Plate Tectonics I: Key Observations

Cosmological Origins and Earth Formation

• Origin of the Universe (The Big Bang Theory):
  • The universe originated approximately $14\,\text{Ga}$ ($14\,\text{billion years ago}$), expanding outward from a single, infinitely dense point.
  • Measuring Cosmic Expansion Velocity:
  • Distant galactic velocities are determined using red shifts (Doppler effect applied to light).
  • As objects move away from an observer, light shifts toward longer, lower-frequency wavelengths within the electromagnetic spectrum (from gamma rays, X-rays, and ultraviolet, through visible light, to infrared, microwaves, and radio waves).

• Measuring Cosmic Distance:
  • Cosmic distances are measured using standard candle stars, specifically Cepheid Variable Stars (e.g., Cepheid Variable Star V1 in M31, discovered by Henrietta Leavitt).
• Hubble's Law & The Raisin Bread Metaphor:
  • Plotting galactic distance ($x$ -axis, in megaparsecs $\text{Mpc}$) against velocity ($y$ -axis, in $\text{km/s}$) yields a linear relationship: the farther away a galaxy is, the faster it travels away from us.
  • This expansion is modeled as a rising loaf of raisin bread: as the dough expands overall (e.g., from $5\,\text{cm}$ to $10\,\text{cm}$ to $20\,\text{cm}$), raisins located further apart move away from each other at higher relative velocities.

• Geologic Time Nomenclature:

  • $\text{Ga}$ = Giga annum ($1\,\text{billion years}$, $10^9\,\text{years}$).
  • $\text{Ma}$ = Mega annum ($1\,\text{million years}$, $10^6\,\text{years}$).
  • $\text{Ka}$ = Kilo annum ($1\,\text{thousand years}$, $10^3\,\text{years}$).

• Nucleosynthesis and Element Distribution:

  • Primordial Composition: Immediately following the Big Bang, the universe consisted almost entirely of Hydrogen ($\text{H}$) and Helium ($\text{He}$), with trace amounts of Lithium ($\text{Li}$) and Beryllium ($\text{Be}$).
  • Stellar Nucleosynthesis: Heavier elements are formed through high-energy nuclear fusion reactions inside stars and released into space during supernovae explosions.

• Nebular Hypothesis for Solar System Formation:

  • Following a supernova, a localized cloud of gas and dust (a solar nebula) collapsed under gravity at approximately $4.55\,\text{Ga}$.
  • Conservation of Angular Momentum: As gravity pulled mass inward toward the center, the collapsing nebula spun faster ($L = I\omega = I'\omega'$), flattening into a rotating protoplanetary disk.

• Proto-Sun Formation: Central collapse triggered nuclear fusion, forming the Sun (surface temperature $>5000^\circ\text{C}$).

• Thermal Gradient & Condensation:

  • Inner Solar Nebula ($>1000^\circ\text{C}$): High-melting-point materials condensed into metallic and silicate dust containing Iron ($\text{Fe}$), Calcium ($\text{Ca}$), and Aluminum ($\text{Al}$), forming the rocky Terrestrial Planets (Mercury, Venus, Earth, Mars).

  • Outer Solar Nebula ($<200^\circ\text{C}$ to $0^\circ\text{C}$): Volatile compounds condensed into frozen ices containing Hydrogen ($\text{H}$), Carbon ($\text{C}$), Nitrogen ($\text{N}$), and Hydrogen Cyanide ($\text{HCN}$), forming the volatile-rich Gas Giants (Jupiter, Saturn, Uranus, Neptune).

  • Geochronological Constraints on Earth's Age:

• System Age: The Solar System and Earth formed simultaneously at $\sim 4.55\,\text{Ga}$.

• Oldest Earth Rocks: Dated to $4.0\,\text{Ga}$ (e.g., Acasta Gneiss).

• Oldest Earth Minerals: Detrital zircon crystals dated to $4.4\,\text{Ga}$ ($4404\,\text{Ma}$ / $4113\,\text{Ma}$).

