ERTH 375- Midterm 1

Natural Disaster

A natural event in which a large amount of energy is released in a relatively short time with catastrophic consequences for life and/or infrastructure.

Cause= natural not man-made, but can result from ignoring hazardous natural conditions.Typically caused by sudden release of energy stored over a much longer time.

Can have significant casualties, societal disruption and/or economic loss.

Energy sources for disasters- Earth’s internal energy

Causes Earthquakes, Tsunamis, Volcanos

Energy sources for disasters- Gravity

Causes Mass movements and snow avalanche

Energy sources for disasters- Solar Energy

Causes Meteorological Storms, Flood, Drought, Wildfire, and Magnetic Storms

Energy sources for disasters- Impact Energy

Impact with Space object

What drives Earth’s internal energy and what does it cause?

• Earth’s internal heat drives plate tectonics → earthquakes, volcanoes, tsunamis.

• Heat mainly comes from radioactive decay (U, Th, K).

• Extra heat left from early impacts + planet formation

What is gravity, and how does it relate to potential and kinetic energy?

• Gravity = force pulling masses together; stronger with more mass, weaker with more distance.

• Higher elevation = potential energy; falling releases kinetic energy.

• Landslides, mudslides, avalanches = gravity‑powered hazards.

How does solar energy affect Earth?

• The Sun makes energy by fusion (H → He), releasing heat + light (solar radiation).

• Uneven heating (warm tropics, cool poles) drives weather, climate, winds → hazards like tornadoes + hurricanes.

• Powers the water cycle → rain, floods, avalanches.

What is impact energy and why was it important early in Earth’s history?

• Space objects (asteroids, meteoroids, comets) hit Earth at ~100,000 km/hr, creating huge kinetic energy on impact.

• These collisions were a major energy source during the Hadean era (~4 billion years ago).

What are some energy sources of natural disasters? (list all that apply)

Gravity (due to attraction between two masses), Solar energy, wind energy, wave energy

The main source of Earth's internal energy is:

radioactive decay

Hazard

Potential for dangerous event.

Vulnerability

Likelihood a community will suffer when exposed to natural hazards.

Risk=

Vulnerability x Hazard

When do natural disasters occur?

when a hazard hits a vulnerable community.

What are hazard and vulnerability?

• Hazards = natural events we cannot control (e.g., earthquakes, storms).

• Vulnerability = how exposed or fragile people are; it can be reduced, but often costs money.

What happens before and after a disaster?

Before a disaster (long‑term):

• Mitigation → reduce future damage

• Preparedness / Adaptation → plan, train, adapt

After a disaster:

• Response (short‑term) → immediate help, rescue

• Recovery (mid‑term) → rebuild, restore

Mitigation and what are examples of structural and non‑structural mitigation?

Actions taken in advance to reduce disaster risk.

Structural mitigation (physical structures):

• Dams, dykes, floodways

• Retrofitting public/commercial buildings

• Earthquake‑proofing homes

Non‑structural mitigation (policies + planning):

• Land‑use rules

• Severe weather warnings

• Building codes

• Public education

What is preparedness, and what are examples of it?

Preparedness = pro‑active steps to get ready for a disaster and ensure you can cope.

Examples:

• Stockpiling essentials (food, water, supplies)

• Household emergency kits

• Evacuation drills

• First‑aid training

What are Response and Recovery after a disaster?

Response (immediate actions after disaster):

• Right after the disaster

• Done by emergency workers, medical teams, police, firefighters

• Must be swift, decisive, coordinated

• Goal: Get the situation under control

Recovery (long‑term to rebuild communities):

• Rebuilding homes, roads, services, communities

• Restoring life to pre‑disaster conditions

• Goal: Get life back to normal

Retrofitting a hospital to better withstand ground shaking during an earthquake is considered to be:

Mitigation

Because it Reduces future risk

• Strengthens structures to limit damage

• Is a long‑term, proactive measure

How are natural disaster trends changing over time?

