Plate Tectonics & The Ocean Floor

Layers of the Earth (1.4)

Layers of the Earth

Structure of the Earth Oceanic Crust –Composed of basalt –Crust beneath oceans –Younger crust –More dense ~ 3 g/cm 3 Continental Crust –Composed of granite –Crust beneath continents –Older crust –Less dense ~ 2.7 g/cm 3 Crust: Thinnest & outermost layer of the Earth

Lithospheric Plates * Lithosphere: made up of the crust and the upper mantle

 Click Sid for a cool music video

Continental Drift Theory First proposed by Alfred Wegener in 1915 Proposed that a supercontinent, Pangaea, once existed and has since broken apart into the continents we know today

Continental Drift Theory Panthalassa “all seas” was the single ocean. Tethys Sea was a small inland sea that eventually divided Pangaea into two land masses. Laurasia in the north and Gonwanaland in the south. Click for video

200 million years ago

What Evidence supports Continental Drift? –Jigsaw puzzle-like continental coastlines –Fossils on opposite coastlines were similar in type and age.

–Similar rock types and structures of matching mountain ranges What Evidence supports Continental Drift?

200 mya Today What Evidence supports Continental Drift? –Evidence of glaciation in tropical areas in low latitudes.

Objections to Continental Drift One of the biggest objection was his explanation of “how” the continents could have drifted apart. He suggested that gravity and tidal forces allowed for continental crust to plow through oceanic crust of the ocean basins.

Plate Tectonics Movement of lithospheric plates due to convection currents and mantle plumes Sinking of dense, cool magma due to gravity is replaced with rising less dense, warmer magma Movement of these huge plates creates mountains, volcanoes, oceanic ridges, and trenches

Evidence for Plate Tectonics Paleomagnetism Sea Floor Spreading Ocean drilling Earthquake Patterns Hot Spots

Paleomagnetism Earth’s magnetic field has reversed many times since Earth’s formation Iron is magnetic. As magma cools to form igneous rocks, the atoms of iron align them selves with the magnetic field of the earth.

Paleomagnetism Using the position of the iron atoms, scientist can determine the location of the magnetic pole which has reversed in the past. Using radiometric dating, they have determined that the Earth’s magnetic field has reversed about 1000 times over the past 76 million years.

Magnetic Pole Reversal Discovery of the Magnetic Reversal In the news….

Sea Floor Spreading Sea floor features were discovered from depth recordings of Capt. Harry Hess (1953.) His soundings (depth recorder) showed –Extensive mountain ranges near the center of ocean basins. –Deep trenches near the edge of ocean basins where crustal plates are subducted and destroyed. Click “Sea Floor Spreading” for an animation of the process

Sea Floor Spreading From the soundings, Hess suggested that new crust forms at the mid-ocean ridges, while older crust is found at the trenches, where the process of subduction occurs. Click Bill Nye for a video about Capt. Hess’ Discovery of sea-floor spreading. 

Ocean Drilling (DSDP) Showed that the age and thickness of sediment increased with increasing distance from the mid-ocean ridge Glomar Challenger

Earthquake Patterns Earthquakes occur in areas of plate movement, mainly along plate boundaries.

But..Let’s not forget volcanoes too.

These are pictures associated with the 9.0 earthquake that hit Japan in 2011, which caused a devastating tsunami.

Hot Spots Shows movement of volcanic activity in middle of oceanic plates

Acceptance of a Theory Finally in the 1960’s the continental drift theory and sea floor spreading were united as the Plate Tectonics Theory. The theory explains that tectonic plates that make up the lithosphere “float” on the somewhat “fluid” asthenosphere. Click MeClick Me for a 10 minute review video