L3 - Layers of the Earth and the Atmosphere

Definitions of Earth Layers

Layers can be defined based on:

• Composition: Differences in chemical makeup (e.g., mantle vs. core).

• Mechanical properties: How layers deform (e.g., lithosphere vs. asthenosphere).

Compositional Layers

• Mantle vs. Core: The mantle is primarily iron-magnesium silicate rock, while the core is mostly iron.

• Crust vs. Mantle: Compositional differences exist between the crust (oceanic or continental) and the mantle.

Oceanic Crust vs. Continental Crust

Oceanic and continental crust differ in age, structure, composition, and rheology.

Mechanical Layers

• Lithosphere: Includes the crust and uppermost mantle, behaving as a rigid plate.

• Asthenosphere: A warmer, more ductile part of the mantle beneath the lithosphere.

• Mesosphere: A more rigid part of the mantle below the asthenosphere (term not universally used).

• Outer Core vs. Inner Core: Primarily iron with some nickel and light elements. The outer core is liquid, while the inner core is solid due to cooling and freezing. The solid inner core may rotate differentially with respect to the mantle. The inner core may exhibit a Schlichter mode where it moves back and forth slightly.

Rigidity vs. Ductility

These properties depend on the timescale:

• Seismic waves: The mantle appears very rigid.

• Mantle convection (millions of years): The mantle behaves like a fluid.

Outer Core

The outer core is convecting and is responsible for generating Earth's magnetic field. Material moves at a rate similar to that of a snail or slug.

Phase Changes

Rocks undergo phase transitions to more compact structures under high pressure and temperature. Examples include:

• 410 km depth: Transition to Wadsleyite.

• 520 km depth: Transition to Ringwoodite.

• 660 km depth: Transition to Bridgmanite.

Earth's Structure

The Earth has distinct layers:

• Crust (thin or thick).

• Lithospheric mantle.

• Upper mantle.

• Transition zone (where phase changes occur).

• Lower mantle (relatively uneventful).

• Core-mantle boundary (complex).

• Outer core.

• Inner core.

How We Know About Earth's Structure

• Seismology: Imaging using seismic waves.

• Electromagnetics: Imaging using electromagnetic properties.

• Xenoliths: Deep mantle rocks brought to the surface.

• Mineral physics: Simulating Earth materials under high pressure and temperature.

• Diamonds: Inclusions within diamonds provide samples from great depths.

Kimberlite Pipes

Diamonds are brought to the surface rapidly in kimberlite pipes, reaching speeds of Mach 2.

Mineral Physics Techniques

1. Diamond Anvil Cell:

   • Material is placed between two diamonds and subjected to high pressure (up to core pressures).

   • Heated with lasers (up to 3000-4000 degrees).

   • Probed with X-rays to determine material properties (e.g., $V_p$, $V_s$).

2. Numerical Simulations (Density Functional Theory):

   • Determine the most stable configuration of materials (e.g., silica, iron, oxygen) under specific pressure and temperature conditions.

   • Requires large computers.

Atmosphere

Atmospheric Layers

• Troposphere: Contains most of the atmospheric mass, crucial for life and cloud formation (up to 10-12 km).

• Stratosphere: Contains the ozone layer, which absorbs solar radiation. The recovery of the ozone layer after CFC reduction demonstrates the possibility of reversing environmental damage through human action.

• Mesosphere and Thermosphere: Contain ionized gas, important for radio communications.

• Exosphere: Extends into outer space (above 100 km).

Ionosphere

• Extends from about 50 km to 1000 km.

• Elements are ionized by solar radiation.

• Important for radio communication (as demonstrated by Marconi's transatlantic radio signals).

Van Allen Belts

Zones of charged particles held in place by the magnetosphere, shielding Earth from solar radiation.

Atmospheric Composition

Comparison of planetary atmospheres:

Formation of the Atmosphere

Possible sources:

• Residual gases after planetary accretion.

• Extraterrestrial sources.

• Degassing of the Earth by volcanism (key factor).

Geochemical evidence suggests the atmosphere primarily formed from degassing, due to the isotopic constraints.

Argon-40 is found on the Earth in abundance, making up over 99% of the argon, compared to the solar and chondritic ratios of abundantly argon-36 and argon-38. This arises as argon-40 is produced via the radiactive decay of potassium-40 and escapes into the atmosphere via volcanism

Primordial Atmosphere

Likely contained hydrogen and helium, but these were lost due to high temperatures and the moon-forming impact. Other gases such as CH₄ are also depleted compared to chondritic ratios suggesting that it wasn’t retained due to low gravity, or it could have been lost due to the Moon forming impact

Archean Atmosphere

• Primarily formed from degassing.

• Some contribution from weathering exposed rocks.

• Gases produced during volcanism: hydrogen, carbon dioxide, sulfur, chlorine, ammonium, methane.

• No free oxygen at the time

Water produced through degassing formed the oceans around 4 billion years ago.

Faint Young Sun Paradox

The sun had lower luminosity in the early stages of planetary formation, leading to colder temperatures. However, geological evidence suggests liquid water existed.

Explanation: Greenhouse gases warmed the Earth, counteracting the lower solar luminosity.

Water and Plate Tectonics

• Water is essential for life and possibly for plate tectonics.

• Subduction recycles major and trace elements, water, $CO_2$, and other gases.

• Subduction is more efficient earlier in Earth's history.

• Hydration impacts mantle rheology and plate tectonic styles.

Building Continental Crust: Accretion of plates in island arc settings, differentiating crust over time.

