundary. Divergent Plate Boundaries How do the two plates move? Seafloor Spreading Occurs at Divergent Boundaries New ocean crust is formed! 1. Divergent Plate Boundary 2. Convergent Plate Boundary 3. Transform Plate Boundary

3 Divergent Boundaries: Mid-Ocean Ridges Iceland Sub-aerial spatter cones along the Mid-Atlantic Ridge Rifting continents can form new ocean basins! Tension causes crust to stretch and crack apart People for scale Rift Valley Continued divergence leads to: 1) Widening of rift valley 2) Formation of mid-ocean ridge! A Rifting Continent The Great Rift Valley of Eastern Africa. Volcanism occurs in the rift valley. Convergent Plate Boundaries Convergence leads to destruction of ocean crust! How do the plates move?

4 Features of a Subduction Zone. Features of a Subduction Zone. Where is the nearest volcanic arc? Trench Volcanic Arc 1_23 Where is the nearest volcanic arc? Mt. Redoubt volcano Anchorage From Subduction to Collision. Transform Plate Boundaries How do the plates move? Bering Sea Aleutian Islands Ocean-to-ocean subduction (a) PACIFIC PLATE NORTH AMERICAN PLATE JUAN DE FUCA PLATE Mt. Saint Helens volcano Cascade Range GORDA PLATE Ocean-to-continent subduction (b) The death of an ocean basin and the formation of collisional mountains.

5 Fracture Zones on the Ocean Floor Offset the mid-ocean ridges Fracture Zones are Transform Boundaries A Transform Boundary on Land mid atlantic ridge.kmz san andreas sf bay.kmz mation/gps/ mid atlantic ridge.kmz san andreas sf bay.kmz A Transform Boundary What Drives Plate Motion? An oversimplified view: Heat transfer by convection of mantle ROCK creates a frictional drag on the plates. What Drives Plate Motion?

6 How Fast Do Plates Move? Summary The Earth has gaseous, liquid, and solid layers. The solid Earth can be divided into 3 layers based on chemical composition. 1 to 15 cm/yr!! The lithosphere and asthenosphere are layers based on their physical properties one flows, one doesn t. Lithospheric plates move around on the underlying asthenosphere and interact in 3 basic ways: divergence, convergence, and by translational motion (transform boundary where plates slide by each other). Most geology happens at or near plate boundaries where they interact.