• Weather‑related disasters (storms, floods, heatwaves) → Increasing in frequency

• Geologic disasters (earthquakes, tsunamis, volcanoes) → Stable frequency over time

Return Period

Average time between events
-Example: A damaging Vancouver Island earthquake every 20 years (on average)

Frequency

1 / Period: Average number of occurrences in a given time
-Example: Vancouver Island experiences 1/20 = 0.05 damaging earthquakes per year

Magnitude

Amount of energy released
-Bigger magnitude → rarer events

• Small events happen often; large events happen rarely

How do disaster impacts differ between developed countries, and what trends are seen in Canada?

• Developed countries:

  • Fewer casualties → better buildings, technology, medical care, education, lower population density

  • Higher economic losses → more expensive infrastructure to damage

• Canada:

  • Recent disasters are mostly weather‑related

  • Likely because geologic disasters have long return periods

How can Earth’s internal structure be described?

• Chemical composition → What it’s made of

• Rheology → How the material behaves or deforms under stress

What are stress and what types of stress exist?

=force per area
Types:

• Compression → pushes together (perpendicular) → contraction

• Tension → pulls apart (perpendicular) → extension

• Shear → slides past (parallel) → distortion

What is strain?

deformation caused by stress

• How much the material changes shape in response

What is rheology, and how do materials behave under stress?

= how materials deform (strain) under stress

• Viscosity = internal resistance to flow

• 1. Liquids:

  • Flow when stressed

  2. Solids:

  • Elastic → deformation is temporary (returns to original shape)

  • Ductile → deformation is permanent

  • Brittle → material breaks/fractures

  • Plastic → flows slowly like a high‑viscosity fluid (e.g., glaciers)

Force per unit area is

Stress

How do time (t), temperature (T), and pressure (P) affect rheology?

Rheology = how materials deform under stress, controlled by t, T, and P.

• Abrupt stress + low T and/or low P → brittle rupture

  • Material breaks rather than flows

• Long‑term stress + high T and/or high P → plastic flow

  • Material deforms slowly and permanently

Examples:

• Glaciers:

  • Flow plastically over long timescales

  • Fracture (crevasses) under sudden stress

• Rock:

  • Brittle near Earth’s surface → earthquakes

  • Plastic at depth where T and P are high

What are the key properties of the Inner Core, Outer Core, and Mantle?

Inner Core (solid)

• T ≈ 5000 °C

• Density: 14–16 g/cm³

• ~2% of Earth’s mass

• Solid metal: mostly iron (Fe) + some nickel (Ni)

Outer Core (liquid)

• T ≈ 4000 °C

• Density: 9.7–14 g/cm³

• ~30% of Earth’s mass

• Liquid Fe/Ni

• Generates Earth’s magnetic field (fluid motion)

What are the key properties of the continental and oceanic crust, and what is the Moho?

Crust–Mantle Boundary

• Moho (Mohorovičić discontinuity)

  • Discovered 1906

  • Marks the compositional boundary between crust and mantle

Continental Crust (brittle)

• T = 0–1000 °C

• Density ≈ 2.7 g/cm³

• ~0.4% of Earth’s mass

• Thickness: ~35 km average (up to 80 km under mountains)

• Composition: granitic; rich in O and Si; ~60% silica (SiO₂)

Oceanic Crust (brittle)

• T = 0–1000 °C

• Density ≈ 3.0 g/cm³

• ~0.1% of Earth’s mass

• Thickness: ~10 km (even thinner at mid‑ocean ridges)

  • Composition: basaltic volcanic rock; ~48% silica

What are the lithosphere, asthenosphere, and mesosphere, and how do they behave?

Lithosphere (rigid)

• Rheology‑based layer: crust + uppermost mantle fused together

• Rigid, brittle solid

• Forms Earth’s tectonic plates

• Controls plate interactions → earthquakes, volcanoes, mountain building

Asthenosphere (weak/plastic)

• Located below the lithosphere

• “Soft,” plastic upper‑mantle layer

• Mostly solid, but with a few percent partial melt

• Flows slowly under stress

• Allows lithospheric plates to move over it

Mesosphere (stiff plastic mantle)

• Beneath the asthenosphere

• Stiffer, stronger plastic behavior

• Still capable of slow flow, but more resistant

Why it matters

• Lithosphere–asthenosphere interactions are key to geologic hazards:

  • Earthquakes

  • Volcanism

  • Plate motion

    • Mountain building

What controls whether a layer inside Earth is solid or liquid, and what are key examples?

