C3 Tectonic Cycle

Compositional Layers of Earth

1. Core: Iron, magnetic poles, dense

2. Mantle: Silicates

3. Crust
   Oceanic Crust: Average of 8km thick
   Continental crust: 30-70km thick, average of 45km thick

Based on chemical properties

Which parts of the Earth are liquid?

The core is partially liquid (outer core)

Rock Strength of Earth’s Layers

1. Inner Core: Solid inner core

2. Outer Core: Liquid outer core

3. 3. Mesosphere (Middle Sphere): Solid mantle, has most of the metal

4. Asthenosphere (Weak Sphere): Mantle that deforms like plastic under pressure

5. Lithosphere (Rocky Sphere): Outermost 100km, includes rigid crust and upper mantle

Continental Drift

The idea that the continents were once all together and they slowly drifted apart

Alfred Wegener

Wegener hypothesized that the continents were once joined as a supercontinent called Pangaea, 300-240 Ma

Evidence for Plate Tectonics

1. Puzzle-like fit of continental margins

2. Distribution of land/fresh outer fossils

3. Distribution of mountain ranges and rock ty[es

4. Distribution of glacial features indicating the direction of the flow

Why wasn’t Wegener’s idea of continental drift accepted?

Mid-Atlantic Ridge

In 1952, Marie Tharp created the first map of the Atlantic seafloor, discovering the ridge

She believed that the rift valley formed when new material welled up to the surface, pushing either side of the ridge apart

Seafloor Spreading

Idea based off Tharp’s maps of the seafloor
Harry Hess proposed the idea of seafloor spreading in 1960

1. Molten material from the mantle is constantly upwelling along mid-ocean ridges (MORs), forming new oceanic crust

2. This pushes continental crust, providing a mechanism for continental drift

Evidence of Seafloor Spreading

1. Rock ages getting older away from mid-ocean ridges

2. Sediment thickens increasing away from mid-ocean ridges

3. Magnetic lineations mirrored on either side of the mid-ocean ridges

Plate Tectonics

The theory that the Earth’s outer shell is divided into several rigid plates that glide over the mantle (the asthenosphere), moving relative to each other

There are 7 major plates, and many smaller ones

If the ocean is spreading, why isn’t Earth getting bigger

Because of seduction (new crust is made, old crust is destroyed)

Types of Plate Boundaries

1. Divergent

2. Convergent

3. Transform

Divergent

Where two plates move apart

Spreading centers, constructive margins

Passive upwelling of the mantle associated with a decrease in pressure and melting

Partial melting of ultramafic mantle material produces mafic melts

Can start on a continent until it becomes an ocean

Convergent

Where two plates move together

Destructive margins

Ocean-ocean convergence = subduction of one plate beneath the other (subduction zone)

Ocean-continent convergence = subduction of the oceanic plate

Continent-continent convergence = neither plate subducts (too buoyant)

Introduction of water with the subducting plate lowers the melting temperature of rock, producing magmas and a chain of volcanoes

Transform

Where two plates slide past each other in a horizontal or strike-slip motion

Constructive margins

Can occur in oceanic or continental crust

What drives plate tectonics?

1. Mantle Convection

2. Slab Pull

3. Slab Push

Mantle Convection

Rising magma at ocean ridges may push plates away from each other

Slab Pull

Lithosphere breaks, sinks into the asthenosphere, dragging the plate away

Slab Push

New rock produced at ocean ridges is less dense and rises above the seafloor, gravity pulls this plate downslope, pushing the entire plate away

Resurfacing

The surface of the Earth slowly evolved, replacing older cratered crust with smoother crust

Where are new crusts being generated on Earth today?

Mid-ocean ridges, subduction zones (volcanoes)