• Evidence for $4.55\,\text{Ga}$ Age:

  • Over $90\%$ of meteorites (unaltered remnants of early solar system planetesimals, such as the asteroid Bennu) yield radiometric ages of exactly $4.55\,\text{Ga}$.
  • Lunar Samples: Oldest moon rocks date to $4.45\,\text{Ga}$. For example, Apollo 17 sample 76015,143 (collected by astronauts Eugene Cernan, Harrison Schmitt, and Ronald Evans in the Taurus-Littrow Valley) weighs $332\,\text{g}$ and is dated to $3.9\,\text{Ga}$, recording the Imbrium Impact event ($1145\,\text{km}$ diameter basin).

• Meteorite Impact Preservation: Earth has far fewer visible impact craters (e.g., Meteor Crater, Arizona) than the Moon or Mercury due to continuous resurfacing via plate tectonics, atmospheric weathering, and erosion.

Matter, Atoms, and Mineral Fundamentals

• Atomic Structure:

  • Protons: Positively charged subatomic particles located in the nucleus.
  • Neutrons: Electrically neutral subatomic particles located in the nucleus.
  • Electrons: Negatively charged subatomic particles orbiting the nucleus.

• Atomic Variations:

  • Isotopes: Atoms of the same element with varying numbers of neutrons.
  • Hydrogen Isotopes: Protium ($^1\text{H}$: 1 proton, 0 neutrons, 1 electron), Deuterium ($^2\text{H}$: 1 proton, 1 neutron, 1 electron), Tritium ($^3\text{H}$: 1 proton, 2 neutrons, 1 electron).

• Ions: Electrically charged atoms formed by changing the number of electrons.
  • Cations: Positively charged ions formed by the loss of electron(s).
  • Anions: Negatively charged ions formed by the gain of electron(s).

• Definition of a Mineral:
  1. Naturally Occurring: Formed by natural geological processes (e.g., liquid water is not a mineral, but naturally frozen water ice and snowflakes are minerals).
  2. Solid: Stable solid state at ambient temperatures.
  3. Inorganic: Not composed of complex organic carbon chains (e.g., wood is organic and non-mineral).
  4. Crystalline Structure: Possesses a regular, highly ordered, repeating internal arrangement of atoms or ions.
  • Amorphous Solids: Non-crystalline solids lacking repeating internal atomic structures (e.g., obsidian / volcanic glass) are non-minerals.
  • Crystal Aggregates: Single mineral crystals, polycrystalline masses (multiple interlocking crystals), or amorphous glass.

• Mineral Classification by Anion Group:

  • Oxides: Anion = $\text{O}^{-2}$.
  • Sulfides: Anion = $\text{S}^{-}$.
  • Halides: Anions = $\text{Cl}^-$, $\text{F}^-$ (e.g., Halite $\text{NaCl}$).
  • Carbonates: Anion = $\text{CO}_3^{-2}$ (e.g., Calcite $\text{CaCO}_3$, main constituent of limestone and marble).
  • Hydroxides: Anion = $\text{OH}^-$.
  • Sulfates: Anion = $\text{SO}_4^{-2}$ (e.g., Gypsum $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$).
  • Native Elements: Pure single-element substances (e.g., Gold, Silver, Graphite, Diamond).

• Crater of Diamonds State Park Case Study:

  • Located near Murfreesboro, Arkansas; it is the only place in the world where the public can mine for diamonds and keep their finds.
  • An average of 3 diamonds per day are unearthed by visitors from eroded lamproite/kimberlite volcanic pipes.

• Elemental Abundance Comparison:

• Earth's Crust (Weight %): Oxygen ($46\%$ - $49\%$), Silicon ($26\%$ - $28\%$), Aluminum ($7.1\%$ - $8\%$), Iron ($5\%$ - $6\%$), Calcium ($2.5\%$ - $3.6\%$), Sodium ($2.8\%$), Potassium ($2.3\%$ - $2.6\%$), Magnesium ($2.1\%$ - $4\%$), All other elements ($1.8\%$ - $3.2\%$).

