Earth’s Internal Structure: Layers, Composition, and Dynamics

Posted on Jan 31, 2025 in Geology

Earth’s Internal Structure

Understanding the Earth’s interior involves gathering information from the surface, both directly and indirectly. Seismic waves reveal that the Earth is heterogeneous, changing speed and direction as they travel through it. These changes indicate that our planet is divided into concentric layers with different properties. Two models explain the internal structure: a geokimico model and a dynamic model.

Geochemical Model of Earth’s Internal Structure

Crust

The crust has two types:

  • Continental Crust: Approximately 70km thick, it’s composed of igneous, metamorphic, and sedimentary rocks. Metamorphism increases with depth, and igneous rock types vary. Higher levels are dominated by sedimentary, volcanic, and low-grade metamorphic rocks. Intermediate levels show more intense metamorphism, while deeper areas are dominated by intensely metamorphosed rocks.
  • Oceanic Crust: A thin, continuous layer consisting of:
    • Sediment Layers: Thickness varies, greater near continents and decreasing towards mid-ocean ridges.
    • Basalt Layer: Formed from solidified lava at spreading ridges.
    • Gabbro Layer: Basalts that solidified more slowly, resulting in better crystallized minerals.

Horizontal Structure of the Crust

The crust has distinct areas:

Continental Crust
  • Cratons or Shields: Tectonically stable areas with minimal earthquake activity.
  • Orogenic or Mountain Ranges: Active areas with significant tectonic and magmatic activity, featuring elevated and folded reliefs.
  • Internal Platforms: Depressions between cratons and orogenic areas where sediments from mountain erosion are deposited.
Oceanic Crust
  • Continental Margins: Submerged extensions of the land surface, formed by continental crust.
    • Continental Shelves: Submerged areas at depths between 20m and 600m.
    • Continental Slope: Extends from the edge of the continental shelf to the deep ocean.
  • Seabed: Formed by oceanic crust.
    • Abyssal Plain: Located at an average depth of 4 km, with few volcanic structures called guyots.
    • Trenches: Long, deep depressions reaching up to 11km deep.
    • Mid-Ocean Ridges: Topographic features rising 1-4 km above the seafloor, with a central rift where basalts from the upper mantle emerge, causing the separation of blocks on either side.
    • Transform Faults: Transverse fractures disrupting the ridges.

Mantle

Extends from the Mohorovičić discontinuity to the Gutenberg discontinuity, composed of peridotitic rocks.

Core

Composed of iron, divided into:

  • Outer Core: Liquid state, extending from 2900km to 4900km.
  • Inner Core: Solid state, with a transition zone around 5000km.

Lithosphere

The outermost layer, including the crust and the upper part of the mantle.

Asthenosphere

Located beneath the lithosphere in the mantle, partially molten.

Mesosphere

The part of the mantle extending below the asthenosphere to a depth of 2900km (Gutenberg discontinuity).

Endosphere

Equivalent to the core.