• Pressure (P)

  • Increases roughly linearly inward due to overburden

  • High P favors solids (raises melting point)

• Temperature (T)

  • Increases nonlinearly inward

  • Driven by radioactive decay of U, Th, K

• Composition

  • Different materials melt at different P–T combinations

  • Controls whether a layer melts or stays solid

Examples

• Inner core vs. outer core

  • Inner core = solid because very high P keeps Fe/Ni solid

  • Outer core = liquid because lower P allows Fe/Ni to melt

• Outer core vs. mesosphere

  • Outer core = liquid Fe/Ni

  • Mesosphere = solid/plastic silicate mantle

  • Difference is due to composition (silicates vs. metals)

What is rheology?

-how material deforms when subject to stress and how a material strains under stress.

What is isostasy, and what examples help explain it?

It is the gravitational balance where the lithosphere “floats” on the denser asthenosphere.

• Elevation depends on thickness and density of the lithosphere.

Analogy: floating objects in water

• Icebergs: the higher they stick up, the deeper they extend below.

• Higher‑density objects float deeper (wood sinks deeper than styrofoam).

• Adding or removing mass changes elevation → isostatic adjustment.

What real‑world examples illustrate isostasy?

• Mountains sit on thicker lithosphere, so they float higher but also extend deep into the mantle.

• Oceanic lithosphere is denser, so it sits lower, creating ocean basins.

What are examples of isostatic adjustment?

= elevation changes when mass is added or removed.

Example 1: Post‑glacial rebound

• Heavy glaciers push crust down (subsidence).

• When ice melts, the crust rebounds upward.

• Ice melts faster than the asthenosphere flows → rebound continues long after melting.

• Formerly glaciated areas rise; bulged margins sink slightly.

Example 2: Volcanic island subsidence → atoll formation

• New volcanic islands slowly subside as the lithosphere adjusts.

• If coral growth keeps pace with subsidence, a ring‑shaped atoll forms around a sinking island.

What evidence tells us about Earth’s internal structure?

A) Density Distribution

• Earth’s average density = 5.5 g/cm³, but crustal rocks are only 2.7–3.0 g/cm³ → density must increase inward.

• Earth doesn’t wobble much and surface gravity is nearly constant → mass must be arranged in concentric layers.

(B) Earthquake Seismology

• Best evidence for interior structure.

• Seismic wave timing (reflection/refraction) reveals rheological & compositional boundaries.

(C) Magnetic Field

• Earth’s magnetic field requires convecting, electrically conducting fluid → proves outer core is liquid metal.

(D) Direct Observation

• Drilling: crust drilled to ~12 km.

• Lavas: show upper mantle composition to ~200 km.

• Kimberlite pipes: bring deep mantle material + diamonds to surface.

(E) Lab Studies

• High temperature & pressure recreated in labs to study mineral behavior.

• Piston‑cylinder apparatus: simulates ~100 km depth.

  • Diamond‑anvil cell: simulates ~2000 km depth.

What is an earthquake, and what can cause one?

Earthquake = shaking of the Earth caused by seismic (vibrational) waves from an initial disturbance.

Main causes:

• Faulting → sudden movement of rock blocks along a fracture (most common)

• Volcanic activity

• Meteorite impacts

• Landslides

• Explosions

• Oil & gas production (induced seismicity)

• Mining

• Caldera collapses

• Glaciers

Fault

A fracture in Earth across which the two sides move relative to each other (ex. in response to tectonic forces)
-Faults are complex irregular surfaces where interlocking rock is held together by friction.

Where can fault rupture occur, and how does temperature affect it?

• Fault rupture only happens where rock is rigid and brittle

  • Rock must store elastic strain long enough to suddenly break

• High temperatures make rock too plastic to rupture

  • Hot rock flows instead of fracturing

• Temperature increases with depth, so:

  • Most earthquakes occur in the crust

    • Deeper rocks are too warm to break (with rare exceptions)

How does the elastic‑rebound theory explain fault rupture and earthquakes?