• Whole Earth (Weight %): Iron ($35\%$), Oxygen ($29\%$ - $30\%$), Silicon ($15\%$), Magnesium ($13\%$), Nickel ($2.4\%$ - $2.7\%$), Sulfur ($2\%$ - $2.7\%$), Calcium ($0.6\%$ - $1.1\%$), Aluminum ($1\%$), Other ($<1\%$).

  • Silicate Mineralogy (The Rock-Forming Minerals):

• Basic structural building block: The Silicon-Oxygen Tetrahedron ($\text{SiO}_4^{-4}$), consisting of 1 central Silicon cation bonded to 4 surrounding Oxygen anions.

• Polymerization Configurations:

  • Isolated / Pairs: No shared oxygens (e.g., Olivine).
  • Single Chains: Tetrahedra share 2 oxygens (e.g., Pyroxene group).
  • Double Chains: Alternating shared oxygens forming twin chains (e.g., Amphibole group).
  • Sheets: Tetrahedra share 3 oxygens in 2D sheets (e.g., Micas like Biotite/Muscovite, Clay group).
  • Framework: All 4 oxygens shared in 3D networks (e.g., Quartz $\text{SiO}_2$, Feldspar group).

Mineral Crystallization and Rock Genesis

• Mechanisms of Mineral Crystallization:

  1. From a Melt: Crystallization occurring as molten rock (magma/lava) cools.
  2. From a Solution (Precipitation): Dissolved ions precipitate as solvent evaporates or chemical saturation is reached.
  3. Solid-State Recrystallization: Preexisting minerals regrow or transform into new minerals directly in the solid state in response to changes in temperature, pressure, or fluid composition.
  4. Vapor Deposition: Direct crystallization from gas/vapor phases (minor geological contribution).

• Definition of a Rock:

  • A solid aggregate composed of one or more minerals or mineraloids with cemented or interlocking grains.
  • Crystalline Rocks: Aggregates of interlocking mineral crystals grown during cooling or precipitation.
  • Clastic Rocks: Aggregates composed of pre-existing mineral or rock grains cemented together.

Igneous Processes and Igneous Rocks

• Igneous Rock Definition and Classification:

  • Formed by the cooling and solidification of molten rock (magma underground, lava at the surface).
  • Classified based on Texture (size, shape, and spatial relationships of mineral grains) and Mineralogy (chemical/mineral composition).

• Cooling Rate and Grain Size Relationships:

• Extrusive / Volcanic: Cools quickly at or near Earth's surface.

  • Aphanitic / Glassy: Tiny, microscopic crystals or no crystals at all.
  • Examples: Obsidian (glassy, zero minerals), Rhyolite, Andesite, Basalt.

• Intrusive / Plutonic: Cools slowly deep underground within magma chambers.

  • Phaneritic: Large mineral grains visible to the naked eye.

  • Examples: Granite, Diorite, Gabbro.

  • Pegmatite: Exceptionally slow cooling or volatile-rich magmas producing extremely large mineral crystals.

  • Magma Composition Types:

• Granitic (Felsic): High $\text{SiO}_2$ content, low Iron/Magnesium ($\text{Fe/Mg}$), lower cooling temperature; light-colored minerals (Quartz, Potassium Feldspar, Muscovite).

• Basaltic (Mafic): Low $\text{SiO}_2$ content, high Iron/Magnesium ($\text{Fe/Mg}$), higher cooling temperature; dark-colored minerals (Olivine, Pyroxene, Plagioclase Feldspar).

  • Igneous Mineral Resources (Platinum Group Elements - PGE):

• PGEs (Platinum, Palladium, Rhodium, Osmium, Iridium, Ruthenium) are critical minerals essential for high-tech industrial applications and catalytic converters.

• Rarity: PGE crustal abundance is extremely low ($\sim 0.5\,\text{ppb}$).

• Geological Origin: Mined almost exclusively from ultra-mafic igneous intrusions produced in Earth's early history ($>1\,\text{Ga}$).

• Global Reserves: Over $90\%$ of global reserves are situated in 3 primary locations, with South Africa's Bushveld Complex holding $\sim 90\%$ of global PGE reserves.

