Plate Tectonics - Part 1 PDF

4 - Plate Tectonics 4.1 – Introduction 4.2 – Modern Plate Tectonics 4.3 – Divergent Plate Boundaries 4.4 – Convergent Plate Boundaries 4.5 – Transform Plate Boundaries 4.6 – Other Plate Features 4.7 – Paleomagnetism 4.8 – How Plates Move 4.1 – Introduction  Alfred Wegener (right)...

4 - Plate Tectonics 4.1 – Introduction 4.2 – Modern Plate Tectonics 4.3 – Divergent Plate Boundaries 4.4 – Convergent Plate Boundaries 4.5 – Transform Plate Boundaries 4.6 – Other Plate Features 4.7 – Paleomagnetism 4.8 – How Plates Move 4.1 – Introduction  Alfred Wegener (right) German meteorologist proposed (1915):  Harry Hess American professor proposed (1960):  By 1968 a model of plate tectonics with seafloor spreading and subduction had been developed. 4.1 – Introduction  Wegener proposed the concept of Continental Drift:  Wegener’s evidence included: 4.1 – Introduction  Continents fit together like puzzle pieces.  Wegener could arrange them all in a single land mass, “Pangaea.”  True edge of continent is continental shelf, not shoreline: “Bullard fit” Figure 2.2 4.1 – Introduction Wegener’s Evidence: Distribution of Climate Belts  Wegener aligned similar ancient climate belts between continents  Ancient climate belts make sense on a map of Pangaea Figure 2.4 4.1 – Introduction Wegener’s Evidence: Distribution of Fossils Fossils common to two or more continents  The continents now are separated  Fossils suggest they were connected in the past Figure 2.5 4.1 – Introduction Wegener’s Evidence: Matching Geologic Units Distinct rock types are found in widely separated regions  Appalachian Mountains match mountains in Great Britain, Scandinavia, and Africa  Rock types in South America match those in NW Africa Figure 2.6 4.1 – Introduction The Opposing View  Fixist view: No known force could cause the movement of the continents Wegener’s proposal of continents “plowing” through the ocean floor made no sense 4.1 – Introduction Mobilist View  A shift in understanding came decades later after WWII. New technology led to evidence of moving crust Recognition of seafloor spreading led to a new 4.1 – Introduction The Discovery of Seafloor Spreading— Bathymetry Evidence for moving plates: Bathymetric features of the seafloor Figure 2.7 4.1 – Introduction Distribution of Earthquakes  Evidence of moving plates: Earthquakes (ch. 8) are not randomly distributed Occur in distinct “seismic belts” Figure 2.8b 4.1 – Introduction Harry Hess and Seafloor Spreading  Stretching apart at a mid-ocean ridge forms new ocean crust  Descending old crust is destroyed at deep-sea trenches  This movement of rock causes seismic belts and the observed bathymetric features Figure 2.9 4.2 - Modern Plate Tectonics Theory  Earth’s outer layers composed of: Lithosphere: Asthenosphere: Lithosphere Plates  Earth is broken up into ~20 main plates Figure 2.11a Identifying Plate Boundaries Plate boundaries have active faults Active faults will generate earthquakes Figure 2.12a Identifying Plate Boundaries  Plates can have: Oceanic crust (mainly basalt—more dense) Oceanic (mainly basalts) and continental crust (mainly granite —less dense) Figure 2.12b Three Types of Plate Boundaries  Divergent:  Convergent:  Transform:  Final Summary: Plate Tectonics Theory Today  Rigid outer shell broken into plates  Plates lithosphere Figure 2.10a 4.3 - Divergent-Plate Boundaries  Bathymetry of South Atlantic  A mid-oceanic ridge appears as an elevated ridge halfway between South America and Africa Figure 2.14a Divergent-Plate Boundaries  Cross section of Atlantic ocean floor  The mid-ocean ridge rises 2–3 km higher than the abyssal plain Figure 2.14b Divergent Boundaries Processes  Magma rises from under the crust  Rising magma forces the crust apart  Results in “spreading” and formation of new ocean crust Figure 2.15b Divergent Boundary Processes  New oceanic crust forms at a ridge.  The crust is youngest and hottest near a ridge  Farther away from a ridge, the ocean crust is older  The ocean gets wider over time  Continents move apart as a result Figure 2.15a Lithosphere Changes  Lithosphere thickens away from the MOR  Cooler & denser away from MOR  “Sinks” into the asthenosphere  Abyssal plains are lower than ridges Figure 2.16 Sediment Variations  Rock near a ridge is new, so no sediments have accumulated  Moving away from a ridge, sediments are older and thicker Figure 2.17b 4.4 - Convergent-Plate Boundaries  Bathymetry of convergent zone typically has deep ocean trenches Figure 2.18a,b Convergent-Plate Boundaries—2  Subduction zones feature: Volcanic arcs (on ocean floor or on continents) Deep-sea trenches All ocean floor eventually undergoes subduction Figure 2.18c  Subduction Zones  Where two plates converge, one is forced down under the other.  The descending plate sinks into the mantle  Earthquakes occur down to a depth of ~660 km Figure 2.21 Convergent-Plate Boundary Variations  Collisions occur between either: Figure 2.20a,c  Unique features associated with subduction zones. Volcanic island arcs. Magmatic arcs Volcanic island arcs = where two oceanic plates converge Aleutian Islands, Japan and Philippines Magmatic arc = oceanic plate is subducted under continental plate Interior Mountain belt = when two continental plates collide Himalayan Mtns Convergent and Divergent Boundaries Close Together  Boundaries can occur close to each other  Crust is created and destroyed at relatively close boundaries  These are tectonically very active regions Figure 2.20d