• Fault is locked by friction → no slip occurs even though plates keep moving

• Elastic strain builds up as the rocks on either side deform and store energy

• When stress > friction, the fault ruptures suddenly

• Stored energy is released as heat and seismic waves → earthquake

How does fault rupture begin and spread, and what are the hypocentre and epicentre?

• Rupture starts at a point called the hypocentre (focus)

• Rupture then spreads rapidly along the fault surface

  • May or may not rupture the entire fault

  • May or may not reach the Earth’s surface

• Epicentre = point on Earth’s surface directly above the hypocentre

How do strike‑slip faults move, and what deformation occurs at step‑overs?

Strike‑slip faults = horizontal motion only

• No vertical component

• Rock units slide past each other

• Often form long linear valleys, marked by streams or long lakes

• Also called transform faults

Types:

• Left‑lateral → opposite side moves left

• Right‑lateral → opposite side moves right

Step‑over deformation (right‑lateral example):

• Left step → compression + uplift

  • Right step → extension + subsidence

What are dip‑slip faults, and how do normal vs. reverse faults differ?

faults where movement is vertical, along the dip (slope) of the fault plane.

Normal Faults

• Tensional forces → pulling apart

• Hanging wall moves down relative to footwall

• Creates a zone of omission (missing section of strata)

Reverse / Thrust Faults

• Compressional forces → pushing together

• Hanging wall moves up relative to footwall

• Creates a zone of repetition (duplicated strata)

Thrust Fault Image

Normal Fault Image

Strike-slip Fault Image

Why is earthquake rupture mostly limited to the Earth’s crust?

the crust is cold and therefore rigid and brittle.

What did scientists believe before plate tectonics, and what shift in thinking made the theory possible?

(1960s)

• Revolutionized understanding of EQs, volcanoes, tsunamis, and global geologic processes

• To understand the theory, start with the historical context

Before the 20th Century, scientists believed:

• Crustal motion was only vertical

• Oceans and continents were similar (except oceans held water)

• Geologic change was catastrophic and rapid, not slow and continuous

19th–20th Century shift:

• Uniformitarianism began replacing catastrophism

• Idea: Gradual processes operating today also shaped Earth throughout geologic time

• This shift in thinking was essential for developing plate tectonics

What is Continental Drift, who proposed it, and why was it originally rejected?

Wegener’s Proposal (1912)

• Proposed by Alfred Wegener, a German meteorologist

• Early precursor to modern Plate Tectonic Theory

Key Ideas:

• All continents once formed a supercontinent Pangaea (“all‑earth”)

• Surrounded by a global ocean Panthalassa (“all‑ocean”)

• Pangaea later broke apart, and continents drifted to their present positions

Why Wegener’s idea was rejected:

• Opposed scientific thinking of the time

• Wegener could not provide a reasonable mechanism

  • He imagined continents plowing through the seabed like a ship through water

  • Physically impossible with known geology

• Without a mechanism, most geologists dismissed the idea

Aftermath:

• Over the next 50 years, evidence steadily accumulated

  • Eventually led to seafloor spreading and the full Plate Tectonics revolution in the 1960s

What continental‑scale evidence supported Wegener’s Continental Drift?

1. Fit of Continents

• South America + Africa coastlines noted to fit as early as the 1600s (Francis Bacon)

• Fit improves when matching continental shelves (true geologic edges), not coastlines

2. Continuity of Geologic Features

If continents were once joined, continuous features should cross today’s ocean gaps — and they do:

• Rock types & ages match across continents

• Mountain belts continue across oceans

  • e.g., Appalachian Mountains (N. America) align with Caledonian Mountains (Europe)

• Glacial striations (scratches) point from oceans toward land

  • Impossible unless continents were once joined and glaciers flowed across them

• Plant & animal fossils match across now‑separated continents

  • Example: Mesosaurus fossils found in South America + Africa

    • Could not swim across 5000 km of ocean

What ocean‑based evidence helped lead to plate tectonics?

