Geography 1000 - Test 4

Theory of Plate Tectonics → Founder/Theorist

• Alfred Wegner

• Theory remains controversial for decades until the concept of seafloor spreading was developed in the 1960s, providing critical evidence for plate movement.

Evidence Supporting Plate Tectonic Theory

• Continental drift

• Mapping of seafloor and mid-ocean ridges

• Magnetic reversals recorded in seafloor spreading

• Ocean crust gets older further from seafloor ridges

Continental Drift

• Matching coastlines were and early observational clue

• Rock, fossils, and ancient environments line up across oceans

Bathymetry

• Mapping and analyzing underwater terrain

• sonar, satellite, LiDAR, etc.

Seafloor spreading

• Seafloor spreading creates new ocean crust; new basaltic crust forms as plates move apart

• Magma rises to fill the gap and solidifies as new oceanic crust

• New crust is added continuously as older crust moves away from the ridge

Magnetic reversal at the seafloor

• Magnetic reversals record seafloor spreading

• Earth’s magnetic field leaves a symmetrical pattern in newly formed ocean crust

• Cooling basalt locks in the direction of Earth’s magnetic field

• The magnetic field has reversed many times through Earth history

• Matching magnetic stripes on both sides of a ridge show that new crust forms and moves outward

Oldest vs youngest crust

• Younger oceanic crust is located along the mid-ocean ridges

• Crust becomes progressively older with distance from the ridge

Earth’s lithosphere is broken into how many plates?

• 14 main plates

• Plates include both oceanic and continental lithosphere

• Most geologic activity is clustered near plate boundaries

3 kinds of plate boundaries

• Convergent

• Divergent

• Transform

Types of convergent boundaries

• Ocean-ocean

• Ocean-continent

• Continent-continent

Divergent plate boundaries

• Plates move apart and new lithosphere forms between them

• Features: ridges, rifts, shallow earthquakes, volcanism

Divergent plate boundary → Ocean ridges

• Seafloor spreading creates basaltic oceanic crust

Divergent plate boundary → On Continents

• Stretching and thinning can create a rift valley

Transform plate boundaries

• Plates slide horizontally past one another

• Conserve crust; no major crust is created or destroyed

• Shear stress accumulates along faults

• Can produce shallow earthquakes

Ocean-continent convergence

• dense oceanic lithosphere descends beneath the continental crust that is less dense; subduction

• builds continental margins

• Trenches, earthquakes, magma generation, mountain building

Ocean-ocean convergence

• one oceanic plate subducts beneath the other

• Builds island arcs

• Marked by deep ocean trenches

• The magma generated produces a curved chain of volcanic islands

Continent-continent convergence

• Continental crust resists subduction and shortens and thickens

• Little to no subduction-arc volcanism

Hazards created by each type of plate boundary

Convergent

• Subduction

• Strong Earthquakes

• Volcanic arcs

• Ocean lithosphere sinks

Divergent

• Rifting/spreading

• Shallow earthquakes

• Basaltic volcanism

• New crust forms

Transform

• Plate slide past

• Shallow earthquakes

• Little volcanism

• Crust is neither made nor destroyed

Hot Spots

• Areas of persistent volcanic activity within a plate

• Localized mantle upwelling

• Do not have to sit on a plate boundary to form

Identify the plate-boundary type and expected landform:

Young basaltic seafloor + ridge

• Divergent boundary

• New ocean crust forms at a mid-ocean ridge

Identify the plate-boundary type and expected landform:

Deep trench + volcanic mountain chain on a continent

• Ocean-continent convergence

• Subduction builds a trench and continental volcanic arc

Identify the plate-boundary type and expected landform:

Very high folded mountains + thickened continental crust

• Continent-continent convergence

• Collison shortens and thickens bouyant continental crust

Identify the plate-boundary type and expected landform:

