How is Earth magnetic? • The Earth is a layered planet consisting of crust, mantle, and core • The outer core is a metallic liquid that moves around, creating a magnetic field • The lithosphere moves around on top of this
What is magnetic reversal? • Every magnet has a north and south pole because magnetic domains are aligned • Earth’s magnetic poles periodically REVERSE • Magnetic North becomes magnetic South and vice-versa EVEN THOUGH the geographic poles don’t move
How was magnetic reversal discovered? • When iron-rich lava cools, its domains align in the direction of the magnetic field • Studies of paleomagnetism provide a rock record of magnetic field reversal
How was magnetic reversal discovered? • In WWII, we developed magnetometers • Sensitive tools to detect steel-hulled submarines • Later, when scientists used the magnetometers to explore the ocean floor, they discovered a banding pattern in the rocks
What is Sea-floor Spreading? • At the mid-ocean ridge, molten material rises from the mantle and erupts. • The molten material then spreads out, pushing older rock to both sides of the ridge. • As the molten material cools, it forms a strip of solid rock in the center of the ridge.
What is Sea-floor Spreading? • The process that continually adds new material to the ocean floor. • Evidences include: molten material, magnetic stripes, and drilling samples.
When was sea-floor spreading theory developed? • Harry Hess, a WWII naval officier, first explained the mechanism in the 1960s • Why the wait??? We did not have the ability to explore the oceans depths previously! • Hess used sonar echo readings to map the ocean floor
How are sea-floor spreading and continental drift related? • They support one another! • One reason continental drift was rejected was because Wegener could not explain “HOW” • The evidence that the sea-floor is younger than continental crust provides a mechanism for continental drift
How are sea-floor spreading and continental drift related? • The evidence that the sea-floor is younger than continental crust provides a mechanism for continental drift • Sea-floor spreading is the force pushing continents apart!
How are sea-floor spreading and continental drift related? • The transfer of thermal energy via convection cells drives seafloor spreading and all tectonic plate movement • Convection cells circulate molten material from areas of high temperature to cooler regions in a circular pattern
How are sea-floor spreading and continental drift related? • As material in the lower mantle warms, it becomes less dense and more buoyant, causing it to rise • As material in the upper mantle cools. It becomes more dense and sinks due to gravity
Partner Demo • Partner up and put your desks together so you are facing each other with almost no space between the two desks • Hold the paper so that only the tiniest bit is exposed on each side • VERY VERYsloooooooooowly, begin pulling the paper up by sliding it across your desk • As you slide, one person should be coloring ALL of the paper as it comes up • When I say “Switch” the second person should start coloring
Partner Demo • Use the diagram to label the following parts on your sea-floor spreading model: • Mid-Ocean Ridge • Strips of alternating normal and reversed polarity (label with “NP” and “RP” • Oldest • Youngest • Subduction Zone
Analysis Questions • Imagine that as you pulled the paper, your hands represented two continents that were once together, but moved apart as the sea-floor grew. How does this model provide evidence for continental drift? • When was the Earth formed? Based on this model, explain why the ocean floor is only 200 million years old.
Analysis Questions • What happens at a subduction zone? What happens to this old rock? • Are the areas of normal and reverse polarity equal in size? What does this lead you to believe about periods of magnetic reversal in history? • Just like on continental crust, the top of the ocean floor is covered in sediments. Do you think the sediments are younger, older, or the same age as the rocks below. WHY?
Bellringer 10/5/2011 “The sea is everything. It covers seven-tenths of the terrestrial globe. Its breath is pure and life-giving. It is an immense desert place where man is never lonely, for he senses the weaving of Creation on every hand. “ ~Jules Verne 20,000 Leagues Under the Sea
Bellringer October 6th: • Watch the demonstration in the front of the room and write down your observations about the movement of the food coloring • Do you remember how heat flows from physical science??? http://education.sdsc.edu/optiputer/flash/convection.htm