The Science of Geology
The Science of Geology
Geology is the science that pursues an understanding of planet Earth.
Physical geology examines the materials composing Earth and seeks to understand the processes that operate beneath and on the surface.
Historical geology seeks an understanding of the origin of Earth and its development through time.
Geology as a field overlaps with natural hazards, resources, and environment;
Natural hazards are part of living on Earth: volcanoes, floods, tsunamis, earthquakes, landslides.
These hazards are natural processes that become threats when they occur where people live.
Megapopulations: More people now live in cities than in rural areas, increasing vulnerability to hazards.
Resources in geology include water, soil, metallic and nonmetallic minerals, and energy resources.
Quotation (contextual): Geologists have been humorously described as
“Little boys who pick up rocks either go to prison or become geologists,”
“Boy Scouts who hated to give up camping… majored in geology,”
“An individual who is in or goes to the Grand Canyon or the Swiss Alps and claims he was working.”
(attributed to Dr. Walter Youngquist, petroleum geologist and professor)
Natural Hazards and the Environment
Natural hazards are part of Earth’s dynamic system and include volcanic activity, floods, tsunamis, earthquakes, and landslides.
Hazards become critical when they intersect with human populations and infrastructure.
Resources in Geology (Geology, Resources, and Environment)
Resources important to geology include:
Water, soil, metallic and nonmetallic minerals, and energy resources.
The study of resources ties geology to practical applications such as water supply, mining, and energy security.
History of Geology
James Ussher (1581–1665): estimated Earth’s age using Biblical genealogies; dated creation to October 22, 4004 BCE (1650).
Catastrophism (17th–18th centuries): major geologic features could be produced by sudden, short-lived, violent events; Earth’s age circa ~6000 years according to some views, with drastic events shaping features.
James Hutton (1726–1797): introduced the principle of uniformitarianism; earth changes slowly through natural processes observable today; argued that past processes were not different from present processes; credited with concept of Deep Time.
Georges Cuvier (1769–1832): foundational figure in catastrophism and extinction; argued for world histories before our own based on catastrophes; presented ideas that revolutionized Earth’s history; famous claim:
“The present is the key to the past” (attributed to Archibald Geikie, 1835–1924, in some contexts).
Key context: while catastrophism emphasized abrupt events, uniformitarianism emphasized continuity of processes; many scientists recognized a synthesis: Earth histories include both gradual change and periodic catastrophic events.
Notable examples and caveats:
Exceptions to simple catastrophism: rock types and formations (e.g., Banded Iron Formations) indicate conditions or time scales not easily observed today; laboratory results can replicate long-term metamorphism even if we cannot observe such processes directly; some conditions require long times to form.
Early evidence of Earth’s interior and stratification began with observations such as unconformities (e.g., Hutton’s unconformity at River Jed, Scotland).
Earth’s Interior and Seismic Probing
Seismic waves from earthquakes provide the primary means to “see” inside Earth; travel times vary with material properties.
Seismic waves travel fastest in stiff, rigid rocks; velocities depend on rock composition and state.
P waves (Primary, compressional) vs S waves (Secondary, shear):
P waves propagate through both solids and liquids.
S waves cannot travel through liquids.
Seismic data reveal Earth as a set of distinct shells defined by composition and properties; abrupt changes in velocity indicate boundaries between layers.
Seismic rays reflect and refract at layer boundaries, producing complex paths that reveal those boundaries.
Earth’s Layered Structure and Phase Changes
Upper mantle and lower mantle have different densities:
Upper mantle density ≈ 3.3 \, \mathrm{g/cm^3}
Lower mantle density ≈ 5.6 \, \mathrm{g/cm^3}
The lower mantle undergoes mineral phase changes under higher pressures (phase changes with depth).
Crust, mantle, and core form the basic structure of Earth’s interior; core–mantle and crust–mantle boundaries are detectable via seismic behavior.
Key internal boundaries include:
The crust–mantle boundary (the Moho)
The core–mantle boundary (at >100° epicentral distance, seismic shadow zones appear)
Probing Earth’s Interior: Seismic Waves and Boundaries
Seismic wave travel times vary with material properties; faster velocities in stiffer rocks; composition affects velocity.
P waves (compressional) vs S waves (shear): P waves travel through liquids and solids; S waves do not travel through liquids, creating shadow zones where S waves are absent.
The Moho (Mohorovičić discontinuity): boundary between crust and mantle; discovered in 1909 by a marked jump in P-wave velocity.
Seismic shadow zones: regions beyond about 100^ ext{^ ext{o}} from an epicenter where P/S waves are absent or weak, indicating a liquid outer core.
Earth’s Layered Structure: Crust, Mantle, Core
Crust:
Oceanic crust: averages 7 \, \text{km} thick; composed of basalt and gabbro; average density \approx 3.0 \, \mathrm{g/cm^3}.
Continental crust: averages 40 \text{ to } 70 \, \text{km} thick; heterogeneous; average density \approx 2.7 \, \mathrm{g/cm^3}.