Linear fault zone + horizontal displacement + earthquakes

• Transform

• Shear produces strike-slip faulting; crust is conserved

Cratons

• old, strong pieces of continental lithosphere

• Tectonically stable

• Where exposed at the surface they form shields

Terrane accretion

• Crust fragments added onto the edge of an existing continent

Stress

• The force applied to rock

• Measured as force per unit-area

• Can pull, push or shear rock

Strain

• The deformation produced by stress

• Response depends on the rock type, temperature, pressure, and stress

3 main stresses

• tension

• Compression

• Shear

Tension

• Pulls rock apart and produces faults

• Common at divergent boundaries and continental rifts

Compression

• Pushes rocks together, producing folds and reverse or thrust faults

• Common at convergent plate boundaries

• Ductile rocks fold; brittle rocks fracture

Shear

• distorts the crust and produces strike-slip faults

• Common at transform boundaries

Normal Fault

• Caused by tensional stress

• Hanging wall moves down

Reverse / thrust fault

• caused by compressional stress

• Hanging wall moves up

Strike-slip fault

• Caused by shear stress

• Blocks move laterally

Folding

• The ductile response to compression

• Rocks bend instead of breaking, layered rocks from folds

• Anticline: arch-shaped fold

• Syncline: tough-shaped fold

Steps of mountain formation

1. Add crust → volcanism, igneous intrusions, etc.

2. Deform crust → compression folds, faults, shortens existing rocks

3. Thicken + uplift → thicker, more buoyant crust rises to create high relief

4. Erosion → water and gravity break down mountains

Earthquake

• A sudden release of stored elastic energy

• When stress exceeds fault resistance, the fault slips abruptly

• Energy radiates outward as seismic waves

Elastic rebound

• stress accumulates gradually, slip and energy releases are sudden

Earthquake focus

• Point inside Earth where rupture begins

Earthquake epicenter

• Point on the surface directly above the focus

P Waves

• Fastest seismic waves

• Arrive first

• Compressional "push-pull” motion

• Travels through solids, liquids, and gases

S Waves

• Slower

• Arriver after P waves

• Shearing motion perpendicular to travel direction

• Travel through solids only

Seismometer

• Records ground motion; seismograms turn shaking into a measurable time series

• Amplitude records the size of ground motion at the station

• Estimate source location and magnitude

Magnitude

• One value for the event

• Based on physical size of rupture /seismic moment

• Does not depend on where you are

Intensity

• Varies from place to place

• Describes observed shaking and damage

• Depends on distance and local conditions

Why can the same magnitude produce very different damage?

Earthquake: magnitude, depth, rupture, direction, distance

Ground conditions: bedrock vs sediment, water saturation, basin effects

Society: building design, density, preparedness, time of day

Earthquake hazards

Ground failure

• Surface rupture

• Liquefaction

• Landslides

Water hazards

• Tsunami

• Dam / levee failure

• Coastal inundation

Built environment

• Building collapse

• Fires

• Infrastructure disruption

Which community is likely to experience the greatest damage, and why?

a. M7.0, shallow, 15 km from a dense city built on water saturated sediment

b. M7.0, shallow, 15 km from a low-density town on competent bedrock

c. M7.0, deep, 120 km from a city with modern seismic construction

d. M6.5 offshore subduction event with strong vertical seafloor displacement

a. likely strongest urban damage. Shallow source + soft saturated sediment + high exposure.

b. lower than A. Same magnitude and distance, but stronger ground and lower exposure

c. typically lower shaking. Greater depth and distance reduce shaking; resilient construction lowers vulnerability

d. special concern. Even with lower magnitude, vertical seafloor displacement can create tsunami hazard

Where do volcanoes erupt?

Divergent boundaries: decompression melting produces basaltic magma

Subduction zones: water from the slab promotes melting above the descending plate

Hotspots: volcanism occurs within plates, away from a boundary

Most volcanoes occur in predictable tectonic settings

Low viscosity

• Hotter, basaltic magma

• Flows easily

• Gas escapes easily

• Less explosive

High viscosity

• Cooler, silica-rich magma

• Slower flow

• Gas can become trapped

• More explosive

• magma traps gas until pressure released violently

• Ash columns

Composite volcanoes

• Steep, tall, layered cone

• Built from lava flow

• More viscous magma

• Explosive eruptions

Shield Volcanoes

• broad, gentle slopes

• Built by fluid basaltic lava

• Effusive eruptions

• Long lava flow that spreads far

Cinder Cones

• small, steep sided cone

• Built from sediment and cinder

• Short-lived eruptions

• Formed around a single vent

Pyroclastic flows

• hot mixture of gas, ash, and rock that move rapidly

• They are dense, fast, extremely hot and leave little time to escape

Volcanic hazards near the vent

• Ballistic blocks / bombs

• Lava flow

• Pyroclastic density currents

Volcanic hazards down valley

• debris avalanches and landslides

• Lahars

• Flood from melted snow / ice

Volcanic hazards downwind

• ash fall

• air-quality + health impacts

How do we monitor volcanoes?

