Earth’s Layers and Plate Tectonics: A Comprehensive Guide

Posted on Oct 17, 2024 in Geology

Earth’s Layers

The Crust

The Earth’s crust is the outermost layer of the Earth and the uppermost part of the lithosphere. Its thickness varies from about 5 kilometers in oceanic regions to up to 70 kilometers in mountain ranges. The crust is composed of rocks and is divided into three main layers:

  • Sedimentary Layer: Located on continents and continental platforms, this layer consists of sedimentary rocks. It includes folded rocks from ancient mountain ranges, the base of current continents and continental shelves, and recent sediments deposited on the continental shelf and seabed.
  • Granitic Layer: This layer is made up of rocks similar to granite, forming the core of emerged continental areas. The Conrad discontinuity separates the granitic layer from the basaltic layer below. Both the sedimentary and granitic layers are in isostatic equilibrium, floating on the basaltic layer.
  • Basaltic Layer: Composed of basalt-like rocks, this layer is continuous around the Earth, unlike the previous two. The Mohorovicic discontinuity separates the basaltic layer from the mantle below, at a depth of 30 to 70 kilometers. Other important discontinuities include the Repetti discontinuity (separating the upper and lower mantle at 670 km), the Gutenberg discontinuity (separating the mantle and outer core at 2900 km), and the Lehman discontinuity (separating the outer and inner core at 5150 km).

Plate Tectonics

Constructive Boundaries

At constructive boundaries, also known as divergent boundaries, plates move apart due to ascending convection currents. New oceanic lithosphere is created from asthenospheric material. These boundaries are located at mid-ocean ridges and continental rifts, such as the East African Rift Valley and the Mid-Atlantic Ridge. Volcanic activity in these areas results from the diverging plates and the formation of new oceanic crust, causing the seafloor to spread and the adjacent plates to separate.

Hot Spots and Mantle Plumes

Mantle plumes are columns of hot material rising from the base of the mantle to the surface. They create areas of intense volcanism called hot spots. Solifluction is the seemingly solid material behaving like a slow-flowing viscous fluid.

Destructive Boundaries (Subduction Zones)

When one plate subducts under another, part of the lithosphere melts, creating volcanic island arcs, such as those in the Pacific Ocean. Several scenarios can occur:

  • If the overriding plate is oceanic, a volcanic arc forms offshore. The area between the arc and the mainland may become an inland sea or marginal sea, like the Sea of Japan.
  • If the overriding plate is continental, marine sediments and the volcanic arc are compressed against the continent, forming a mountain system (orogeny), such as the Andes Mountains.
  • If both plates carry continental crust, a continental collision occurs, creating a large orogeny between them, like the Himalayas. The zone where the continents join is called a continental suture.

The Lithosphere and Asthenosphere

Lithosphere

  • Rigid outer layer of the Earth.
  • Composed of the crust and the uppermost part of the mantle.
  • Thickness up to 200 km.
  • Thinnest under mid-ocean ridges, thickest under ocean trenches.
  • The Mohorovicic discontinuity marks its base.

Asthenosphere

  • Lies beneath the lithosphere, with the same chemical composition.
  • Solid but with some plasticity, allowing slow convective movement (2-4 cm/year).
  • Convection currents rise under mid-ocean ridges and descend under ocean trenches.
  • Extends from about 200 km to 800 km depth.

Subduction

A subduction zone is a convergent boundary where one tectonic plate slides beneath another. These zones are where plates disappear, compensating for the creation of new lithosphere at mid-ocean ridges. The subducting plate melts, inducing volcanism and earthquakes. Major subduction zones are found around the Pacific Ocean, notably along the western coasts of North and South America. The Andes Mountains are a prime example of volcanism associated with subduction. The friction between plates can cause sudden releases of energy, resulting in devastating earthquakes, such as those common in Mexico.