Mantle: extends from crust to ~2900 \, \text{km} depth; upper mantle density ~3.3 \, \mathrm{g/cm^3}; lower mantle density ~5.6 \, \mathrm{g/cm^3}; experiences mineral phase changes with depth.
Core:
Outer core: liquid iron–nickel; thickness \approx 2270 \, \text{km}; density \approx 9.9 \, \mathrm{g/cm^3}; its liquid state explains S-wave shadow zones.
Inner core: solid sphere; radius \approx 1216 \, \text{km}; density \approx 13 \, \mathrm{g/cm^3}; grows as the Earth cools and rotates, and may rotate independently of crust and mantle.
Lithosphere–asthenosphere–mesosphere:
Lithosphere: outer rigid shell consisting of crust plus the uppermost mantle; thickness ~100$-$200 \, \text{km}.
Asthenosphere: beneath lithosphere to ~660 \, \text{km}; more plastic and capable of flow, enabling plate motion.
Mesosphere (lower mantle): rigid layer from ~660 \, \text{km} to 2900 \, \text{km} depth; rocks can undergo very gradual flow.
Plate tectonics requires a partially molten, ductile layer (asthenosphere) to allow plate movement atop a rigid lithosphere.
Plate Tectonics: Mechanisms, Boundaries, and Driving Forces
Plate tectonics unifies:
Continental drift (Wegener’s concept)
Seafloor spreading (new ocean floor formation at mid-ocean ridges)
Paleomagnetism (record of magnetic reversals in rocks)
The system is powered by Earth’s internal heat and mantle convection; the asthenosphere is more plastic than surrounding layers.
Plates move as rigid slabs and interact at their margins, where most geological activity occurs.
The plates include blocks of continental crust that can resist subduction due to lower density.
Subsurface Boundaries: Key Global Boundaries
Plate boundaries types:
Divergent boundaries (constructive margins)
Convergent boundaries (destructive margins)
Transform boundaries (conservative margins)
Divergent boundaries (mid-ocean ridges):
Oceanic ridge system surrounds the globe; new ocean floor is created at ridges; rocks are mafic (basalt and gabbro).
Spreading rates vary: slow, intermediate, fast.
Spreading rates classification:
Slow: < 5 \, \text{cm/yr}
Intermediate: 5$-$9 \, \text{cm/yr}
Fast: > 9 \, \text{cm/yr}
Ridge system length exceeds 7.0 \times 10^4 \, \text{km} (70,000 km).
Rift valleys and continental rifting can lead to the formation of new oceans (e.g., Thingvellir Rift Valley in Iceland).
Passive margins are coastal regions along the Atlantic where there is no active plate boundary, little volcanism, and few earthquakes.
Convergent boundaries (destructive margins):
Ocean–continent convergence: oceanic lithosphere subducts beneath continental lithosphere; forms ocean trenches and continental volcanic arcs (e.g., Andes, Cascades).
Ocean–ocean convergence: subduction of one ocean plate beneath another; volcanic island arcs (e.g., Japan, Aleutians).
Continent–continent convergence: collision of two continents; forms lofty mountain belts (e.g., Himalayas) and sutures; little to no volcanism due to depleted hydrated mantle in subduction zones.
Subduction zones generate earthquakes and magma that contributes to volcanism and metamorphism; contact and regional metamorphism zones are common.
Example volcanic arc: Cascades – a continental volcanic arc above subduction zone.
Example mountain belt: Himalayas – result of continental collision and accretion of terranes.
Transform boundaries (conservative margins):
Transform faults connect divergent and convergent boundaries (e.g., San Andreas Fault connects a spreading center to a subduction zone).
Transform faults allow plates to slide past one another; Ridge–ridge, ridge–trench, and trench–trench transform configurations exist.
Transform boundaries facilitate plate motion and accommodate differential movement between adjacent plates.
San Andreas Fault and related fault systems illustrate transform boundary motion.
Plate boundary concepts and dynamics:
Plate margins are the most active regions for volcanism, earthquakes, and crustal deformation and mountain building.
The overall plate motion is driven by mantle convection beneath the lithosphere and deep mantle processes (e.g., mantle plumes).
Paleomagnetism and Sea-Floor Spreading
Paleomagnetism describes how rocks capture the Earth’s magnetic field direction at the time of their formation via magnetic minerals (e.g., iron-bearing minerals).
The magnetic field periodically reverses; the north magnetic pole becomes the south magnetic pole and vice versa.
Rocks record the orientation of Earth’s magnetic field when they form, which helps reconstruct past plate motions and positions.
Ocean floor magnetic stripes: symmetric high- and low-intensity magnetism parallel to mid-ocean ridges; reflect normal and reversed polarity of the Earth's field over time.
Seafloor spreading (Hess, 1962): new crust forms at mid-ocean ridges and old crust is consumed at plate margins, consistent with the magnetic stripes observed.
Deep-sea drilling and sediment data show ocean floor is youngest at ridge axes and oldest away from ridges, supporting seafloor spreading.
Magnetic reversals recorded in ocean crust provide a time record of polarity changes for the past ~200 million years.
The Moho and Core–Mantle Boundary
The Moho (Mohorovičić discontinuity) is the boundary between crust and mantle; P-wave velocities increase abruptly at this boundary, discovered in 1909.