• US Supply: $90\%$ of US PGE supply is imported; the remaining domestic production originates from a single mining operation in Stillwater, Montana.

Weathering and Clastic Sedimentary Processes

• Sedimentary Processes:

  • Weathering: Physical breakdown and chemical alteration of rock at or near Earth's surface.
  • Erosion: Mobilization and removal of sediment by water, wind, ice, or gravity.
  • Transportation: Movement of sediment across distances.
  • Deposition: Settlement of sediment in a new environment.
  • Lithification: Burial, compaction, and cementation transforming loose sediment into clastic rock.

• Physical Weathering Mechanisms:

  • Jointing: Fracturing caused by pressure release during overburden removal.
  • Frost Wedging: Mechanical splitting caused by the expansion of water as it freezes inside rock fractures.
  • Root Wedging: Physical expansion of fractures caused by growing plant roots.

• Chemical Weathering of Granite:

  • Granite weathering produces specific residual minerals, secondary clay minerals, and dissolved ions:

• Sedimentary Transport Media:

  • Wind (Aeolian): Transports fine-grained silt and small sand grains. Extensive wind-blown silt deposits (loess) were laid down in the US Midwest $\sim 15\,\text{kya}$ during glacial periods.
  • Ice (Glaciers): Transports all sediment sizes non-selectively, building landforms including lateral, medial, end, terminal, ground, and recessional moraines, plus outwash plains.
  • Water: Transport capability depends directly on flow velocity.
  • High Velocity / High Energy: Rivers and rocky coastlines deposit large rounded clasts, forming Conglomerate.
  • Moderate Velocity: Waves and streams deposit sand, preserving Ripple Marks in Sandstone.
  • Low Velocity / Quiet Water: Floodplains and ephemeral lakes deposit mud and fine clay, preserving Desiccation Cracks (mud cracks).

• Depositional Environments & Coastal Plain Dynamics:

  • Sedimentary Facies: Carbonate facies (deep marine) $\rightarrow$ Shale facies (shallow marine/shelf) $\rightarrow$ Sand and silt facies (nearshore/beach) $\rightarrow$ Shale and coal facies (delta/swamp/floodplain).
  • Sea Level Fluctuations: Global sea level curves (Exxon curve, Hallam curve) show major sea level transgressions and regressions over Earth history, with a general sea level drop over the last $100\,\text{Ma}$.
  • Georgia Coastal Plain Case Study:
  • Extends from the Fall Zone (near Augusta) down to Savannah.
  • Stratigraphic cross-section shows Cretaceous, Paleocene/Eocene, Miocene/Pliocene, and Quaternary coastal plain sediments resting atop crystalline basement rocks.

• Clastic Mineral Resources (Heavy Mineral Sands):

  • Heavy mineral sand deposits contain economically valuable concentrations of Titanium ($\text{Ti}$) and Rare Earth Elements ($\text{REE}$) contained in minerals like Monazite ($0.1\%$ to $0.3\%$ weight concentration).
  • Okefenokee / Trail Ridge Formation: Trail Ridge sand formation along the Georgia/Florida border hosts major heavy mineral sand mining sites (e.g., Twin Pines Mine site near Okefenokee National Wildlife Refuge).

Chemical and Biochemical Sedimentary Rocks

• Chemical Sedimentary Rocks:

  • Formed by the inorganic precipitation of dissolved ions directly from water solutions (evaporites).
  • Examples:
  • Halite ($\text{NaCl}$): Rock salt precipitated during evaporation of enclosed lakes (e.g., Death Valley, California).
  • Gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$): Primary evaporite mineral component of commercial drywall.
  • Banded Iron Formations (BIFs): Alternating layers of iron oxides and chert.
  • Lithium Mineral Resources:
  • Lithium is mined from igneous pegmatites containing Spodumene (Li-bearing pyroxene, e.g., North Carolina) or extracted from evaporated salt flats (salars).
  • Australia ($61\,000\,\text{t}$) and Chile ($39\,000\,\text{t}$) account for over $70\%$ of annual global production.
  • Largest world reserves: Bolivia ($21\,\text{Mt}$), Argentina ($20\,\text{Mt}$), US ($12\,\text{Mt}$), Chile ($11\,\text{Mt}$), Australia ($7.9\,\text{Mt}$), China ($6.8\,\text{Mt}$).

