Earth Sciences (Tectonics 1)

Continental Crust

Rich in non-ferromagnesian silicates (felsic)
• Felsic comes from feldspar and silica
• Rich in silica (silicon and oxygen - SiO2
40-70km thick

Oceanic Crust

Rich in ferromagnesian silicates (mafic)
• Mafic comes from magnesium and ferric

0-8km thick

Mantle

Consists of Ferro-Magnesian Silicates (Ultra-Mafic)

Mohorovičić Discontinuity

A chemical composition boundary between the mafic or felsic crust and the ultramafic (upper) mantle.

Lithosphere

1st mantle layer

Includes the crust and uppermost mantle.

Rigid and brittle

Asthenosphere

• 2nd mantle layer

• Hotter and Weaker layer in the upper mantle

• Solid (<3% liquid) but behaves like a fluid over long geologic time periods

• Partly melted and Ductile

• Permits the overlying lithosphere to mechanically detach from the layer below

Mesosphere

3rd lower sphere

- In the lower mantle, increased pressure counteracts the weakening effects of high temperatures.

• Rocks strengthen with depth in the lower mantle.

• Rock in the mesosphere is still able to have plastic flow.

• Solid and Ductile

Core

• Composed primarily of iron (Fe) and a lesser amount of
nickel (Ni) as well as oxygen and sulphur mostly in the liquid outer core.

Theory of Plate Tectonics

Movement of lithospheric plates that shift continents and causes volcanism, earthquakes, and mountain building.

Divergent plate boundaries

Plates spreading apart.

Transform plate boundaries

Plates sliding past one another, forward to back

Occurs between two offset segments of a mid-
ocean ridge (seafloor spreading centres)

Causes earthquakes but no volcanoes

Convergent plate boundaries

Plates pushing together, one above the other (subduction or collision zones).

Oceanic plate

-seafloor spreading centers or ‘Mid-
ocean ridges’ (MORs)
-divergent plate boundary within
oceanic crust

Continental Plate

- Divergent plate boundary within
continental crust
- Will eventually form oceanic crust

Continental rift zone

Elongated depression generated when continental crust is pulled apart

Step 1 of tectonic plate movement

Hot, less dense, buoyant magma forms at the outer
core-mantle boundary and rises to the base of the
lithosphere where it cools.

Step 2 of tectonic plate movement

This pushing apart plates at divergent boundaries as
new rock forms (divergence)

Step 3 of tectonic plate movement

Overtime, the cool, dense crusts subducts and sinks into
the mantle – as the cycle begins again (convergence)

Trench

Deep, valley on the ocean floor, created by the subduction of oceanic lithosphere

Accretionary prism/wedge

Wedge of sediment accumulated onto the continental plate as the oceanic plate subducts.

Orogeny

Process where two continents collide, compressing, shortening, and thickening them by folding and faulting.

Oceanic-continental convergent plates

• Subduction plates

• These interactions lead to volcanic activity and earthquakes

• Forms a trench and accretionary wedge

Oceanic-Oceanic convergent plates

• Subduction plates

• Abundant volcanism and earthquakes

• Older oceanic plate subducts (sinks down) since its colder and denser

Continental-Continental convergent plates

• Creates very thick continental plate
  • Large mountain belts (e.g Himalayas)
  • Abundant earthquakes with rare volcanism

Transform plate boundaries

Fault lines that cause earthquakes but not volcanism, occurs in oceanic-oceanic, continental-continental and oceanic-continental convergent plates

Paleomagnetism

Magnetic field of the earth recorded in rocks. Helps understand sea floor spreading

Sea floor spreading (Mid-Oceanic Ridge)

Ridges in the ocean prove they are sites of new tectonic plate growth. Oceanic lithosphere is constantly being destroyed and re-grown

Formed where heat from asthenosphere causes crust to melt. Magma rises to form new oceanic crust from divergent plate boundary

Mantle plume

Pipe-like, plastic upwelling of abnormally hot rock within the mantle which creates volcano

Hotspot

Magma generated by this rises through the rigid plates of the lithosphere and produces active volcanoes
at the Earth's surface

Continental shelf

extends from the shore to the point where the sea floor steepens sharply, gently sloping ~0.1, thick accumulations of shallow water sediment (several km’s thick

Continental Slope

the bottom marks the boundary between the continental and oceanic crust

Abyssal Plain

The nearly flat plain on the deep ocean floor.

Active continental margin

Continental margin that is a plate boundary.

• Thin continental shelf

• Volcanism and earthquakes due to convergence

• A deep offshore trench

• An accretionary wedge where ocean floor sediment accumulate

Passive continental margin

Continental margin that is not a plate boundary but marks the shift from continental to oceanic lithosphere.

• wide continental shelf

• No volcanism or earthquakes because there is no
  tectonic activity

• thicker accumulation of sediment on shelf than active margins

Average ocean deapth

4,000metres

Theory of continental drift

The hypothesis that continents have moved over geologic time, resulting in their current positions. Proposed by Alfred Wegener, it suggests that continents were once part of a single supercontinent.