Earth's Crust, Core, and Plate Tectonics

Earth's Structure and Tectonic Processes

Continental and Oceanic Crust

• Continental Crust: Thickness varies between 30 and 70km. Forms the continents and is composed mainly of granite.
• Oceanic Crust: Thickness is about 10km. Forms the deep ocean floor and is composed of basalt and gabbro.

Inner and Outer Core

• Inner Core: A solid area with a radius of over 1220km.
• Outer Core: Ranges from 2900km (base of the mantle) to 5150km and is in a liquid state.

Technological Discovery: Mapping the Ocean Floor

Sonar Applications

Using sonar, the ocean floor and deep trenches were mapped, revealing reliefs over 2000m above sea level, thousands of km long, and areas with few sediments.

Magnetic Remanence

Remanence: Magnetic crystals align their orientation with Earth's magnetic field.

Continental Drift and Plate Tectonics

Movement Mechanisms

• Continental Drift: The movement of continents arising from the ocean floor.
• Plate Tectonics: Plates slide over the asthenospheric mantle, moving both continents and oceans.

Driving Forces

The force driving continental movement differs: Continental drift suggests movement due to Earth's rotation, while plate tectonics posits that the force comes from the ocean floor pushing continents at mid-ocean ridges.

Causes of Relief

• Continental Drift: Wrinkles produced at the advancing front of the continents.
• Plate Tectonics: Caused by collisions between lithospheric plates.

Asthenosphere: Past and Present Beliefs

Initially, the asthenosphere was thought to be the layer emitting the sliding of the lithosphere, where convection occurred.

Currently, it appears that convection affects the entire lithosphere; subduction represents the descending limb of this convection.

Benioff's Discovery and Plate Tectonics

Benioff discovered that earthquakes occurring along the North American Pacific coast show a peculiar distribution:

• Shallow seismic foci are next to the shoreline.
• Deeper foci occur inland, suggesting an incline.

According to plate tectonics, this corresponds to the oceanic lithosphere sinking (subducting) into the mantle beneath the continental plate.

Manifestations of Convection Movement on Earth's Surface

Item 9: Surface Effects

• Terrestrial Magnetism: Originates from violent convection currents excited by the fluid in the inner core at high temperatures.
• Movement of Continents: Convective motions occur in the mantle, moving horizontally across the surface layer.
• Volcanism: Heat is transported from the base of the mantle to the most superficial land layers, giving rise to volcanic mountain ranges.
• Seismicity: Movements in both deep oceans and continents form large fractures in the lithosphere, causing earthquakes.
• Segregation of Materials by Density: Less dense materials are brought to the surface, causing the formation of granitic continental crust.