Rate of plate velocity movement is a few cmyr The rate at which

# Rate of plate velocity movement is a few cmyr the

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January 22, 2007 -Rate of plate velocity (movement) is a few cm/yr . The rate at which fingernails grow. Actual values 2-20 cm/yr -First Approximation: all points on a given plate move in the same direction with the same speed. -South Atlantic Ocean. 5000 km wide. Created over the last 120 million years. Africa and South America were joined as a part of Pangaea. -Physiography (Physical Features) Continents and Oceans Height/Depth – measured from sea level Sea level varies from place to place -Continents- average elevation 1 km. highest at 9 km. Mount Everest -Oceans- average depth 3.5 km . greatest ones are associated with convergent plate boundaries called deep sea trenches which are narrow belts in oceans caused by subduction. Deepest one at 11km depth -on average continents ride 4.5 km higher than the ocean floor. Light, low density- 2.7g/cc, continental crust “floats” higher than denser, 3.2g/cc oceanic crust
-Floats on what? -asthenosphere: behaves like bowl of jello, with denser oceanic material “sitting down” and continental material riding higher -note: at convergent plate boundaries, heavier material (oceanic) bends and goes into the interior- subduction -continental material is too light to subduct. Cannot push cork into water and keep it down. When two plates with continental material collide, neither wants to subduct. Ex: India colliding with Eurasia. Rocks are crumpled up into highest mountain chain (Himalayas have over 1000 peaks higher than any other mountain chain- Rockies, Alps, and Andes). Chapter 1 (continued) -Scientific Method Make observations, do experiments to explain phenomena Use observations to make hypothesis -untested theory See if hypothesis can explain observed “facts” If so and it holds up for a long time, now have theory . This will continued to be tested against observations/experiments. Ex: 1. Floatation- Archimedes 2. Gravitation- Newton 3. Relativity- Einstein -Life on Earth -Began quite early about 4 billion years ago. For a long time remained fairly simple in form. Atmosphere consisted mostly of “poisonous” gases -about 2.5 billion years ago, oxygen began to collect in the atmosphere. Some of this formed an ozone layer above the Earth protecting life forms from harmful incoming radiation. -about 600 million years ago, life forms exploded on earth and from that time on fossils, traces of past organisms in rocks, are common. -geological record shows that at particular times I the past many life forms have gone extinct in a short period of time. -Mass Extinction Events causes are being investigated one about 65 million years ago appears to have been caused by a collision of 10 km meteorite with earth. About 60% of life forms became extinct.
Another about 250 million years ago is more enigmatic. About 90% of life forms became extinct A third (today) is mostly triggered by mans actions and could out do the other two.

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