• Seismicity

  • Fracturing magma can produce earthquake swarms

• Deformation

  • GPS and satellite measurements

• Gas+heat

  • Changes in gas chemistry and thermal output

Caldera

• form after large magma systems erupt and collapse

• Lava hotspot system

Endogenic landform processes

• Endogenic processes are geological events powered by energy originating inside Earth

• Tectonic uplift

• Volcanism

• Crustal deformation

Exogenic landform processes

• Originate from external forces acting on the Earth

• Weathering

• Erosion + transport

• Mass movement

Steady state system

• Conditions fluctuate around a stable average

• Small adjustments are frequent

Dynamic equilibrium

• The landscape adjusts while the system remains in balance

• A longer-term trend can occur as conditions change

Weathering

• The breakdown of rock in place

• NO transport required

Erosion

• Removal + Transport

• Water, ice, wind, gravity

Mass movement

• Gravity-driven downslope motion

• Rock, debris, or soil

Deposition

• Material comes to rest

• Sediment accumulates

Physical Weathering

• Breaks rock into smaller pieces

• Physical breakdown increases surface area for chemical weathering

• Does not change mineral composition

Chemical weathering

• Dissolves or alters minerals

• Weakens rock and creates material that can later be transported

Why do rocks not weather at the same rate?

• differences in mineralogy, fractures, exposure and climate

• Weak or soluble layers retreat faster

• Joints expose more surface area to water and air

Regolith

• loose surface material that become sediment and often the parent material for soil

5 controls of rock weathering

1. Rock + Joints

2. Climate

3. Water

4. Slope Aspect

5. Vegetation

How does rock composition and structure affect weathering?

• Different minerals weather at different rates

• Hard/insoluble rock generally persists longer

• Joints and fractures increase exposed surface area

• More pathways → more water and oxygen for chemical reaction = faster weathering

How does climate affect weathering?

• Warm + wet → rapid chemical weathering

• Repeated freezing + thawing → frost cracking

• Dry climates can favor salt crystal growth

• The same rock in different climates will have a different rate of weathering.

How does subsurface water affect weathering?

• water enters joints, bedding, planes and pores

• Water dissolves minerals and transports dissolved ions

• Acid in soil / water can accelerate erosion

• Drives both chemical and physical weathering

How does slope orientation affect weathering?

• different slopes have different sun exposure, temperature, moisture, snowfall, vegetation, etc.

• South-facing slopes = warmer and drier

• North-facing slopes = cooler and moister

How does vegetation affect weathering?

• Roots reinforce regolith and can reduce erosion

• Tree canopies can shield the surface

• Roots can pry open existing cracks

• Organic acids can lead to chemical weathering.

• Plants can stabilize and weather rock at the same time

Examples of physical weathering processes

• frost cracking

• Salt crystal growth

• Root wedging / exfoliation

Examples of chemical weathering processes

• dissolution

• oxidation

• Hydrolysis

Frost cracking

• water et]enters cracks; repeated freezing can generate enough stress to detach blocks

• Common in cold and high elevation environments

Salt crystal growth

• physical weathering process

• water evaporates

• crystals grow in rock pores

• Crystal pressure weakens the rock

Exfoliation / sheeting

• physical weathering process

• Outer sheets detach

Hydrolysis

• Chemical weathering process

• Water reacts with silicate minerals and forms clay

Oxidation

• Chemical weathering process

• Oxygen reacts with iron-bearing minerals

• Forms rust-like oxides that weaken the rock

Dissolution

• chemical weathering process

• Acidic water dissolves minerals, weakening and removing rock

Triggers that can cause a slope to fail

• Rain / Snowmelt

• Earthquake

• Erosion

• Volcanic activity

• Human activity

Explain this image

• Gravity drives slope motion; friction and cohesion resist it

Driving factors

• Gravity

• Weight of material

• Angle of slope

Resisting factors

• Friction

• Cohesion

• Root reinforcement

Relationship between resisting strength/driving stress and slope failure

Stable = resisting strength > driving stress

Failure = driving stress > resisting strength

How do people affect slope stability?

• can increase or reduce

• Construction changes slope, drainage and vegetation

4 types of slope failure

1. fall

2. slide

3. flow

4. creep

Falling slope

• material detaches and falls through the air

• Triggered by weathering, undercutting or shaking

• Rapid

• Happens at steep/vertical slopes

Translational slide

Movement along a planar surface

Rotational slide

Movement along a curved surface

Debris flow

• Triggered by intense rain or rapid snowmelt

• extremely fast and far reaching

Creep slope failure

• Slow deformation accumulates over years

solifluction

Slow, downslope flow of waterlogged soil over frozen ground or permafrost