Geology 15 Lecture 7 Schedule: Hazard Update: Review Lecture 6 Activity 2 cont: Plate Boundaries and Their Motions Cover Material/Objectives Plate Tectonics (continued) Earth s Structure Evidence for Continental Drift Earth s Magnetic Field Differential Rotation between the inner core and the outer core. Matching sequences of rocks and mountain chains Similar rocks on different continents
Why do these anomalies look like this? They are parallel to the mid ocean ridge. They alternate in polarity from the ridge symmetrically. Several lines of evidence: (1) at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest; (2) the youngest rocks at the ridge crest always have presentday (normal) polarity; and (3) stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times. Early in the 20th century, paleomagnetists -- such as Bernard Brunhes in France (in 1906) and Motonari Matuyama in Japan (in the 1920s) -- recognized that rocks generally belong to two groups according to their magnetic properties: (1) normal polarity and (2) reversed polarity ocean floor shows a zebra-like pattern of alternating stripes of magnetically different rock creating magnetic striping. Bruhnes-Matuyama ~ 780 ka
Evidence for Continental Drift Cascadia subduction zone Magnetic anomalies What Drives Plate Motions? (1) Density vs. Gravity: causes oceanic crust to sink in subduction zones, causes crust to extend at spreading ridges (called ridge push, but the ridge is not pushing, the crust is pulling as it sinks into subduction zones) (2) Thermal Convection: exerts drag force to base of crust, circulates heat and mantle material... Atwater, 1970
Bonus Activity This is very similar to parts of Activity 2. Do this on your own. This is an optional exercise that was created to help you practice these calculations. Do not be surprised if an exam question looks very much like this one.
How Seismology! do we know about the structure of the earth s interior? Differences in physical properties and strength. Differences in mineral and chemical composition.
Refraction (Snell s Law) Can you think of a refraction example? Refraction Example Why are there different layers? Why is some crust above sea level? Oceanic Crust Mean = 7 km Continental Crust Mean = 40 km http://earthquake.usgs.gov/research/structure/crust/download.php
~2.7 g/cc ~3.3 g/cc ~4.5 g/cc Isostasy is the vertical movement of the crust to attain buoyancy in the mantle. The height a block of wood floats in water depends on it s density and thickness. The height of the earth s crust also depends on it s density and thickness. Crustal Age Bathymetry Thermal Control on Crustal Depth Average depth ocean ridges = 2.5 km As plates cool, they get more dense and sink Depth = 2.5 km + 0.33 x (sq root age in Ma)
Types of Plate Boundaries Spreading Centers Divergent (More Vectors!) Convergent Mid Atlantic Ridge = ~3.5 cm/yr East Pacific Rise = ~16.5 cm/yr Transform East Pacific Rise = ~16.5 cm/yr Mid-Atlantic Ridge = ~3.5 cm/yr
Transform Margins Connect spreading ridges Strike slip Transform Margins Connect spreading ridges Strike slip http://www.geocaching.com/seek/cache_de tails.aspx?guid=f6a2bf15 d163 4f3f a383 0f647bf374a4 Transform Boundaries Transform Margins Strike slip Loma Prieta 1989 Ms 7.1 36 mm/yr NORTH AMERICA Davidson et al., 2002 PACIFIC San Andreas Fault, Carrizo Plain
Convergent Margins a) Ocean Continent Subduction b) Ocean Ocean Subduction c) Continent Continent Collision Ocean Continent Subduction Anyone have an example? Ocean Ocean Subduction Anyone have an example? Continent Continent Collision Anyone have an example?
Convergent Margins Ocean Continent Subduction Ocean plate is subducted Continental arcs generated Explosive andesitic volcanic eruptions Cascadia subduction zone Juan de Fuca and Gorda plates subduct northeastwardly beneath the North America plate Subduction zone earthquakes can cause tsunami Ocean Continent Subduction Cascadia subduction zone Cross Section Water content fluxes melt to generate volcanism USGS Ocean Continent Subduction Subduction zone earthquakes can generate tsunami Mt St Helens, WA March 1980 USGS Mt St Helens, WA May 18,1980