Geology of the Hawaiian Islands Class 3 20 January 2004 Any Questions? IMPORTANT Big Island Field Trip We need a firm commitment PLUS $164 payment for airfare BEFORE January 29th Plate Tectonics Fundamental Concept and Unifying Theory in Earth Science Idea is > 100 yrs old Acceptance only within the past 30 yrs 1
Early geologists saw similarities between the coasts of Africa and South America. This map from 1858 shows that Africa and South America can be fit together very well. But, if we move the continents together, it is easy to explain the glacial evidence. If the northern continents are fit together, rock units match very well. 2
Again, a nice match if the continents are fit together During the Permian Period (225 Million Years Ago), there was one super continent called Pangea The Theory of Continental Drift proposed that Pangea began to break up in the Triassic (200 Million Years Ago) Gondwanaland (southern continent) began to break up in the Jurassic Period (135 Million Years Ago) 3
The south Atlantic opened in the Cretaceous (65 Million Years Ago) And the continents continued to drift to their Present Day positions Why was Continental Drift not accepted? Because it was difficult to under- stand how continents could move. During the last half of the 20 th century, we learned many things that helped us understand how continents could move The Earth has several layers. 4
Changes in P- and S-S wave Velocity Reveal Earth s Internal Layers The Lithosphere is very important in Plate Tectonics Note that crust under continents is thicker (45 km) than under oceans (8 km). Distribution of earthquakes Not random Focused in linear zones 5
Divergent (Spreading) Convergent (Subduction Zone) World Seismicity, 1963 2000 Transform Three Types of Plate Boundaries Types of plate boundaries Age of Seafloor Crust Divergent: mid-ocean ridges : collision zones volcanic arcs Convergent: collision zones Strike-slip slip: San Andreas Fault (California) Anatolian Fault (Turkey) 6
Age of Seafloor Crust Realizing that the ocean basins are very young was key to acceptance of Plate Tectonics Theory Earth s Magnetic Field Another key to understanding the puzzle. Earth s Magnetic Field The Earth behaves like a magnet whose poles are nearly coincident with the spin axis (i.e., the geographic poles). Earth s Magnetic Field Magnetic lines of force emanate from the magnetic poles. A suspended magnet is inclined due to the magnetic field. 7
A compass points North because the needle is made of iron. It becomes aligned in the Earth s magnetic field. Magma formed at spreading centers contains minute pieces of iron that become aligned in the direction of the Earth s magnetic field. When the magma cools, the iron pieces remain frozen in this alignment, pointing toward the Earth s magnetic North Pole. 8
Magnetic reversals A special property of the Earth s magnetic field is that it has reversed its direction many times in the past. When this happens, rocks formed at that time are magnetized pointing toward the South Pole. Magnetic reversals The last reversal was about 700,000 years ago. These reversals appear to be abrupt (probably only take about 1000 years or so). Magnetic reversals A period of time in which magnetism is dominantly of one polarity is called a magnetic epoch. We call north polarity normal and south polarity reversed. 9
Formation of Magnetic Anomalies Magnetic Anomalies in the Atlantic 10
The Geomagnetic Time Scale Based on determining the magnetic characteristics of rocks of known age (from both the oceans and the continents). We have a good record of geomagnetic reversals back to about 60 Million years ago. Formation of Magnetic Anomalies Rates of plate motion Age of Seafloor Crust Mostly obtained from magnetic anomalies on seafloor Fast spreading: 10 cm/year Slow spreading: 3 cm/year 11
The moving plates describe a process called Convection Relative Velocity and Direction of Plate Movement Water boiling in a pan on your stove is an example of convection Convection Hot water rises Cold water sinks Convection 12
Convection May Be The Driving Mechanism of Plate Tectonics Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere. Convection within the Earth The Lithosphere is created at spreading centers It is destroyed at Trenches (Subduction Zones) Upper Mantle Convection as a Possible Mechanism for Plate Tectonics What Plate Tectonics Theory explains Distribution of earthquakes and volcanoes Age distribution of oceanic crust (and why the oceans are MUCH younger than the continents) Magnetic information in rocks 13
Plate Tectonics Video Questions? Planet Earth, part 1 (38 minutes) Homework #1 Due Thursday Find it on the Web Site Homework #2 Due 27 January Don t forget to go to the web site and fill out the Student Information form! 14
Next Time Please read p. 141-142 142 in the text 15