• Biochemical Sedimentary Rocks:

  • Formed from the skeletal accumulations or organic activities of living organisms.
  • Examples:
  • Limestone: Composed primarily of Calcite ($\text{CaCO}_3$) derived from coral reefs, stromatolites (algae), or shell debris. Crucial for agricultural lime, cement, and concrete.
  • Chert / Diatomite: Microscopic silica-shelled plankton (diatoms, radiolarians) accumulate as $\text{SiO}_2$ sediment on ocean or lake floors; freshwater diatomite is harvested commercially for cat litter.
  • Chalk: Microscopic calcite-shelled plankton (coccoliths) accumulate on ocean floors forming soft white limestone.
  • Coal: Formed from the accumulation, burial, anaerobic decay, and compaction of terrestrial plant remains (lycopods, early gymnosperms, horsetails, seed ferns, ferns) in ancient coastal swamps.

Metamorphic Processes and Rock Transformation

• Metamorphic Principles:

  • Preexisting parent rocks (protoliths) are altered in the solid state by combination of heat ($T$), pressure ($P$), and chemically active hydro-thermal fluids.

• Glacial Ice Metamorphism Analogy:

  • Glacial ice accumulation represents low-temperature solid-state recrystallization:
  • Snow: Loose crystal flakes ($90\%$ air space).
  • Granular Ice: Snowflakes change shape to rounded grains ($50\%$ air space).
  • Firn: Compacted granular ice ($20\%$ to $30\%$ air space).
  • Glacial Ice: Recrystallized solid ice mass ($20\%$ air space remaining as trapped air bubbles).

• Shale Protolith Metamorphic Grade Sequence:

  • Progressive metamorphism of a Shale protolith (composed of clay minerals + quartz) under increasing P-T conditions:
  1. Slate: Low-grade metamorphism; fine-grained clay minerals align to form slatey cleavage.
  2. Schist: Medium-grade metamorphism; coarse alignment of visible micas (Biotite, Muscovite) + Quartz.
  3. Gneiss: High-grade metamorphism; segregation of minerals into alternating dark (mafic) and light (felsic Feldspar + Quartz) metamorphic bands.
  • Carbonate Protolith: Limestone alters to Marble via recrystallization of Calcite.

• Foliation Texture:

  • Foliation: Preferred planar alignment of platy mineral grains perpendicular to the principal direction of differential stress/pressure.

• Index Minerals and Metamorphic Facies:

  • Index Minerals: Key minerals that form only within restricted ranges of pressure and temperature, serving as geological thermometers/barometers.
  • Aluminosilicate Polymorphs ($\text{Al}_2\text{SiO}_5$):
  • Kyanite: High pressure, moderate temperature mineral.
  • Andalusite: Low pressure, moderate/high temperature mineral.
  • Sillimanite: High temperature, moderate/high pressure mineral.
  • Triple Point: Kyanite, Andalusite, and Sillimanite coexist in equilibrium at $T = 530^\circ\text{C}$ and $P = 0.42\,\text{GPa}$ ($\sim 15\,\text{km}$ depth).

• Types of Metamorphic Environments:

  • Regional Metamorphism: Widespread elevation of P and T produced during mountain-building continental collisions or deep burial.
  • Metamorphic Zones in NE Scotland (Glen Clova to Stonehaven): Progressive regional zones marked by index mineral entry (Isograds): Chlorite $\rightarrow$ Biotite $\rightarrow$ Garnet $\rightarrow$ Staurolite $\rightarrow$ Kyanite $\rightarrow$ Sillimanite.
  • Contact Metamorphism: Localized high-temperature heating of host rock adjacent to intrusive magma bodies.

