Sea floor Spreading Aim: Is the ocean getting bigger?

1. SONAR (it’s an acronym) a. In the 1960s, SONAR was used to map the ocean floor. b. A ridge was found in the Atlantic Ocean – longest and tallest mountain chain on Earth. (mid- Atlantic Ridge) c. Trenches were found along the coastlines –one is 6x deeper than the Grand Canyon (Mariano -11 km)

A ridge runs all through the oceans

Mid-ocean Ridge and Marianna Trench Trench Ridge

2. Core Drillings a. Rock samples taken from the crust indicate that the continental crust is older than the ocean crust. b. Also, ocean crust sampled near the ridge is younger than farther from the ridge.

Relative Age of Ocean Floor  Red is younger  Yellow and Green are older

3. Magnetic Striping of Crust a. A magnetometer detected the magnetite (a mineral made of iron) in the basalt (an ignesus rock) in the oceanic crust. b. Earth’s magnetic field has reversed its polarity many times, called a magnetic- reversal when the north pole swaps polarity with south pole. c. Pattern (isochron) on one side of mid-atlantic ridge is a mirror of other side.

4. Hydrothermal Vents a. ROVs (remote operated vehicles) saw steam and lava coming out of the ocean floor around the ridge. b. Vents are underwater geisers c. Interesting sea life found around the heated water of the vents

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5. Theory of Sea-Floor Spreading a. The process by which molten material adds new oceanic crust to the ocean floor. b. The ocean floor is widening because new ocean floor is being made at the ridges, while old ocean floor is being destroyed when it is pushed back down in a trench. c. Finally a possible explanation for the continents moving.

Textbook pages  Page 71 Questions # 1- 4  Page 77 Questions #1 – 5  Section Review pages #1-14 AND #1-4  Exclude page 76 (Microplate Terraines)

Theory of Plate Tectonics Continental Drift + Sea-floor Spreading = Theory of Plate Tectonics

Do Now: Lava Lamp  Can you explain how the lava lamp is working:  What is the heat doing?  What is happening to the molecules of lava?  What is happening to the lava’s density?

1. What are plates? a. The Earth’s lithosphere is cracked into about 20 pieces, like a cracked egg shell. b. The less dense plates float on the more dense magma of the asthenosphere.

2. What is driving sea-floor spreading? a.CONVECTION: Heat transfer by movement of a fluid. b.Convection Currents in the asthenosphere are the forces that move the plates of the lithosphere c.This movement is the cause of continental drift

3. How does convection work? a. During convection magma heated from the core rises, then cools and sinks again (like a lava lamp) b. Heat is being transferred from the inner core to asthenosphere

c. Sometimes lava comes out of the crack at the mid-ocean ridge, pushing the plates apart. d. This movement causes the plates above to move around. e. Convection makes the plates move about 2 cm/year.

4. What happens when plates move around? a. Geological features (landforms) are formed as some plates push apart, push together, or slide past one another. b. The features are called tectonic, formed from the movement of lithospheric plates. c. Plate boundaries are formed where 2 pieces of plate meet.

3 Types of plate boundaries  Divergent  Convergent  Transform

5. Divergent Boundaries  Boundary between two plates that are moving apart or rifting    Rifting causes Sea-floor Spreading

Ex. 1 : Mid Ocean Ridge – ocean crust diverging from ocean crust

Ex. 2: Iceland tallest mountain on the mid-Atlantic ridge

Ex. 3 : African Rift Valley 2 continental plates are pulling apart

6. Convergent Boundaries a. A place where two plates come together. b. Collision between plates. c. Three types of convergent boundaries:  Continent-continent collision  Continent-oceanic crust collision  Ocean-ocean collision

Convergent Type 1 a. Ocean plate collides with less dense continental plate b. Subduction Zone: where the more dense plate slides under the less dense plate c. Volcanos and trenches occur at subduction zones

Andes Mountains, South America

Convergent Type 2 a. Ocean plate collides with another ocean plate b. The more dense plate slides under the less dense plate creating a subduction zone called a deep ocean trench and an island arc

Ex. 1: Mariana Trench

Bathyscape Triest

Ex. 2 -Aleutian Islands, Alaska

Convergent Type 3 a. A continental plate colliding with another continental plate b. Have Collision Zones:  a place where folded and thrust faulted mountains form.

Ex. - Himalaya Mountains

7. Transform Boundary a. Boundary between two plates that are sliding past each other in opposite directions b. Stress builds up and earthquakes occur along faults (cracks)

The lithosphere cracks from the stress (pressure) from the friction of the plates grinding against one another. Large faults (cracks) are created.

Ex. 1: San Andreas Fault

Plate Tectonic Map

Summary:  The movement of the tectonic plates of Earth’s crust is caused by convection in the mantle.  The movement causes the plates to either:  Converge (collide)  Diverge (move apart)  Transform (slide past)