The Crust

Summary

General Characteristics

• Represents <1% of Earth’s mass
• Two main types:
  • Oceanic crust — basaltic, dense, thin (≈ 6–7 km)
  • Continental crust — granitic/intermediate, thick (up to 70 km)
• Enriched in incompatible and lithophile elements (K, Rb, Th, U, LREE)

Oceanic Crust Composition

• Dominant rocks: MORB basalt, gabbro, underlying depleted peridotite
• Typical major-element composition:
  • SiO₂ ~ 50 wt%
  • MgO ~ 7–10 wt%
  • FeO ~ 8–10 wt%
  • CaO ~ 10–12 wt%
• Chemically depleted in incompatible elements compared to continental crust.

Continental Crust Composition

• Dominated by granites, granodiorites, tonalites, gneisses
• Enriched in:
  • Alkalis (K, Na)
  • Incompatible trace elements (LREE, Th, U)
• Highly heterogeneous due to crustal recycling and magmatic differentiation.

Overview

The Crust is the outermost layer of earth. And is also the outer part of the Lithosphere next to the upper part of the Mantle. Isostasy describes the physical, chemical and mechanical differences between the mantle and crust, that allow the crust to "float" on the more malleable mantle. Not all regions of Earth are balanced in isostatic equilibrium, it depends on the density and thickness of the crust, and the dynamic forces at work in the mantle.

The temperature in the crust also varies. The temperature near the Moho ranges from 200° to 400°.

Oceanic Crust

Extend ca. 5 km (3-6 km) under the ocean floor. Mostly composed of different types of Basalts. The most abundant minerals are silicate and magnesium.

Oceanic crust is formed at mid-oceanic ridges where tectonic plates are tearing apart from each other. Magma wells up from these rifts, cools and becomes young oceanic crust. Therefore, changes the age and density of oceanic crusts with distance to the mid-ocean ridges.

The oceanic crust gets destroyed in subduction zones. At these zones, the crust gets pushed under the continental crust. It sinks in to the mantle, becomes plastic and ductile. The minerals maybe recycled as crust-making lava.

Continental Crust

Mostly composed of different types of Granite. Mostly composed of silicate and aluminum. It is thicker than the oceanic crust (70 km) but less dense.

It is also getting created by plate tectonics. Plates crush into each other, thrusts up and builds mountains. The thickest parts of the crust are at mountain ranges. Like icebergs the tall peaks the crust extends deep under the mountains.

Continental crust is mostly much older that oceanic crust. Because it gets rarely destroyed and recycled.

Boundary between Crust and Upper Mantel

The Crust is the outermost layer of earth. The knowledge of the layers of the earth comes from the study of P- and S-Waves.

The boundary between crust and mantle is the Moho. It separates rocks with 6-7 km/s P-Wave speed and rocks with 8 km/s P-Wave speed.

Question and Answers

What percentage of earths mass is the Crust?::1 %

What are the two crust types?::Oceanic Crust and Continental Crust

What are the most common minerals in the oceanic crust?::Silicate and Magnesium

What are the most common minerals in the continental crust?::Silicate and Aluminum

What is the oceanic crust mostly made of?::Basalt

What is the continental crust mostly made of?::Granite

Typical major-element composition in the oceanic crust?
?
Element compositions:

• SiO₂ ~ 50 wt%
• MgO ~ 7–10 wt%
• FeO ~ 8–10 wt%
• CaO ~ 10–12 wt%