Plate Tectonics, Lithosphere, and Earth Structure

Continental Drift and Plate Tectonics

Mechanism of Continent Movement

• The concept suggests oceanic sliding; the ocean floor remains motionless.
• Lithospheric plates slide over the mantle as the continents move. The oceanic floor changes.

Forces Driving Continent Movement

• Not clear. One mechanism suggests Earth's rotation could be the cause of motion.
• Mantle convection currents. Seafloor spreading at the ridges pushes the continents.

Causes of Reliefs

• Wrinkles produced in the leading edge of continents.
• Lithospheric plate collisions.

Oceanic and Continental Lithosphere

• Thickness

  • Oceanic: 100 km
  • Continental: 300 km

• Crust Composition

  • Oceanic: +90% of its mass are basaltic metamorphic rocks. Rocks poor in quartz.
  • Continental: +85% of its mass are granitic metamorphic rocks. Rocks rich in quartz. Volcanic and sedimentary rocks.

• Density

  • Oceanic: Basalt is a dense rock. The oceanic lithosphere can sink into the mantle.
  • Continental: Granite and quartz-rich rocks are less dense. The low density of continental crust prevents the lithosphere from sinking into the mantle.

• Movement

  • Oceanic: Actively moving part of the pattern of mantle convection.
  • Continental: Moves passively, driven by the oceanic lithosphere or dragged by convection currents.

Earth's Internal Structure and Seismic Discontinuities

Seismic waves occur in an earthquake, travel inside the Earth, and are received worldwide by seismographs. When the waves move from one layer to another with different material characteristics, their speed changes, and their trajectory deviates; part reflects. By analyzing the seismic wave records, seismologists can locate at what depth are the separations between these layers. Because these separations are identified and located by seismic studies, they are called seismic discontinuities.

Seismic Discontinuities

• Mohorovicic Discontinuity (Moho): Located between the crust and mantle, between 30 and 70 km depth.
• Repetti Discontinuity: Separates the upper mantle from the lower mantle, at a depth of 670 km.
• Gutenberg Discontinuity: Separates the mantle from the outer core, at 2900 km depth.
• Lehmann Discontinuity: Between the outer core and the inner core, at a depth of 5150 km.

The Lithosphere

Inside the mantle, there is another discontinuity (sometimes referred to as the S discontinuity) which is at a variable depth within the mantle. The part of the mantle above this discontinuity is united with the crust, forming a rigid layer. The outermost part of the mantle is firmly attached to the crust, forming the rigid lithosphere. There are 2 types:

Types of Lithosphere

• Continental: Formed by continental crust and upper mantle. Reaches 300 km thick under low mountain ranges, while in flat areas it is 100 km inland.
• Oceanic: Formed by oceanic crust and upper mantle part. Thickness is less than 100 km in ancient oceans and less than 20 km in young oceans.

The rest of the upper mantle beneath the lithosphere, even though it is also solid, is subjected to temperatures and pressures so high that it can flow slowly like an extremely viscous liquid, similar to the way ice moves in glaciers. The lithosphere is dragged by such movements in the sublithospheric mantle, breaking into large fragments called lithospheric plates.