The Theory of Plate Tectonics 6 th Grade

Remember what we learned yesterday about continental drift? Alfred Wegener found a variety of evidence that suggested that the seven continents used to be joined together as one large supercontinent. What was the ONE PART of his theory that he was not able to explain?

Alfred Wegener was not able to explain the force that moved the continents

What force is great enough to move the heavy continents?

Geologists think that the movement of convection currents in the mantle is the major force that causes plate motion!

The Theory of Plate Tectonics: States that pieces of the Earth’s lithosphere called plates are in slow constant motion because of convection currents in the mantle

What are plates?

Plates: The lithosphere is broken into separate sections called plates Plates move around on top of the mantle like rafts

Plates fit together along cracks in the lithosphere

Review: What is the Lithosphere?

The solid upper part of the mantle = lithosphere –Less dense than the material below it so it “floats” on top of the asthenosphere

Review: What is the Asthenosphere?

Asthenosphere: semi-solid layer below the lithosphere ***The plates of the lithosphere float on the asthenosphere***

2 Types of Plates Oceanic plates - plates below the oceans Continental plates - plates below the continents

Plate Boundaries

Remember that the convection currents (heat transfer) that are occurring in the mantle are causing these plates to move

Scientists use instruments on satellites to measure plate movement. Plates move at slow rates: ex. The North American and Eurasian plates are moving apart at 2.5 cm per year –about as fast as your fingernails grow

3 types of tectonic plate movement : 1. Divergent 2. Convergent 3. Transform

Divergent Boundaries Boundary between two plates that are moving apart or rifting   RIFTING causes SEAFLOOR SPREADING

What is seafloor spreading? When oceanic plates diverge, (move apart) leaving room for molten material to erupt and cool This leads to the constant creation of new ocean floor This always occurs at the mid-ocean ridge

Seafloor Spreading at the Mid- Ocean Ridge

Mid-Ocean Ridge

Life at the Mid-Ocean Ridge

Features of Divergent Boundaries Mid-ocean ridges Rift Valleys

Rift Valley: when divergent boundaries occur on land

Great Rift Valley in Africa Volcanic mountains found along the rift include Mount Kilimanjaro, Mount Kenya and Mount Elgon extending from Jordan in southwestern Asia through eastern Africa to Mozambique 4,000 miles long, 30 miles wide

Great Rift Valley

Convergent Boundaries Boundaries between two plates that are colliding  

Oceanic plate colliding with a less dense Continental plate the dense oceanic crust sinks under the continental crust Causes TRENCHES

Oceanic plate colliding with another Oceanic plate The more dense plate slides under the less dense plate, creating a TRENCH

When 2 continental plates collide, both are about the same density Collision squeezes the crust into mountain ranges

Transform Fault Boundaries Boundary between two plates that are sliding past each other Creates “faults” or breaks in the Earth’s crust What do you think that this movement causes?

Earthquakes!

San Andreas Fault, CA

Questions... What are the three types of boundaries? What direction do plates go for each?

Understanding Plate Tectonics nTVkhttp:// nTVk