Plate Tectonics VSI

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Lithosphere takes tens of millions of years to warm up when subducted into the asthenosphere.
Author

Stephen J. Mildenhall

Published

2024-06-21

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Plate Tectonics

  • p.35: speed of spread mid-Ocean affects the depth of the ocean and accounts for some of the different “sea levels” observed historically.
  • p.97: there is a big difference between continental and oceanic earthquakes
  • Subduction: think of piece of paper bending over the edge of a table; it rises before it falls, amount of rise depends on how thick it is; in extension at the top and compression at the bottom around the bend
  • Subducted crust is colder and takes 10M+ years to warm up!
  • Understand oceanic rigid plates much better than continental plates
  • Water impurities, sucked down by subduction, lower melting points, resulting in liquid rock that then forms volcanoes
  • GPS confirms plates are rigid
  • India scrunching into Eurasian plate, Himalayas “skim” top surface off
  • Rockies are tricky to understand: mystery why the great planes and Denver “float” so high - where is the lost mass?

GPT Notes follow.

Compensating Deficit of Mass

Unlike the Andes, where thick crust provides compensating deficit of mass, the compensating deficit of mass at depth beneath the Rockies seems to lie largely within the uppermost mantle.

Concept of Buoyancy and Isostasy

Think of the Earth’s crust as floating on the denser, semi-fluid mantle, similar to how icebergs float on water. The principle of buoyancy, as described by Archimedes’ principle, states that an object immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the object.

Application to the Andes

  1. Thick Crust: The Andes have a significantly thick crust. This thickened crust, although less dense compared to the mantle, provides a larger volume of material that needs to be supported by buoyant forces from the mantle.

  2. Isostatic Equilibrium: To maintain isostatic equilibrium (gravitational balance), the thicker crust means there is more crustal material “floating” above the mantle. Since the crust is less dense than the mantle, the thicker it is, the higher it will stand, similar to how a thicker piece of foam will float higher in water than a thinner one.

  3. Mass Deficit: The thicker crust effectively creates a mass deficit at depth because the crustal material is less dense than mantle material. This deficit in mass at depth (relative to the surrounding mantle) helps balance the high elevation of the Andes.

  • The thick crust is less dense than the mantle and creates a buoyant force that helps the Andes stand high.

  • The thick crust itself creates a compensating mass deficit needed for isostatic equilibrium.

Application to the Rockies

  1. Less Thick Crust: The Rockies have a crust that is not as thick as that of the Andes.

  2. Upper Mantle Contribution: Instead of relying solely on a thick crust to provide the necessary mass deficit for isostatic equilibrium, the Rockies achieve this balance through a mass deficit within the uppermost mantle. This means there are regions within the upper mantle that are less dense or have lower mass, contributing to the buoyancy required to support the elevation of the Rockies.

  • The crust is not as thick, so the compensating mass deficit is largely within the upper mantle.

  • The less dense upper mantle regions provide the buoyancy required to maintain the Rockies’ elevation.

In both cases, the principle of isostasy ensures that there is a balance between the mass above (mountains) and the deficit below (in the crust or mantle), maintaining gravitational equilibrium.

The Earth’s Layers

Here’s a detailed description of the Earth’s layers from the surface down to the core.

  1. Crust
    • Continental Crust:
    • Oceanic Crust:
  2. Lithosphere
  3. Asthenosphere
  4. Mantle
    • Upper Mantle:
    • Lower Mantle:
  5. Outer Core
  6. Inner Core

Note there are overlapping terms:

  • Crust: The outermost solid layer of the Earth.
    • Not part of the mantle.
  • Lithosphere: A rigid layer that includes:
    • Crust (both continental and oceanic).
    • Uppermost part of the mantle.
  • Asthenosphere: A semi-fluid layer beneath the lithosphere.
    • Part of the upper mantle.
|-----------------------|
|       Crust           |  <- Not part of the mantle
|-----------------------|
|  Lithospheric Mantle  |  <- Part of the lithosphere, uppermost mantle
|-----------------------|
|     Asthenosphere     |  <- Part of the upper mantle
|-----------------------|
|    Rest of Mantle     |  <- Includes lower mantle
|-----------------------|
  • The lithosphere contains the crust and the uppermost mantle.
  • The asthenosphere is a part of the upper mantle, just below the lithosphere.
  • The crust is outside the mantle, but it is included in the lithosphere.

