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what is the compositon, temperature, thickness, and density of earths crust
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Short answer up front
- Crust composition: two main types — continental (felsic to intermediate: granite-like) and oceanic (mafic: basalt/gabbro). Major elements are O, Si, Al, Fe, Ca, Na, K, Mg.
- Thickness: ~5–70 km total; oceanic ~6–10 km, continental ~25–70 km (average ~35–40 km).
- Density: continental ≈ 2.6–2.9 g/cm3 (≈2600–2900 kg/m3); oceanic ≈ 2.9–3.1 g/cm3 (≈2900–3100 kg/m3).
- Temperature: highly variable; near-surface is cold (0–20 °C), increases with depth at a geothermal gradient typically ~20–30 °C/km; typical temperatures at the base (Moho) are roughly 400–900 °C depending on location.
Details
1) Composition
- Continental crust: predominately felsic to intermediate rocks (granite, diorite, sedimentary rocks). Chemically dominated by oxygen and silicon, then aluminum, iron, calcium, sodium, potassium, magnesium. Typical bulk (upper continental) by weight: O ~46%, Si ~28%, Al ~8%, Fe ~5%, Ca ~3–4%, Na ~3%, K ~2–3%, Mg ~2%.
- Oceanic crust: mafic rocks (basalt at the surface, gabbro deeper). Higher Fe and Mg and Ca than continental crust; lower Si, Al, K. Typical mafic bulk composition gives higher density than continental crust.
- Note: crust is compositionally layered (sediments and igneous rocks at top, intermediate/mafic lower crust) and changes with tectonic setting and alteration.
2) Thickness
- Oceanic crust: typically ~6–10 km (thin, formed at mid-ocean ridges).
- Continental crust: averages ~35–40 km, ranges ~25 km in stable cratons to 60–70 km or more under major mountain belts (e.g., Himalaya).
- Overall crustal thickness worldwide: roughly 5–70 km.
3) Density
- Continental crust average: ≈2.6–2.9 g/cm3 (2600–2900 kg/m3). Upper crust typically lower (~2.6–2.7 g/cm3), lower crust somewhat higher.
- Oceanic crust average: ≈2.9–3.1 g/cm3 (2900–3100 kg/m3); oceanic lithosphere as a whole can be a bit denser as it cools and thickens.
- By comparison, the upper mantle (peridotite) has density ≈3.3 g/cm3, which is why denser oceanic crust sits lower and is subducted more readily.
4) Temperature
- Surface: ambient (0–25 °C).
- Typical continental geothermal gradient: ~20–30 °C per km (varies widely). Oceanic gradients can be higher near ridges and lower in old, cold lithosphere.
- Example rough temperatures: at 10 km depth ~200–350 °C typical; at 35 km (typical continental Moho) ~500–1000 °C depending on gradient and heat flow. Moho temperatures are often quoted in the 400–900 °C range.
- Temperatures depend strongly on tectonic setting (high under rifts and hotspots, lower in stable cratons).
If you want, I can give specific examples (e.g., crustal thickness, Moho temperature, and composition under a continent like North America versus an oceanic plate) or provide plots/tables of typical geothermal gradients.
- Crust composition: two main types — continental (felsic to intermediate: granite-like) and oceanic (mafic: basalt/gabbro). Major elements are O, Si, Al, Fe, Ca, Na, K, Mg.
- Thickness: ~5–70 km total; oceanic ~6–10 km, continental ~25–70 km (average ~35–40 km).
- Density: continental ≈ 2.6–2.9 g/cm3 (≈2600–2900 kg/m3); oceanic ≈ 2.9–3.1 g/cm3 (≈2900–3100 kg/m3).
- Temperature: highly variable; near-surface is cold (0–20 °C), increases with depth at a geothermal gradient typically ~20–30 °C/km; typical temperatures at the base (Moho) are roughly 400–900 °C depending on location.
Details
1) Composition
- Continental crust: predominately felsic to intermediate rocks (granite, diorite, sedimentary rocks). Chemically dominated by oxygen and silicon, then aluminum, iron, calcium, sodium, potassium, magnesium. Typical bulk (upper continental) by weight: O ~46%, Si ~28%, Al ~8%, Fe ~5%, Ca ~3–4%, Na ~3%, K ~2–3%, Mg ~2%.
- Oceanic crust: mafic rocks (basalt at the surface, gabbro deeper). Higher Fe and Mg and Ca than continental crust; lower Si, Al, K. Typical mafic bulk composition gives higher density than continental crust.
- Note: crust is compositionally layered (sediments and igneous rocks at top, intermediate/mafic lower crust) and changes with tectonic setting and alteration.
2) Thickness
- Oceanic crust: typically ~6–10 km (thin, formed at mid-ocean ridges).
- Continental crust: averages ~35–40 km, ranges ~25 km in stable cratons to 60–70 km or more under major mountain belts (e.g., Himalaya).
- Overall crustal thickness worldwide: roughly 5–70 km.
3) Density
- Continental crust average: ≈2.6–2.9 g/cm3 (2600–2900 kg/m3). Upper crust typically lower (~2.6–2.7 g/cm3), lower crust somewhat higher.
- Oceanic crust average: ≈2.9–3.1 g/cm3 (2900–3100 kg/m3); oceanic lithosphere as a whole can be a bit denser as it cools and thickens.
- By comparison, the upper mantle (peridotite) has density ≈3.3 g/cm3, which is why denser oceanic crust sits lower and is subducted more readily.
4) Temperature
- Surface: ambient (0–25 °C).
- Typical continental geothermal gradient: ~20–30 °C per km (varies widely). Oceanic gradients can be higher near ridges and lower in old, cold lithosphere.
- Example rough temperatures: at 10 km depth ~200–350 °C typical; at 35 km (typical continental Moho) ~500–1000 °C depending on gradient and heat flow. Moho temperatures are often quoted in the 400–900 °C range.
- Temperatures depend strongly on tectonic setting (high under rifts and hotspots, lower in stable cratons).
If you want, I can give specific examples (e.g., crustal thickness, Moho temperature, and composition under a continent like North America versus an oceanic plate) or provide plots/tables of typical geothermal gradients.
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