Geology of the Hawaiian Islands Class 2 15 January 2004 Any Questions? Origin of solar system Pick a theory, any theory, but it must be consistent with these facts: Planets all revolve around the Sun in the same direction in nearly circular orbits. The angle between the axis of rotation and the plane of orbit is small (except Uranus). Origin of solar system Pick a theory, any theory, but it must be consistent with these facts: All planets (except Venus and Uranus) rotate in the same direction as their revolution; their moons do, too. Each planet is roughly twice as far as the next inner planet is from the Sun. Origin of solar system 99.9 % of mass is in the Sun; 99 % of angular momentum is in the planets. Planets in two groups: terrestrial (inner): : Mercury, Venus, Earth, Mars Mercury is mostly Fe (ρ( = 5.4) Jovian (outer): : Jupiter, Saturn, Uranus, Neptune. Jupiter mostly gas and ice ( ρ = 0.7) Pluto???? Origin of solar system Terrestrial planets are mostly O, Si,, Fe, Mg. The Sun is almost entirely H & He (also important in Jovian planets). Nebular hypothesis 1) Primeval nebula (slowly rotating cloud of He and H gases + dust) Nebular hypothesis 2) Gas cloud contracts under the force of gravity and flattens Nebular hypothesis 3) Sun forms and dust particles collide and clump together to form planetesimals 1
Nebular hypothesis 4) Terrestrial planets build up by accretion of planetesimals Our Solar System Timeline for the Sun, Earth, and Moon The Earth Differentiates Early Earth was a homogenous body The Earth Differentiates Irons sinks to the interior and lighter material floats upward The Earth Differentiates Now Earth is a zoned planet with a dense core and lighter crust Relative Abundance of Elements Interacting Earth Systems Volcanoes contribute gases to the atmosphere and solids to the crust Lithosphere Hydrosphere Atmosphere Biosphere 2
Plate Tectonics Fundamental Concept and Unifying Theory in Earth Science Idea is > 100 yrs old Acceptance only within the past 30 yrs Plate Tectonics How and why did it come about? During the 1800 s geologists recognized many strange things that seemed to imply that the continents had once been together. 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. There is evidence that huge glaciers occupied large parts of the southern continents about 250 million years ago. This distribution is very difficult to explain if the continents were in this position during the time of glaciation. 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. The same kinds of fossils appear on both sides of the Atlantic 3
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) 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. What did we learn to make us more willing to accept the idea that the continents have moved? During the mid-20 th Century, we began to learn about the Earth s Interior. The Earth has several layers. 4
How did we learn about the Earth s Interior? By studying Meteorites By studying earthquake waves (Seismology) Meteorites have struck the Earth in the past. Many meteorites are made of iron and nickel. We think this represents material of planets similar to Earth. Earth s interior (core) is probably also composed of iron and nickel. Seismology Study of the propagation of mechanical energy released by earthquakes. When energy is released in this fashion, waves of motion (like the effect of a pebble tossed into a pond) are set up in the Earth. Seismic Waves Radiate from the Focus of an Earthquake Seismograph Record and Pathway of Three Types of Seismic Waves Structure of the Earth Seismic velocity depends on the composition of material and pressure. We can use the behavior of seismic waves to tell us about the interior of the Earth. Changes in P- and S-S wave Velocity Reveal Earth s Internal Layers P- and S-S wave Velocities Increase with depth within each layer Structure of the Earth When waves move from one material to another they change speed and direction. 5
Refraction Reflection Refraction and Reflection of a Beam of Light Reflection Refraction Note: Earth Systems Today CD has a nice demo of this concept. Check out: Earth s Layers Reflection/refraction Seismic (earthquake) waves travel through the Earth. P-wave Shadow Zone S-wave Shadow Zone Check out Earth Systems Today CD for a discussion of Shadow Zones: Earth s Layers Core Studies Study of the behavior of seismic waves tells us about the shape and composition of the interior of Earth: Crust : ~10 70 km, intermediate composition Mantle: : ~2800 km, mafic composition Outer core: : ~2200 km, liquid iron Inner core: : ~1500 km, solid iron Lithosphere Asthenosphere The Lithosphere is very important in Plate Tectonics 6
Note that crust under continents is thicker (45 km) than under oceans (8 km). Distribution of earthquakes Not random Focused in linear zones World Seismicity, 1963 2000 Earthquake distribution defines plate boundaries See Earth Systems Today CD for an exercise on Earthquake distribution: Earthquakes + tsunamis Earthquakes in Space and Time Plates Rigid Lithosphere with definite boundaries Can have both oceanic and continental crust or just one kind. Earthquakes in mid-ocean areas Mid-Atlantic Ridge The Earth s Major Plates 7
Iceland is being pulled apart as it sits astride the Mid-Atlantic Ridge. Benioff- Wadati Zone Earthquakes in subduction zones Divergent (Spreading) Convergent (Subduction Zone) Gudmundur E. Sigvaldason, Nordic Volcanological Institute Transform Three Types of Plate Boundaries Types of plate boundaries Age of Seafloor Crust Divergent: mid-ocean ridges Convergent: : collision zones volcanic arcs Strike-slip slip: San Andreas Fault (California) Anatolian Fault (Turkey) Mosaic of Earth s Plates Age of Seafloor Crust Realizing that the ocean basins are very young was key to acceptance of Plate Tectonics Theory The moving plates describe a process called Convection Water boiling in a pan on your stove is an example of convection Convection 8
Hot water rises Cold water sinks Convection Convection within the Earth The Lithosphere is created at spreading centers It is destroyed at Trenches (Subduction Zones) Questions? Homework #1 Due January 22nd Find it on the Web Site Next Time Short Film Application of Plate Tectonics to the origin of the Hawaiian Islands Please finish reading Chapters 1 and 3 in the text 9