Age by Layer

Continental Crust

  • Average Age: The continental crust is generally much older than the oceanic crust. The age of continental crust varies widely, but large portions are over 1 billion years old. Some parts, known as cratons, can be as old as 3.5-4 billion years.
  • Formation: Continental crust forms through a combination of processes, including volcanic activity, sedimentation, and tectonic plate interactions. It accumulates and is recycled less frequently than oceanic crust, which contributes to its greater age.
  • Key Regions: The oldest continental rocks are found in cratons, such as the Canadian Shield, the African Shield, and parts of the Australian Shield.

Oceanic Crust

  • Average Age: The oceanic crust is significantly younger than the continental crust. It is continuously created at mid-ocean ridges and recycled into the mantle at subduction zones. The oldest oceanic crust is about 200 million years old.
  • Formation: Oceanic crust forms at mid-ocean ridges through the process of seafloor spreading. As tectonic plates pull apart, magma rises from the mantle, cools, and solidifies to form new oceanic crust.
  • Recycling: Due to subduction, where one tectonic plate sinks beneath another, oceanic crust is constantly being recycled into the mantle. This recycling process ensures that oceanic crust does not get as old as continental crust.

Lithosphere

  • Age Variation: The lithosphere, which includes both the crust and the uppermost solid part of the mantle, varies in age depending on whether it is beneath continents or oceans.
    • Continental Lithosphere: Corresponds in age to the continental crust, often exceeding 1 billion years.
    • Oceanic Lithosphere: Corresponds in age to the oceanic crust, generally less than 200 million years old.

Upper Mantle

  • Age and Dynamics: The upper mantle beneath the lithosphere is involved in convective processes that drive plate tectonics. While individual mantle rocks can be very old, the convective movement means that the material in the upper mantle is continually being mixed and recycled over geological time scales.
  • Isotopic Studies: Studies of isotopes in mantle-derived rocks (like basalt from mid-ocean ridges and volcanic islands) indicate that the mantle source regions can have ages reflecting long-term chemical differentiation, often in the range of hundreds of millions to billions of years.

Age Summary

  • Continental Crust: Average age > 1 billion years, with some regions up to 4 billion years.
  • Oceanic Crust: Average age < 200 million years, continuously recycled.
  • Lithosphere: Age varies by type (continental or oceanic) and reflects the age of the crust.
  • Upper Mantle: Material is ancient but dynamically mixed and recycled.

Temperature Distribution in the Continental Crust

  • Surface: The temperature at the Earth’s surface is highly variable and depends on the local climate, but it generally ranges from -50°C in polar regions to 50°C in deserts.
  • Shallow Crust (0-10 km depth):
    • Temperature Range: 0°C to 200°C
    • Details: The temperature increases with depth due to the geothermal gradient, which averages about 25-30°C per kilometer of depth in the continental crust.
  • Mid to Deep Crust (10-35 km depth):
    • Temperature Range: 200°C to 400°C
    • Details: The temperature continues to increase with depth. By the time we reach the base of the continental crust, temperatures can range from 400°C to even higher, depending on the local geothermal gradient and tectonic activity.

