1. DEVELOPMENT OF A THEORY Chapter 14 Lesson 2

2. Mapping of the Ocean Floor WWII played an important role in mapping of the ocean floor- Why? They were able to observe and determine the depth of the ocean floor and the features of the ocean floor. The mountains in the middle of the oceans were called Mid- Ocean Ridges

3. Sea Floor Spreading The 1960’s – Harry Hess was credited for explaining sea floor spreading. Sea Floor spreading- the process by which new oceanic crust forms along mid-ocean ridges and the older oceanic crust moves away from the ridge.

4. Sea Floor Spreading cont. When the seafloor spreads:  The mantle below melts and forms magma  The magma rises in cracks because it is less dense than solid mantle  The magma rises along the mid ocean ridge.  The older oceanic floor is pushed away from the mid ocean ridge  This process forces the continents to move apart as well.  Remember the continents are floating on the asthenosphere

5. Topography of the sea floor The mountains along the mid- ocean ridge can form in two different ways Large amounts of lava can erupt along the ridge forming mountains or The lava can cool and cracks occur creating jagged, rough mountains

6. Topography of the ocean floor Sediment can build up on the ocean floor. The newer oceanic crust has very little sediment, but the older oceanic crust can have a thick, smooth layer of sediment This smooth layer is the ABYSSAL PLAIN

7. Development of a Theory Magnetic Reversals- scientists studied the minerals of the rocks in the ocean floor. The iron- rich, liquid outer core is a like a magnet that creates Earth’s magnetic field. The magnetic field is not constant. Normal polarity- the state in which magnetized objects like compass points will point north.

8. Magnetic reversals cont Reversed polarity- the state in which magnetized objects would reverse direction and orient themselves to point south. Magnetic reversals occur every few hundred thousand to few million years. Basalt is iron rich and it is a common igneous rock found along the mid ocean ridge.

9. Magnetic reversals cont. As the lava erupts and cools, the minerals in basalt act like a magnetic and record the direction of the magnetic poles. Scientist have noticed parallel patterns in the magnetic signatures of the rocks on either side of a mid ocean ridge.

10. Evidence to support the theory Scientist use a magnetometer to record the magnetic signature of the rocks on the seafloor.  They observed parallel magnetic stripes on either side of the mid ocean ridge  Each pair of stripes have a similar composition, magnetic signature, and age

11. Evidence cont Thermal energy measurement  By drilling a hole in the seafloor, the temperature can be measured.  More thermal energy leaves at the mid ocean ridge then at the abyssal plain. Dating of sediment  Using radioactive dating- rock closest to the mid ocean ridge is younger than at the abyssal plain.  Sediment is also thicker at the abyssal plain.