Plate Tectonics

Plate Tectonics and the Ocean Floor Plate tectonics – Theory that the outermost portion of the Earth is composed of plates that move on a liquid molten layer (similar to icebergs floating on water) Alfred Wegener a German meteorologist developed the theory of continental drift in 1912 Continental drift- Continents slowly drift across the globe

Evidence of Continental Drift Pangaea – (pan= all; gaea= Earth) A large landmass that the continents fit together to form Evidence Matching rocks on different continents Similar rocks form Appalachian mountains match rocks in British Isles Distribution of organisms Finding fossils of the same species on different continents Glacial Activity in now tropical areas

The continents fit together

Fossils of same species found on different continents.

More on Continental Drift As the seafloor spreads the lithosphere (continental and oceanic crust) moves in sections on a viscous liquid layer- asthenosphere On average these tectonic plates move 1-5 inches/year

More on Continental drift If new crust is being formed all the time why isn’t the earth covered in crust? Because as new crust is formed old crust is destroyed Let’s start by looking at the 2 types of crust Both types of crust make up the outer layer of earth, called the lithosphere Continental Crust Granite Lower density than continental Oceanic Crust Igneous rock + basalt Higher density than continental

Continental drift The crust sits on top of a viscous layer – the asthenophere A Viscous substance is a thick liquid What are some examples of a viscous substance that you are familiar with? The asthenosphere is part of the mantle called the upper mantle

Convection Convection is the heat transfer in a gas or liquid by the circulation of currents from one region to another Convection cell is a circular-moving loop of matter (gas or liquid) involved in convection movement

Convection Convection causes movement in the liquid mantle layer which the crust sits upon As a result the crust slowly moves on this layer Kind of like an iceberg floating on water As this occurs the tectonic plates move slowly over the asthenosphere

Tectonic plates

Forming Crust Mid-Ocean ridge- Underwater mountain range winds through all oceans Seafloor spreading occurs at the ridge This is where molten rock reaches the surface, cools, and forms new crust As this new crust forms the magnetic minerals in the crust align in relation to Earth’s current magnetic field Scientists can look at the sea floor to get a history of Earth’s magnetic reversals (we will talk about how this works in a minute)

Seafloor Spreading Harry Hess (1906-1969)- left his depth recorder on while traveling as sea in the Navy. He noticed mountain ridges at the center of the ocean basins and deep, narrow trenches at the edges Published his idea of sea floor spreading which suggested that new ocean crust is formed at ridges, split apart and moves away from ridges

Seafloor Spreading It was confirmed that sea floor spreading does occur at mid-ocean ridge Crust is also being destroyed at ocean trenches (we will talk about this in a minute) This is called subduction

New crust is made at the mid-ocean ridge

Magnetic Field Reversals Fredrick Vine and Drummond Matthews (1963) Combined the pattern of magnetic sea floor stripes with sea floor spreading. Can see fluctuations in rock from normal polarity to reverse polarity.

Magnetic field reversals- This is the piece of Evidence that made The continental drift Theory widely accepted Among scientists. As the sea floor spreads (at mid-atlantic ridge) the Magnetism of the rocks Formed reverses. These Magnetic shifts can be Observed in crust.

Being able to identify these magnetic pole reversals in the rock Confirmed the theory of continental drift.

Plate Boundaries- 3 types Divergent Boundaries Convergent Boundaries Ocean to Ocean Ocean to Continental Continental to Continental Transform Boundaries

Divergent boundaries Plates are moving apart Ocean crust – Ocean crust Sea floor created Mid-Atlantic Ridge Continental crust – Continental crust Continental rifting Continents are splitting apart Red Sea, Gulf of California

This is how the Mid-Atlantic ridge started!

Convergent Boundaries Plates are moving together Ocean –Continent Old sea floor destroyed Subduction zone (continental arcs) Subduction- A process in which one plate descends below another (volcanoes) Ocean –Ocean Subduction zone (island arcs) (volcanoes) Continent –Continent Collision Zone Mountains form Why does subduction occur? Hint: composition of plates

Examples of Convergent Plate boundaries Continental/Oceanic

Transform Boundaries Plates are moving past each other Oceanic Continental Both of these neither create nor form sea floor Causes Earthquakes

What do we know about all of the locations of these active volcanoes?