Notes on Earth’s Internal Structure, Plate Tectonics, and Natural Hazards

Internal Structure of Earth

• Earth is layered and dynamic.

• Internal structure can be differentiated in two ways:

  • By composition and density (heavy or light; mafic or silicic).

  • By physical properties (solid or liquid; weak or strong).

• Radius: R \approx 6730 \,\text{km}

• Core:

  • Thickness: t_{core} > 3440 \,\text{km} (≈ 2120 mi)

  • Very high temperature

  • Composition: iron (~90% by weight) with sulfur, oxygen, nickel

  • Density: \rho_{core} \approx 10.7 \,\text{g/cm}^3

• Mantle:

  • Solid

  • Thickness: t_{mantle} \approx 2900 \,\text{km} (≈ 1800 mi)

  • Composition: iron- and magnesium-rich silicate rocks

  • Average density: \rho_{mantle} \approx 4.5 \,\text{g/cm}^3

• Crust (outermost layer):

  • Variable thickness

  • Average density: \rho_{crust} \approx 3.0 \,\text{g/cm}^3

  • Moho discontinuity separates crust from mantle

• By physical properties:

  • Inner core: solid; thickness > 1200 \,\text{km}

  • Outer core: liquid; thickness > 2200 \,\text{km}

  • Composition similar to inner core

  • Mesosphere: strong & rigid

• Lithosphere vs Asthenosphere:

  • Asthenosphere: hot, weak and plastic; capable of slow flow

  • Lithosphere: cool, strong and rigid; includes crust + outer mantle; thickness 20-400 \,\text{km} (variable; thinner beneath mid-ocean ridges)

Continental vs Oceanic Crust

• Continental Crust:

  • Thickness: averages 35 \,\text{km} (range 20–70 km)

  • Old: up to several billion years

  • Density: \rho_{continental} \approx 2.7 \,\text{g/cm}^3

  • Composition: granitic basement rocks

• Oceanic Crust:

  • Thickness: 6-7 \,\text{km}

  • Young: < 200 \,\text{million years} old

  • Density: \rho_{oceanic} \approx 3.0 \,\text{g/cm}^3

  • Composition: basaltic

How We Know About the Internal Structure of Earth

• Primary method: seismology (study of earthquakes)

• Seismic waves reveal

  • Movement through solids, not liquids for some wave types

  • Reflection and refraction patterns indicate internal boundaries

  • Travel times and paths infer structure, physical properties, and composition

• Key conclusions:

  • Earth’s internal structure is more complex than once thought.

  • Magma generation in the asthenosphere.

  • Existence of sinking lithospheric slabs.

  • Lithosphere thickness is highly variable due to age/history.

  • Plate tectonics is driven by internal processes.

Plate Tectonics: Basics

• Plate Tectonics: large-scale geologic processes that deform Earth’s lithosphere.

• Plates: pieces of Earth's lithosphere that move over the asthenosphere.

• Major plates (seven):

  • North American, South American, Pacific, Eurasian, African, Indo-Australian, Antarctic

• Minor plates: a dozen or so (not listed exhaustively here).

• Core questions: Is this process internal or external? (Plate motions are driven by internal processes.)

Evidence for Plate Tectonics

• Continental Drift (proposed by Alfred Wegener, 1915):

  • Evidence:

  • The fit of Africa and South America

  • Restricted plant and animal fossils across now-distant landmasses

  • Ancient glacial deposits on southern continents

  • Problem: driving mechanism for continental movement was not plausible, leading to temporary abandonment of the hypothesis.

• Subsequent evidence integrated into Plate Tectonics theory:

  • Seafloor spreading (1950s–1960s) and sea-floor investigations

  • Paleomagnetism and magnetic reversals

History of Plate Tectonics: Key Milestones

• 1950s–1960s: Exploration of the seafloor (Glomar Challenger) using sonar for bathymetry

  • Discovery of oceanic ridges and deep-sea trenches

• 1962: Theory of Sea Floor Spreading (Harry Hess)

  • New ocean crust is created at oceanic ridges

  • Crust moves laterally from ridges to trenches

  • Ocean crust is destroyed at trenches

• Paleomagnetism: study of rock remnant magnetism; Earth’s magnetic field has polarity reversals

• 1960s: Magnetic surveys revealed stripes of alternating normal and reversed magnetic fields on the seafloor

  • Stripe pattern: polarity changes correspond to geomagnetic reversals over time

• Consequences: support for Sea Floor Spreading and Plate Tectonics; older oceanic crust is no older than ~2.0 imes 10^2 ext{ million years}; crust is destroyed at trenches

• Paleomagnetic dating and ocean-floor mapping confirmed seafloor spreading and plate motions

Paleomagnetism and Magnetic Reversals

• Paleomagnetism: rocks preserve the direction of Earth’s magnetic field when they cool