- *Contact Zoning around Granite Intrusions*: Shale/Sandy Shale transforms into Muscovite-Chlorite zone $\rightarrow$ Biotite-Andalusite zone $\rightarrow$ Cordierite-Sillimanite zone $\rightarrow$ Hornfels at contact; Limestone transforms into Fine Marble $\rightarrow$ Coarse Marble $\rightarrow$ Skarn.

• Metamorphic Ore Resources:
  • Beryllium: Mined at the Spor Mountain mine in Utah, which is the only active beryllium mine in the United States and the world's primary commercial source.

Plate Tectonics: Observations and Evidence

• Key Global Topographic & Bathymetric Observations:
  • Bimodal Elevation Distribution: Earth's surface crust displays two distinct median elevation peaks: Continental landmasses centered at $\sim +0.5\,\text{km}$ above sea level, and ocean basins centered at $\sim -4.5\,\text{km}$ below sea level.

• Continuous Mid-Ocean Ridge System: Earth-encircling underwater mountain chains.

• Linear Mountain Chains: High-elevation narrow mountain belts situated along continental margins (e.g., the Andes).

• Deep-Sea Trenches: Deep, narrow linear depressions in the ocean floor parallel to volcanic chains (e.g., Peru-Chile trench west of the Andes, Aleutian Trench south of Alaska).

  • Seismicity and Volcanism Distributions:

• Earthquakes and active young volcanoes are non-randomly concentrated in narrow, linear or arcuate belts across the globe.

• Earthquake epicenters occur at predictable depth zones ($0-35\,\text{km}$, $35-70\,\text{km}$, $70-150\,\text{km}$, $150-300\,\text{km}$, $300-500\,\text{km}$, $500-800\,\text{km}$).

  • Hot Spot Volcanism & Seamount Chains:

• Hawaiian-Emperor Chain: Active volcanism is localized at the southeastern tip (Kilauea/Hawaii, Loihi active submarine volcano). Extinct volcanic seamounts extend northwestward, increasing progressively in age:

  • Hawaii: $0.8\,\text{Ma}$ to present.
  • Maui: $<1.0\,\text{Ma}$.
  • Molokai: $1.3$ - $1.8\,\text{Ma}$.
  • Oahu: $2.2$ - $3.3\,\text{Ma}$.
  • Kauai: $3.8$ - $5.6\,\text{Ma}$.
  • Nihoa: $7.5\,\text{Ma}$.
  • Necker: $11.3\,\text{Ma}$.
  • Midway: $25\,\text{Ma}$.

• Plate Motion Calculation:
  • Distance vs Age data along the chain yields a linear velocity: $4000\,\text{km}$ over $4.6 \times 10^7\,\text{years}$ gives an average plate rate of $8.7\,\text{cm/yr}$ (or $8.6 \pm 0.02\,\text{cm/yr}$ via Potassium-Argon $\text{K-Ar}$ radiometric dating).

• Crustal Age Discrepancy:

  • Ocean Floor Rocks: Remarkably young ($<208\,\text{Ma}$). Symmetrically arranged around mid-ocean ridges, with the youngest rocks ($0-2\,\text{Ma}$) located directly along ridge axes.
  • Continental Rocks: Much older and heterogeneously distributed ($0-0.6\,\text{Ga}$, $1.0-2.5\,\text{Ga}$, $2.5-3.0\,\text{Ga}$, up to $4.0\,\text{Ga}$).

• Continental Drift & Paleontological Evidence:

  • Jigsaw Fit: Continental margins fit together tightly (e.g., South America and Africa coastline fit, with minor overlaps and gaps along continental shelves).

• Fossil Distributions: Identical terrestrial fossil species spanned across separated southern continents (Gondwana):
  • Glossopteris: Extinct seed fern found across South America, Africa, India, Antarctica, and Australia.
  • Mesosaurus: Freshwater aquatic reptile found in matching rock layers in South America and Africa.

• Glacial Striations: Matchable Permo-Carboniferous glacial ice flow directions and tillites align perfectly when Gondwana landmasses are reassembled at the South Pole.