1. Mapping the Ocean Floor

• Before 20th century: ocean depth measured by lead lines (slow, difficult; avg depth ~5 km)

• Echo‑sounding sonar developed by the 1920s (WWI submarine warfare + Titanic sinking)

• By 1940s–50s, sonar revealed:

  • Mid‑Ocean Ridge (MOR) — a continuous 65,000 km mountain chain in all ocean basins

  • Deep‑sea trenches near continental margins

    • Up to 5900 km long (Peru–Chile Trench)

    • Up to 11 km deep (Marianas Trench)

2. Seafloor Sampling & Age Patterns

• Ocean basins are young: always < 200 million years

  • Continents are billions of years old

• Radiometric ages, fossil ages, and sediment thickness all increase systematically away from MORs

  • Indicates new crust forms at ridges and moves outward → key evidence for seafloor spreading

How do global earthquake and volcano patterns support plate tectonics?

• By the 1960s, a worldwide seismograph network could record and locate all major EQs

• EQ foci (hypocentres) are concentrated in linear belts along:

  • Mid‑Ocean Ridges (MORs)

  • Deep‑sea trenches
    → These belts mark plate boundaries

• At trenches, EQ foci dip beneath continents

  • This dipping zone is the Wadati–Benioff zone

  • Indicates subduction of one plate beneath another

Oceanic crust is youngest

near mid-ocean ridges.

Earthquakes occur predominantly:

along plate margins (trenches and mid-ocean
ridges).

What is paleomagnetism, and why is it important for plate tectonics?

• he study of the geologic record of Earth’s magnetic field (MF) through time

• Developed in the first half of the 20th century as instruments improved

• Measures how magnetic minerals in rocks record the direction and strength of Earth’s magnetic field when they formed

Why it matters

• Provided key evidence for plate tectonic theory

• Showed patterns like:

  • Magnetic stripes on the seafloor

  • Symmetry about mid‑ocean ridges

  • Magnetic reversals preserved in basalt

• Confirmed seafloor spreading and plate motion

What does Earth’s magnetic field look like, and what are magnetic field lines and magnetic poles?

• Earth’s MF resembles a bar magnet, but Earth is not a permanent magnet

  • The interior is too hot to retain permanent magnetization

  • The field is generated by the liquid outer core (geodynamo)

Magnetic Field Lines

• Arrows show the direction of the magnetic field

  • A compass aligns with these lines

• The density of lines indicates field strength

  • More lines = stronger MF

Magnetic Poles

• Locations where the magnetic field is vertical

• Not fixed — they wander over time due to changes in the geodynamo

If Earth is not a permanent magnet, what generates its magnetic field?

• Thermal convection + Earth’s rotation drive the motion of liquid iron in the outer core

• This liquid iron is electrically conductive, so its movement creates electric currents

• These currents generate a self‑sustaining magnetic field

• The flow patterns are rotational, so the magnetic poles form near the rotation poles

• This process is called the geodynamo

What are geomagnetic reversals, and what do “normal” and “reversed” polarity mean?

• Earth’s magnetic field is unstable because the outer‑core flow patterns are unstable

• The magnetic field flips direction at irregular intervals

  • North and South magnetic poles switch places

  • Occurs at random intervals from thousands to millions of years

Polarity Terms

• Normal polarity → same orientation as today

• Reversed polarity → opposite orientation

Earth's magnetic field originates in the:

outer core

What is thermal‑remanent magnetization, and how do rocks record Earth’s magnetic field?

Thermal‑Remanent Magnetization (TRM)

• Earth’s magnetic field can induce TRM in certain minerals (e.g., magnetite, Fe₃O₄)

• As volcanic rocks cool, electron domains align with the magnetic field, giving the rock a net magnetization

• When the rock cools below the Curie temperature (T₍C₎ = 550 °C), this magnetization becomes permanent

  • It won’t change even if Earth’s magnetic field later reverses or shifts

Why it matters

• Rocks preserve the direction and polarity of Earth’s magnetic field at the time they formed

• These rocks can be dated radiometrically, allowing scientists to reconstruct magnetic history and support plate tectonics

How does the paleomagnetic record preserve Earth’s history of magnetic reversals?