Crust: Continental vs. Oceanic

Continental Crust

• 33% is in the continental shelf.

• Varies in thickness.

• Thick continental roots under regions like Southern Africa and Siberia.

• Compositionally different from oceanic crust.

• Mechanically resistant to erosion.

Difficult to drill deep into due to high temperatures and borehole instability. Plasma drilling is a potential solution.

How we study it:

• Seismic experiments.

• Xenoliths.

• Exhumed rocks from ancient mountain belts.

Formation: Island arc settings, differentiation of crust as volatiles rise through the system.

Structure of Upper Continental Crust (UCC)

• Most accessible part, but heterogeneous and differentiated.

• Shields and cratons: Isostatically equilibrated, well-defined structure.

• Orogenic belts: Complex.

• Makes up 33% of the crustal thickness

• Uppermost section is sedimentary/volcanic

• Lowermost UCC is low grade metamorphic and granites

• Average composition of granodiorite (dominated by quartz, feldspar, micas, amphiboles, chlorite).

Structure of the Lower Continental Crust (LCC)

• Not directly accessible except in rare circumstances

• Middle crust (MC) - 10-25km

• Lower crust (LC) - Below 25km

• LC composed of granulite facies rocks and MC of amphibolite facies rocks.

  • Both common as xenoliths and Precambrian terranes

• Much debate about bulk composition of LC as it may be more felsic than once though based on geophysical data.

• Variable thickness in different tectonic settings

Composition

• Upper crust: Granodiorite

• Middle/Lower crust: Granulate facies and amphibolite facies

Oceanic Crust

• Studied through seismic studies and the Ocean Drilling Project.

• Ophiolites provide insights into oceanic crust structure (e.g., Oman, Cyprus).

• Consistent thickness of 6-7 km with 3 principle layers

Structure

• Sediments (Layer 1).

• Pillows and dykes (Layer 2)

  • Pillow basalts (extrusive volcanics) (Layer 2a).

  • Sheeted dykes (volcanic intrusions) (Layer 2b).

• Sheeted dyke complex and gabbros (Layer 3).

Mohorovičić Discontinuity

• Named after Andrija Mohorovičić, who identified it using seismic waves.

• Represents an abrupt increase in seismic velocity.

• Seismological and petrological Moho may not always coincide.

Layer thickness varies depending on spreading rate. Fast-spreading centers have thinner crust; slow-spreading centers have thicker, more complex crust.

Compositional Layers:

• Layer 1: Sediments

  • Thin, ~400 meters, increasing away from the ridge.

  • Extensive sampling through cores and drilling

  • Unconsolidated deposits

    • Terrigenous sediments carries by tubidity currents

    • Pelagic deposits originating from the pelagic column

  • Differences in thicknesses and lithotypes between Atlantic/Indian and Pacific Oceans.

• Layer 2: Pillow basalts and dykes

  • Variable thickness: 1-2.5 km

    • Basaltic extrusives and shallow intrusives

    • Layer 2A

      • Pillow basalts and sheeted lavas that are porous and permeable

    • Layer 2B

      • Less porous basalts with a higher seismic velocity than 2A. Old, cold 2A converted via precipitation of secondary minerals

    • Layer 2C

      • Sheeted dykes

• Layer 3: Plutonics.

  • Main component of oceanic crust by volume

  • Upper layer (3A) of isotropic gabbro and lower layer (3B) of cumulate (layered) gabbro and ultramafic rocks due to gravity settling

  • Possible ‘plagiogranites’ formed at the top of horizon being the last felsic material to crystallise out of the melt. Contain zircons in abundance.

• The Moho

  • Dividing line between crust and mantle seen in both oceanic and continental crust

  • Defined based on sharp increase at crust-mantle boundary (seismic moho)

  • Also attributed to a change in mineralogy (petrological moho)

  • Sometimes coincide, sometimes don’t

Slow vs. Fast Spreading Ridges

• Fast-spreading: Simpler structure, smoother, longer segments.

• Slow-spreading: Axial valleys, more fissure volcanism, complicated geometry.

Fundamental Differences between Crusts

• Continental has ill defined layers due to complex history and high variable thickness.

• Oceanic has well defined horizons with consistent thickness despite differences in geological setting

Age of Rocks

Oldest known rocks on Earth are in Canada found as the Acasta Gneiss.

Metamorphism occured about 4 Ga.

Oldest known mineral are zircons from Australia found in metasediments. Age is 4.4 Ga

Oceanic crust is generally <200 Ma, but a fragment has been found as old as 340 Ma

Plate Boundaries

Divergent, convergent, transform.

Divergent Boundaries

• Plates move apart, melts upwell, new crust forms.

• Ocean-ocean spreading (e.g., Mid-Atlantic Ridge).

• Continent-continent rifting (e.g., East African Rift).

Continental Rifting

Driven by gravitational tension energy due to upwelling of hot mantle.

Convergent Boundaries

• Ocean-ocean (e.g., Mariana Trench): One plate subducts beneath the other, forming an island arc.

• Continent-ocean (e.g., Andes): Oceanic plate subducts under continental plate, causing volcanism.

• Continent-continent (e.g., Himalayas): Intense deformation, thickest crust, debates over subduction vs. interlayering.

Transform Boundaries

• Oceanic (e.g., ocean ridge transform faults): Connect ridge segments, high seismicity.

• Continental (e.g., San Andreas Fault): Continents slide past each other, strong seismicity, low volcanism.

Intraplate Seismicity

• Seismicity within plates (continental, oceanic).

• Hawaiian hotspot: Seismicity along the hotspot trend, volcanism.