  • Iron-bearing minerals record the field at the Curie Point

  • Provides a history of magnetic field direction and polarity

• Magnetic reversals:

  • Rocks show instances of reversed magnetism (N-S vs S-N)

  • Magnetic history shows field reversals many times in the past

  • Reversals are cyclic but random in timing; average interval is on the order of a few hundred thousand years

  • Polarity change takes a few thousand years

• Evidence from seafloor magnetic anomalies: support for time-based stripe patterns used to infer ages of oceanic rocks

Magnetic Mapping and Seafloor Evidence for Plate Tectonics

• Key observations:

  • Ocean ridges have associated magnetic anomalies that run parallel to ridges

  • Seafloor rocks at ridges are youngest; rocks become progressively older away from ridges

  • Sediment thickness increases with distance from ridges

• Implications:

  • Seafloor spreading at mid-ocean ridges pushes newer rocks outward

  • Subduction recycles older oceanic crust at trenches

• Age of ocean floor:

  • No oceanic crust older than about 2 imes 10^2 ext{ Ma}

Mapping Tectonic Boundaries and Plate Motions

• Boundary types (three):

  • Divergent: plates move apart; new lithosphere created at ridges

  • Convergent: plates move toward one another; subduction or collision

  • Transform: plates slide past one another

• Key boundary features and examples:

  • Divergent: Mid-Atlantic Ridge; Great Rift Valley (Africa); rift zones in Papua New Guinea

  • Convergent: Nazca Plate with South American Plate; Andean mountain chain; Himalayas (continent-continent collision)

  • Transform: San Andreas Fault (North American vs Pacific plates)

• General boundary dynamics:

  • Divergent: spreading, ridge creation

  • Convergent: subduction zones with volcanic arcs and trenches, or continental collision with uplift of mountains

  • Transform: earthquakes due to friction and slipping

Oceanic-Oceanic, Oceanic-Continental, and Continental-Continental Convergence

• Ocean-Ocean Convergent: one oceanic plate subducts beneath another creating trenches, volcanic island arcs, and earthquakes

• Ocean-Continent Convergent: oceanic plate subducts beneath continental plate; volcanic arcs form on the continent; trenches develop

• Continent-Continent Convergent: no subduction; crustal thickening and high mountain ranges (e.g., Himalayas)

• Transform boundaries: plates slide past; examples include San Andreas Fault

Subduction, Volcanoes, and Trench Formation

• Subduction zones: deep and shallow earthquakes; important for mountain building and volcanism

• Subducting plate is cool, dense, brittle; friction at contact triggers earthquakes; bending and angle of subduction can be inferred from earthquake depth data

• Wet sediments dragged down; subduction contributes to magmatic activity and volcanic arcs

• Volcanic activity associated with subduction zones due to melting of down-going crust and mantle material

Trench Regions and Deep Earthquakes

• Deep ocean trenches mark subduction zones and are among the deepest oceanic waters on Earth

• Earthquake depth and subduction angle can be constrained by seismic data

Transform Boundaries and Plate Boundary Interactions

• Transform boundaries: San Andreas Fault; transform faults offset mid-ocean ridges and continental boundaries

• Plate motion along transform boundaries can accompany divergent or convergent interactions elsewhere on the plate

Triple Junctions and Hot Spots

• Triple junctions: locations where three plate boundaries meet

• Hot spots: volcanic centers formed by hot mantle plumes rising through plates

  • Plates move over hot spots, creating volcanic chains

  • Seamounts can form as volcanoes grow underwater and then subside after movement over the hotspot

What Drives Plate Movement?

• Plate motion is driven by convection and related mantle dynamics

• Convection cycles:

  • Hot, low-density mantle material rises at mid-ocean ridges; forms new crust

  • Plates are carried by newly formed lithosphere away from ridges

  • At subduction zones, lithosphere is destroyed as it sinks back into mantle

• Heat sources:

  • Upwelling mantle at ridges

  • Heat from the core and mantle convection

  • Subduction zones feed back into mantle circulation

• Two proposed driving mechanisms:

  • Ridge push: gravitational push away from the crest of mid-ocean ridges

  • Slab pull: dense, cool oceanic slabs sinking into the mantle exert a pulling force on the trailing plate

• Evidence suggests that slab pull is the more important process in driving plate motion

The Plate Tectonics and the Rock Cycle

• Plate tectonics shape the present distribution of continents and oceans

• The most recent major break-up of a supercontinent occurred about 180 Ma, forming Laurasia (northern continents) and Gondwana (southern continents)

• Continental crust vs oceanic crust play different roles in tectonics and topography

The Rock Cycle

• A global recycling system of rocks driven by internal heat and plate tectonics and surface processes driven by solar energy

• Rock types:

  • Igneous: formed from crystallization of molten rock (magma or lava); usually silicate minerals; interlocking crystals