Recording Magnetic Reversals

• By measuring TRM directions in rocks of different ages (e.g., stacked volcanic flows), geologists can reconstruct the sequence of geomagnetic reversals

• Each rock layer preserves the magnetic polarity at the time it cooled below the Curie temperature

Magnetic Polarity Time‑Scale

• Represented as alternating black (normal) and white (reversed) bars

• Shows polarity changes through time

• Provides a global reference timeline used in plate tectonics and seafloor‑spreading studies

What are magnetic seafloor anomalies, and what pattern do they form around mid‑ocean ridges?

Magnetic Anomalies

• 1950s–60s ship‑board surveys measured magnetic anomalies on the seafloor

• These anomalies are deviations from expected magnetic field strength caused by TRM in oceanic crust

Types of Anomalies

• Positive anomaly → TRM aligned with Earth’s current magnetic field

• Negative anomaly → TRM opposite to Earth’s current magnetic field

Magnetic Seafloor Stripes

• Surveys revealed regular, alternating bands of positive and negative anomalies

• These stripes are symmetric about the mid‑ocean ridge (MOR) axis

• This symmetry reflects seafloor spreading and geomagnetic reversals recorded in basalt as it cools

What are transform faults, and where do earthquakes occur along mid‑ocean ridges?

Transform Faults

• Mid‑ocean ridge (MOR) segments are offset by long linear features called fracture zones (FZs)

• Only the portion of a fracture zone between the offset MOR segments is active

• This active, earthquake‑producing segment is the transform fault

Earthquake Activity

• Earthquakes occur only on the active transform fault, not along the entire fracture zone

• Outside the active segment, fracture zones are inactive scars on the seafloor

What are hot spots, and what does the Hawaiian–Emperor chain show about plate motion?

• Hot spots create linear chains of volcanic islands and seamounts in ocean basins

• Caused by stationary mantle plumes that melt through the moving lithospheric plate above

• As the plate moves, volcanoes age and go extinct in a line trailing away from the active hot spot

Hawaiian–Emperor Chain

• Extends ~4000 km NW, then bends north for another ~2000 km toward the Aleutian Trench

• Hawaii (Big Island) is the youngest and volcanically active

• Islands become older, extinct, and more eroded to the NW

• Beyond Kauai, volcanoes are submerged seamounts

• The bend in the chain records a change in Pacific Plate motion

How do hot spot tracks form, and what do they reveal about plate motion?

Hot Spot Tracks (Tuzo Wilson, 1963)

• Hot spot chains form as oceanic lithosphere moves over a fixed mantle plume

• Mantle plumes supply pulses of magma that create volcanoes in a linear chain

• As the plate moves, older volcanoes become extinct, eroded, and eventually submerged (seamounts)

Evidence for Plate Motion

• Hot spot tracks provide direct evidence of lithospheric motion driven by seafloor spreading

• The bend in the Hawaiian–Emperor chain marks a change in Pacific Plate direction at ~43 Ma

Why Hot Spots Are Useful

• Over 70 hot spots are recognized worldwide

• Hot spots appear stable relative to the deep mantle

• Because they barely move, they form a stable reference frame for measuring absolute plate motions

Where to earthquakes predominantly occur along mid-ocean ridge systems (MOR)?

Between points B and C (the transform fault).

What key discoveries led to modern plate tectonics, and what are the basic principles of the theory?

From Seafloor Spreading to Plate Tectonics

• Recognition of seafloor spreading explained how new oceanic lithosphere forms at mid‑ocean ridges (MORs)

• The question “What happens to old seafloor?” led to the discovery of subduction zones, where old oceanic lithosphere descends and is recycled into the mantle

• These insights drove acceptance of modern plate tectonic theory

Basic Principles of Plate Tectonics

• Earth’s surface is divided into interlocking lithospheric plates

  • Boundaries include mid‑ocean ridges, subduction/collision zones, and transform faults

• Plates are internally rigid

• Plates move relative to each other on the deformable asthenosphere beneath them

State the type of plate boundary (trench, mid-ocean ridge,
no plate boundary) for locations A, B, and C on the map

A — Trench (Subduction Zone)

• Located off the west coast of North America

• Characterized by deep ocean trench + active subduction

B — Mid‑Ocean Ridge

• Located in the central Atlantic Ocean

• This is part of the Mid‑Atlantic Ridge, a classic divergent boundary

• Marked by seafloor spreading and shallow volcanic activity

C — Mid‑Ocean Ridge

• Located in the South Atlantic Ocean

• Also part of the Mid‑Atlantic Ridge

• Another divergent boundary with new crust forming

What does plate tectonics explain, and what mechanism does it provide for continental movement?

