Plate Tectonics + Earth’s Internal Layers: Chemical

Earth’s Internal Layers: Chemical Classification Scheme

• Rocky Crust → Lower density rocks

• Rocky Mantle → Denser rocks

• Metal Core → Mostly iron, some nickel

Rocky Crust can be further distinguished into what?

Continental and Oceanic Crust

Continental Crust

Average rock type is granite, low density, and thicker

Oceanic Crust

Typically basalt; higher density, thinner

Earth’s Internal Layers: Physical Classification Scheme

• Lithosphere

• Asthenosphere

• Mesosphere (almost entirely makes up the mantle)

• Liquid Outer Core

• Solid Inner Core

Lithosphere

• The plates in plate tectonics

• Composed of the crust and uppermost mantle (~3%)

• Thickness varies due to age of the crust

Asthenosphere

• The “soft” layer that the lithosphere rocks slide over

• Not liquid but easily deformed rock is close to its melting point

Three Types of Plate Boundaries

• Divergent

• Convergent

• Transform

Divergent Boundaries

Plates move away from each other (ex) mid-ocean ridges, Iceland, East Africa)

Convergent Boundaries

Plates move toward each other (Ex, Pacific NW, West Coast of South America, the Himalaya)

Transform Boundaries

Plates move past each other (Ex) The San Andreas Fault in CA, Alpine Fault in NZ)

Plate motions are driven by what?

The weight of the hanging slab of crust being pulled down by gravity into the subduction zone

Other forces of Plate Tectonics (decreasing importance)

• Slab pull

• Ridge push (hot, new material slides down and away from mid-ocean ridges)

• Basal drag

Resisting Forces of Plate Tectonics

• Mantle resistance

• Continental resistance

• Transform resistance

• Basal drag

3 Ways to Melt a Rock

• Increasing temperature- most obvious but least common method

• Decrease pressure- reducing pressure lowers melting part of rock

• Adding water to the rock- lowers the melting temperature of the rock

Partial Melting

• Rocks are composed of different minerals which have different melting points

• Si-rich minerals have a lower melting point, so it melts first and turns into magma

Step one of continental rifts (divergent plate boundaries)

Magma rises from the mantle, meeting continental crust. This magma extension breaks the crust, creating faults. Magma rises up in dikes and sills, and can also produce volcanoes.

Step two of continental rifts (divergent plate boundaries)

Extension and rifting continues with more faulting and a more complex band of fault blocks. Rift valley forms, and magma fills it in, creating a new crust.

Step three of continental rifts (divergent plate boundaries)

If rifting continues for a long time, the rift valley becomes so wide it opens up to the sea, forming a linear sea

Step four of continental rifts (divergent plate boundaries)

Expansion continues, and passive margins form (continental margins that aren’t tectonically active)

Info about Continental Rifts

• 4-step formation process (faults → rift valley → linear sea → ocean basin)

• Typically have 3 arms

Mid-ocean Ridges (oceanic-oceanic) (divergent boundaries)

Seafloor spreading generates a new oceanic lithosphere created by partially melting the mantle through decompression and resolidifcation

Three Types of Convergent Plate Boundaries

• Oceanic-oceanic

• Oceanic-continental

• Continental-continental

Oceanic-oceanic (convergent boundaries)

• Older, colder, denser plates get subducted

• Subducted water-rich minerals dehydrate and release water, thus lowering the melting point of the rocks in the asthenosphere

• Liquid magma rises to the surface and releases from volcanoes

Oceanic-Continental (convergent boundaries)

• Oceanic plate subducts under the continental plate

• Similar process occurs in the asthenosphere, however most of the liquid magma will cool before reaching the surface and forming more continental crust (granite)

• Some magma will eventually move up further to the surface through volcanic eruption

Continental-Continental (convergent boundaries)

• Continents too thick and buoyant to subduct

• Continental collisions form mountain belts

• Suture zone → Zone of convergence between 2 continental plates

• Himalayas → India and Asia plate collision