The core–mantle boundary is revealed by seismic shadow zones; P- and S-waves change behavior across this boundary, indicating a transition from mantle to core.
Plate Motions and Global Tectonics
Plate motions are slow, typically a few centimeters per year, driven by mantle convection and ridge push/slab pull dynamics.
GPS data provide modern plate-motion vectors; paleomagnetic data provide historical motion context.
Plate tectonics integrates continental drift, seafloor spreading, and paleomagnetism into a unified framework for understanding Earth’s dynamic surface.
Plate Configurations: Breakup of Pangaea and Global Reassembly
Alfred Wegener proposed continental drift; Pangaea began breaking apart about 200 million years ago (Ma).
Timeline highlights (illustrative):
~150 Ma: North America and Africa begin to separate, opening the North Atlantic.
~90 Ma: South Atlantic opens; Africa, India, and Antarctica begin to separate.
~50 Ma: Southeast Asia accretes with Eurasia; India continues northward.
~20 Ma: India–Eurasia collision begins to form the Himalayas and Tibetan Plateau.
By ~50 Ma, Southeast Asia docked with Eurasia; India still moving northward.
By ~20 Ma, ongoing collision between India and Eurasia continues to shape the Himalayas.
Plate configurations continue to evolve; models project future configurations:
50 million years from now: Southeast Asia– Eurasia remains a key interaction; India–Asia collision persists; mountain belts and sutures evolve.
250 million years from now: hypothetical future arrangement of continents based on continued plate tectonics.
Visuals provided show progressive breakup and collision patterns from 150 Ma to the present and speculative futures.
Plate Motions and Channels of Reconfiguration
Plate motions are depicted with red arrows (GPS-based) and black arrows (paleomagnetic-based velocities) indicating spreading rates and directions.
The dynamic tectonic system involves constant reconfiguration: plates split, collide, and slide past one another, reshaping continents and ocean basins over geological time.
Hotspots and Volcanism Away from Plate Boundaries
Hawaii is a classic hotspot example: not located at a plate boundary.
A plate moves over a stationary mantle plume (hotspot); successive islands form as the plate moves, creating a chain with ages progressively older away from the hotspot source.
Hawaii track: youngest at the current hotspot location, progressively older toward the northwest (older islands such as Kauai).
Other notable hotspot tracks exist (Iceland, Yellowstone, Azores, Cape Verde, etc.).
The hotspot concept is supported by the presence of volcanic island chains that do not align with plate boundaries.
Mantle plumes and hotspot tracks provide insight into deep mantle processes and plate motion history.
Geology Review and Core Concepts
Key summary points from the material:
Plates move slowly; internal convection drives motion.
Plate boundaries are the primary sites of tectonic activity: divergent, convergent, and transform.
The mid-ocean ridge system is the dominant divergent boundary and a major engine of seafloor spreading.
Continental rifting can create new ocean basins and passive margins.
Ocean–continent convergence produces subduction zones, trenches, volcanic arcs, and metamorphism.
Ocean–ocean convergence yields island arcs; continent–continent convergence yields major mountain belts (with sutures) and limited volcanism.
Isostasy explains how gravity-driven balancing of crustal blocks shapes mountains and basins; mountains have deep roots.
Transform boundaries connect other boundary types and accommodate lateral plate motion (e.g., San Andreas Fault).
Paleomagnetism and magnetic reversals on rocks provide a record of past plate motions and support for seafloor spreading.
The concept of plate tectonics integrates and extends the ideas of Wegener, Hess, and paleomagnetism to explain Earth’s surface dynamics.
Notation and Key Numerical References (for quick recall)
Upper mantle density: 3.3 \, \mathrm{g/cm^3}; Lower mantle density: 5.6 \, \mathrm{g/cm^3}
Oceanic crust thickness: \sim 7 \, \mathrm{km}; Continental crust thickness: 40-70 \, \mathrm{km}
Crust–mantle boundary (Moho): boundary discovered in 1909 via P-wave velocity increase.
Outer core thickness: 2270 \, \mathrm{km}; Outer core density: 9.9 \, \mathrm{g/cm^3}; Inner core radius: 1216 \, \mathrm{km}; Inner core density: 13 \, \mathrm{g/cm^3}
Lithosphere thickness: 100-200 \, \mathrm{km}; Asthenosphere depth: \sim 660 \, \mathrm{km}
Plate motion rates: typically a few \text{cm/yr}; divergent boundaries: slow (<5 cm/yr), intermediate (5-9 cm/yr), fast (>9 cm/yr)
Global ridge length: > 7.0\times 10^4 \, \mathrm{km}
Spread pattern evidence: symmetrical magnetic stripes on the ocean floor; seafloor youngest at ridges; oldest away from ridges
Major time markers: breakup of Pangaea began ~200 Ma; key breakpoints at ~150 Ma, ~90 Ma, ~50 Ma, ~20 Ma
Plate boundary types: Divergent, Convergent, Transform
Notable features: trench zones, volcanic arcs, sutures, hotspot tracks (Hawaii and other tracks)
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