Plate Tectonics Explains:

• Fit of continents and geologic/fossil continuity

• Mid‑ocean ridges and deep‑sea trenches

• Age patterns of the seafloor

• Global earthquake and volcano distribution

• Magnetic seafloor stripes (symmetry + age patterns)

• Transform faults and hot‑spot tracks

Mechanism for Continental Movement:

• Continents ride on moving lithospheric plates

• Plates move like a conveyor belt driven by seafloor spreading and mantle processes

• NOT by continents plowing through ocean crust (Wegener’s incorrect mechanism)

What forces drive plate tectonics, and how do ridge push, slab pull, and mantle convection work?

Plate tectonics is driven by Earth’s internal heat, through three main mechanisms:
1. Ridge Push

• Has two components:

  • Outward push from upwelling hot mantle at mid‑ocean ridges

  • Gravitational sliding of elevated lithosphere down the ridge flanks

• Ridges are high because hot mantle heats + expands the lithosphere → lower density → isostatic uplift

2. Slab Pull

• As lithosphere spreads away from the ridge, it cools, contracts, and becomes denser

• Eventually becomes denser than the underlying asthenosphere

• At subduction zones, the cold, dense slab sinks, pulling the rest of the plate behind it

• Most powerful of the driving forces

3. Mantle Convection

• Thermally driven convection cells circulate in the mantle

  • Upwelling beneath MORs

  • Downwelling at subduction zones

• Plates are carried along like a conveyor belt by this circulating mantle flow

What happens at a spreading centre, and what earthquakes occur there?

Process

• Plates pull apart at a divergent boundary

• Magma upwells from the asthenosphere

• Creates new oceanic lithosphere as it cools

• Forms either a mid‑ocean ridge (MOR) or a continental rift valley

Earthquake Characteristics

• Small magnitude: typically M < 5

• Very shallow: < 10 km depth

• Tensional earthquakes on normal faults

• Maximum size and depth are limited because high temperatures beneath MORs prevent large fault rupture

What are slow, intermediate, and fast spreading rates, and what do full vs. half‑spreading rates mean?

Spreading Rates

Slow spreading: 1–5 cm/yr

• Example: Mid‑Atlantic Ridge

Intermediate spreading: 5–9 cm/yr

• Example: Southeast Indian Ridge

Fast spreading: 9–18 cm/yr

• Example: East Pacific Rise

Full vs. Half‑Spreading Rates

• Full‑spreading rate:

  • The rate at which plate A moves away from plate B

  • Total separation rate across the ridge

• Half‑spreading rate:

  • The rate at which one plate (A or B) moves away from the MOR axis

  • Half of the full rate

What is rifting, and what are the four stages in the evolution of a continental rift?

Rifting

• Process where a continent stretches, fractures, and begins to break apart

• Driven by mantle heat, extension, and faulting

• Can eventually form a new ocean basin
  
  1. Centering

  • Continent is centered over a hot region in the mantle

  2. Doming

  • Heat causes expansion and uplift

  • Leads to stretching and fracturing

  3. Rifting

  • Gravity causes fractures to fail, forming faults

  • The centre sags

  • Volcanism begins as mantle material rises

  4. Spreading

  • The deep rift floods with ocean water

  • Continued divergence forms new oceanic crust

  • Marks the transition from rift → young ocean basin

What are triple junctions, and how do they form during rifting?

• Rifting process often creates a triple junction of 3 new spreading centres

• A triple junction is a point where three plate boundaries meet

• In rift settings, these are typically three diverging (rift) arms

• Common in early continental breakup, where mantle upwelling promotes three‑armed rift geometry

Why They Matter

• Help determine which rift arm becomes an ocean and which become failed rifts (aulacogens)

• Classic example: Afar Triple Junction (Red Sea, Gulf of Aden, East African Rift)

What are the three types of convergent margins?

1. Ocean–Continent Convergence

2. Ocean–Ocean Convergence

3. Continent–Continent Convergence

What are the key features of Ocean-continent convergence?