  • Sedimentary: formed from weathering debris and lithification; includes clastic and chemical classes

  • Metamorphic: rocks formed from pre-existing rocks under heat, pressure, and chemically active fluids; may be foliated or non-foliated

Igneous, Sedimentary, and Metamorphic Rocks: Key Details

• Igneous rocks:

  • Form from crystallization of molten material (magma or lava)

  • Typically silicate minerals; may contain dissolved gases and water

• Sedimentary rocks:

  • Formed from weathering, erosion, deposition, and lithification

  • Subtypes:

  • Clastic: derived from mechanical weathering; grain sizes vary

  • Chemical: minerals precipitate from solution (e.g., limestone, halite, calcite, gypsum)

• Metamorphic rocks:

  • Formed from existing rocks or minerals subjected to heat, pressure, and/or chemically active fluids

  • Metamorphic agents drive changes in texture and mineralogy

• Rock cycle concept: igneous, sedimentary, and metamorphic rocks transform from one type to another over geologic time via tectonics, weathering, and burial

Metamorphic Rocks: Classification and Examples

• Foliated metamorphic rocks (directed pressure):

  • Slate

  • Phyllite

  • Schist

  • Gneiss

• Non-foliated metamorphic rocks:

  • Marble

  • Quartzite

Weathering and Sedimentary Rocks

• Weathering processes:

  • Chemical weathering: chemical decomposition of rocks

  • Physical weathering: physical disintegration into smaller pieces

• Sediment types:

  • Particulate (clastic) sediment

  • Dissolved (chemical) sediment

• Clastic sedimentary rocks: formed from weathered fragments; examples include sandstone, shale, conglomerate (not exhaustively listed here)

• Chemical sedimentary rocks: formed by chemical precipitation from solution; examples include limestone, halite, calcite, gypsum

Earthquakes and Natural Hazards at Plate Boundaries

• Plate motion rates: typically a few centimeters per year

• Earthquake generation: due to stress build-up and sudden release of energy along plate boundaries

• Divergent boundaries (e.g., MORs): earthquakes can occur but are typically shallow and lower magnitude away from continental regions; generally lower hazard at MORs except where continental plates diverge

• Transform boundaries (e.g., San Andreas): frequent earthquakes due to sliding motion

• Convergent boundaries (subduction zones): high earthquake activity; strong earthquakes and volcanic activity; associated with mountain building (e.g., Himalayas)

• Weather and climate: topography (e.g., tall mountain ranges) can influence climate patterns, rain shadows, and regional weather

Conceptual Frameworks for Hazards and Risk

• Concept 1: Science helps predict hazards through plate tectonics evidence and hazard impacts (earthquakes, tsunamis, volcanoes, landslides, floods)

• Concept 2: Knowing hazard risks informs decision-making; hazards are linked to tectonic movement

• Concept 3: Linkages exist between natural hazards; tectonics lead to earthquakes and volcanoes, which can trigger tsunamis, landslides, and floods; topography also influences weather patterns

• Concept 4: Human activities can exacerbate disasters (e.g., population growth on hazard-prone coasts; proximity to volcanoes such as Pompeii, Kilauea, Mt. St. Helens elevates risk)

• Concept 5: Hazard consequences can be minimized with mitigation strategies (e.g., shake-resistant buildings, monitoring and evacuation planning at volcanoes, landslide prevention on steep slopes)

References and Notable Figures

• Jyotirmoy Paul (jyotirmoyp@iisc.ac.in), Indian Institute of Science

• Key citation: Paul, J. (2021), “Cratons, why are you still here?”, Eos, 102. https://doi.org/10.1029/2021EO156381

• Additional resources include classroom videos and articles on plate boundaries, seafloor spreading, and crustal evolution.

Quick Reference: Key Numerical Values

• Radius of Earth: R \approx 6730 \text{ km}

• Core thickness: t_{core} > 3440 \text{ km}

• Inner core thickness: > 1200 \text{ km}

• Outer core thickness: > 2200 \text{ km}

• Mantle thickness: \approx 2900 \text{ km}

• Crust densities: \rho{continental} \approx 2.7 \text{ g/cm}^3; \rho{oceanic} \approx 3.0 \text{ g/cm}^3; \rho_{crust} \approx 3.0 \text{ g/cm}^3\ (average)

• Lithosphere thickness: 20-400 \text{ km}

• Continental crust thickness: 20-70 \text{ km}

• Oceanic crust thickness: 6-7 \text{ km}

• Ocean floor age: no older than \sim 200 \text{ Ma}

• Magnetic reversals: average every few hundred thousand years; polarity change takes a few thousand years

• Plate motion rate: typically \text{a few cm/year}

• Timeframe for major break-up of Pangaea: ~180 \text{ Myr} ago

• Age of oldest exposed continental rocks: >3 \times 10^9 \text{ years} in cratons