Example of Temperature Gradient

A simplified temperature gradient in the continental crust could be summarized as follows:

  • 0 km (Surface): ~15°C (average global surface temperature)
  • 5 km: ~125-150°C
  • 10 km: ~250-300°C
  • 20 km: ~400-600°C
  • 35-40 km (Base of Continental Crust): ~700-900°C

Factors Affecting Temperature

  1. Geothermal Gradient: The rate at which temperature increases with depth. This gradient can vary significantly based on geological settings, such as tectonic plate boundaries, hotspots, or stable cratons.
  2. Heat Flow: The movement of heat from the Earth’s interior to the surface. Areas with high heat flow, such as volcanic regions or rift zones, have higher temperatures at shallower depths.
  3. Radioactive Decay: The decay of radioactive isotopes within the crust generates heat, contributing to the temperature increase with depth.

Temperature Summary

  • Surface Temperature: Varies based on climate, generally -50°C to 50°C.
  • Shallow Crust (0-10 km): 0°C to 200°C.
  • Mid to Deep Crust (10-35 km): 200°C to 400°C, with temperatures reaching 700-900°C at the base of the crust.

Thickness, State, Temperature, Density, Pressure, Volume and Mass of Earth’s Layers

Layer Thickness (km) State Temperature (°C) Density (g/cm³) Pressure (GPa) Relative Volume (%) Relative Mass (%)
Continental Crust 35-40 Solid 200-400 2.7 0.2-1.0 ~0.4 ~0.6
Oceanic Crust 5-10 Solid 0-400 3.0 0.1-0.5 ~0.1 ~0.2
Lithosphere 70-100 (incl. crust) Solid Up to 500 2.7-3.3 0.1-1.5 ~1.0 ~2.0
Asthenosphere ~100-200 Plastic 500-1300 3.3-3.5 1.5-3.5 ~6.0 ~7.5
Upper Mantle To 660 Solid/Plastic 500-1600 3.3-4.5 1.5-23.0 ~10.3 ~10.3
Lower Mantle 660-2900 Solid 1600-4000 4.5-5.6 23.0-136.0 ~72.9 ~67.1
Outer Core 2250 Liquid 4000-5700 9.9-12.2 136.0-330.0 ~15.6 ~30.8
Inner Core 1220 radius Solid 5700-7000 12.6-13.0 330.0-360.0 ~0.7 ~1.7

Common Elements of Earth’s Layers

Layer Thickness (km) Common Elements
Continental Crust 35-40 Oxygen (O), Silicon (Si), Aluminum (Al), Iron (Fe), Calcium (Ca), Sodium (Na), Potassium (K), Magnesium (Mg)
Oceanic Crust 5-10 Oxygen (O), Silicon (Si), Aluminum (Al), Iron (Fe), Magnesium (Mg), Calcium (Ca)
Lithosphere 70-100 Oxygen (O), Silicon (Si), Magnesium (Mg), Iron (Fe), Aluminum (Al), Calcium (Ca)
Asthenosphere ~100-200 Oxygen (O), Silicon (Si), Magnesium (Mg), Iron (Fe)
Upper Mantle To 660 Oxygen (O), Silicon (Si), Magnesium (Mg), Iron (Fe)
Lower Mantle 660-2900 Oxygen (O), Silicon (Si), Magnesium (Mg), Iron (Fe)
Outer Core 2250 Iron (Fe), Nickel (Ni), Sulfur (S), Oxygen (O)
Inner Core 1220 radius Iron (Fe), Nickel (Ni)

Notes

  • Relative Volume (%): Represents the volume of each layer as a percentage of the Earth’s total volume.
  • Relative Mass (%): Represents the mass of each layer as a percentage of the Earth’s total mass.
  • Density of Iron at STP:
    • Standard Temperature and Pressure: Defined as 0°C (273.15 K) and 1 atmosphere (101.325 kPa).
    • Density: Approximately 7.87 g/cm³.
  • Density of Iron in the Center of the Earth:
    • Conditions: Extremely high pressures (330-360 GPa) and temperatures (5700-7000°C).
    • Density: Approximately 12.6-13.0 g/cm³.

Deets

  • Peter Molnar
  • Volume 425
  • Published 2015