• Produces a subduction zone

• Oceanic plate subducts beneath continental plate

• Forms trenches, volcanic arcs, and powerful earthquakes
  
  Denser oceanic lithosphere subducts beneath continental lithosphere

• Trench forms from down‑bending oceanic plate

• Continental volcanic arc forms

  • Caused by dehydration of the subducting slab, triggering melting in the mantle

• Produces the largest earthquakes on Earth

  • Up to M 9+

What are the key features of Ocean–Ocean Convergence

• Also a subduction zone

• One oceanic plate subducts beneath another

• Forms deep trenches and volcanic island arcs

Older, colder, denser oceanic plate subducts beneath the younger plate

• Forms a volcanic island arc

• Generates very large earthquakes

  • Up to ~M 8.5

What are the key features of Continent–Continent Convergence

• A collision zone

• No subduction once buoyant continents meet

  • Forms massive mountain belts (e.g., Himalaya)

Low‑density continental crust resists subduction

• Results in:

  • Uplift → formation of compressional mountain belts

  • Down‑warping → creation of extra‑thick crust

• Produces shallow, compressional earthquakes

  • Up to M 8.5

Where do deep-focus earthquakes occur?

subduction zones

What characterizes transform faults?

• Conservative boundary (no crust created or destroyed)

• Strike‑slip motion between plates

• Shallow EQs (<35 km)

• Can produce major EQs (up to M ~8)

Where do most transform faults occur?

• Mostly on the seafloor, offsetting MOR segments

• EQs here are rarely harmful

• On land, they can be highly destructive (San Andreas, Alpine Fault, North Anatolian Fault)

What causes earthquakes at hot spots?

• Rock expansion/fracture from hot magma

• Harmonic tremors from moving magma

• Fault failure from uplift/subsidence due to repeated magma injection/withdrawal

• Range from small to large EQs

Why does western Canada have the highest earthquake risk?

• Close to active plate boundaries

• Major hazard to Victoria, Vancouver, Seattle, Portland

• Cascades = active volcanic arc (Mt. Garibaldi, Mt. Baker, Mt. Rainier, Mt. St. Helens)

Where do inter‑plate earthquakes occur?

• On the shear interface between subducting and overriding plates

  • Includes megathrust earthquakes (largest on Earth, M 8–9+)

What evidence shows past megathrust EQs in Cascadia?

• Coastal marsh layers (peat → mud → tsunami sand)

• Drowned coastal forests

• Seabed turbidite layers

• Japanese tsunami records

• First Nations oral histories

What sediment sequence indicates past megathrust EQs?

• Peat (land plants)

• Mud (submerged coastal sediment)

• Sand (tsunami deposit)

What do turbidite layers show?

• Alternating coarse sand and fine mud in deep‑sea cores

• Each sand layer = submarine landslide triggered by a megathrust EQ

What happens during the inter‑seismic period of a megathrust cycle?

• Elastic strain builds in overriding plate

• Toe dragged down

• Coastal uplift (decreasing inland)

• Crustal shortening

What happens during the megathrust earthquake itself?

• Toe jumps up (1–5 m), triggering tsunami

• Coastal subsidence (1–2 m)

• Crustal extension (10–20 m)

hat do geodetic measurements reveal about Cascadia?

• Uplift up to 6 mm/yr

• Uplift decreases inland

• Matches the inter‑seismic deformation expected in megathrust zones

What is the megathrust earthquake risk in Cascadia?

• 13 past megathrust EQs

• Recurrence: 300–900 yrs (avg. 400–600 yrs)

• Last one: AD 1700 (319 yrs ago)

• 5–10% chance of M9+ in next 50 yrs

• M9+ shaking lasts several minutes and affects a huge area

What’s the difference between a seismometer and a seismograph?

• Seismometer: Detects ground vibrations

• Seismograph: Records those vibrations (now digital, in 3D: N‑S, E‑W, vertical)

Basic Wave Properties-Key wave terms to know

• Amplitude: Maximum height

• Wavelength (λ): Length of one cycle

• Period (T): Time for one cycle

• Frequency (f): Cycles per second (Hz)

• Velocity: V=λf