Plate Boundaries and Tectonic Dynamics

• Earth Structure:

  • Lithosphere: Brittle, rigid outer shell ($50-100\,\text{km}$ thick) comprising the crust and uppermost solid mantle ("skin of the avocado").
  • Asthenosphere: Deformable, ductile partially molten mantle layer directly beneath the lithosphere ("flesh of the avocado"). Post-Glacial Rebound ($\text{PGR}$ mass rate $-4$ to $+4\,\text{mm eq H}_2\text{O/yr}$) demonstrates mantle flow under changing ice sheet loads.

• Crustal Types & Densities:

  • Continental Crust: Granitic composition; average density = $2.7\,\text{g/cm}^3$; thickness = $40-70\,\text{km}$.
  • Oceanic Crust: Basaltic composition; average density = $3.0\,\text{g/cm}^3$; thickness = $5\,\text{km}$.
  • Mantle Density: $3.3\,\text{g/cm}^3$.

• Tectonic Plate Map & Relative Motions:

  • Earth's lithosphere is segmented into major plates moving at rates of $2$ to $10\,\text{cm/yr}$ (e.g., $5\,\text{cm/yr}$ relative motion between North American and Eurasian/African plates).

• Plate Boundary Types:
  1. Divergent Boundaries (Plates pulled apart):
  • Oceanic Divergence (Mid-Ocean Ridges): Sea-floor spreading creates new basaltic oceanic crust as mantle pressure is released and magma intrudes (e.g., Mid-Atlantic Ridge, East Pacific Rise). In Iceland, spreading is exposed above sea level at Thingvellir and Krafla.
  • Continental Divergence (Rift Valleys): Continental rifting forms down-faulted rift valleys with basaltic volcanism, eventually evolving into narrow linear seas and ocean basins (e.g., East African Rift System including the Ethiopian Rift, Red Sea, Gulf of Aden, Eastern and Western Branches).

   - *Ancient Failed Rifts*: Triassic/Mesozoic rift basins along eastern North America formed during the breakup of Pangea (e.g., Newark basin, Connecticut basin, Fundy basin, Gettysburg basin, Culpeper basin, Danville basin, Richmond basin, Deep River basin, South Georgia rift).

2. Convergent Boundaries (Plates collide):
   • Ocean-Continent Convergence: Dense oceanic lithosphere subducts beneath continental lithosphere.
     • Features: Deep ocean trench, Wadati-Benioff inclined earthquake zone (shallow to deep focal depths down to $800\,\text{km}$), partial melting of overriding mantle wedge creating volcanic continental arcs (e.g., Andes Mountains / Peru-Chile Trench; Cascade Volcanic Chain / Juan de Fuca plate subducting under North America).
   • Ocean-Ocean Convergence: Older, colder, denser oceanic plate subducts under younger oceanic plate.
     • Features: Deep sea trench, subduction zone earthquakes, volcanic island arc (e.g., Aleutian Islands in Alaska, Lesser Antilles in the Caribbean).

 - **Continent-Continent Convergence**: Occurs after intervening ocean basin subducts completely; low-density continental crusts collide without subducting.
   - *Features*: Intense folding, faulting, crustal thickening, high-grade metamorphism, and extreme mountain building (e.g., Himalayas and Tibetan Plateau created by India colliding with Eurasia).

3. Transform Boundaries (Plates slide past horizontally):
   • Plates slide past each other along strike-slip faults, accommodating relative motions of adjacent divergent or convergent plates.
   • Classic Example: San Andreas Fault in California (segments include the 1906 San Francisco rupture, 1857 Southern break, and central creeping segment).

• Triple Junctions & Continental Margins:
  • Mendocino Triple Junction: Intersection of 3 plate boundaries (Divergent, Convergent, and Transform) off northern California.
  • Active Continental Margin: A continental boundary that coincides with an active tectonic plate boundary (e.g., US West Coast, characterized by trenches, earthquakes, and volcanism).
  • Passive Continental Margin: A boundary marked by a gradual transition from continental to oceanic crust within the interior of a single plate, lacking active boundary dynamics (e.g., US East Coast).