Physical Geography Week 8+

Harry Hess

Theorized mid-oceanic ridges by way of seafloor spreading

• 1960s

Subduction

When one portion of the lithosphere descends beneath another portion

• Dense ocean floor (basaltic) subducts beneath lighter continental crust and is recycled as magma

Ridge Push

Magma rises and pushes plates apart at oceanic ridges

Slab Pull

Slab sinks back into mantle when colliding with a continental plate, and the force of gravity pulls the rest of the plate with it too

Plate Boundary Movements

Transform, Divergence, Convergence

Convergent Plate Boundaries

When plates collide and subduct

• Occurs between oceanic plates

  • Creates oceanic trenches and volcanic chains

• Occurs between continental plates

  • Mountain building, earthquakes

• Occurs between continental + oceanic

  • Continental volcanic arc as melted oceanic crust forms magma which rises to the surface

Divergent Plate Boundaries

Plates moving away from each other

• Continental and Oceanic

  • Oceanic occurs due to upwelling of magma which pushes plates apart

Transform Boundaries

Sections of plates sliding horizontally past each other

• Creates powerful earthquakes

Hot spots

Areas of upwelling magma not associated with plate boundaries

• Some are fixed to lower mantle and create island chains as plates slide over the hot spot

• Some move with plate motion

Characteristics of Old and Young Rocks

Old

• More weathered and eroded (smoother)

New

• More jagged and rough as less erosion

Relief

Vertical elevation differences on the Earth’s surface

Topography

Shape of relief of Earth’s surface

Measurement of Relief

Radar + LiDAR

• Radar is less detailed than LiDAR

Orders of Relief

First order

• Coarse details, like entire continents or oceans

Second Order

• Areas within oceans or continents

Third Order

• Individual landforms

Crustal Formation

1. Residual Mountains on Stable Continental Cratons

2. Tectonic Mountains and Landforms

3. Volcanic Mountains

Cratons

Stable crystalline “nucleus” on which a continent grows

Continental Shield

Where craton is exposed at the surface

• Lack of relief due to billions of years of weathering

Orogenesis

Mountain building at plate boundaries

Mountain Building on Stable Cratons

Ocean plate subduction brings sea water and other continental sediment which creates viscous magma.

• Magma either reaches surface in explosive volcanos or

• Becomes subsurface intrusive rocks

Over time, fragments of ocean floor and volcanic islands become forced against the edges of cratons and move inwards (terranes)

Mountain Building by Crustal Deformation

Causes folding and broad warping of the Earth’s crust

Types of Stress + Definition

Force that affects an object

• Tension (resulting in stretching + normal fault)

• Compression (resulting in shortening + reverse/thrust fault)

• Shear (resulting in lateral twisting + strike-slip fault)

Types of Strain + Definition

How rocks respond to stress

• Folding (bending)

• Faulting (breaking)

Normal Fault

Rocks are pulled apart

• Hanging wall = crust that drops down

• Foot wall = continental crust that does not drop

Reverse/Thrust Fault

When rocks move upward along fault plane

• Hanging wall “thrusts” above continental footwall

Strike-Slip Faults

Horizontal movement of plates along a transform boundary

• Either right lateral or left lateral

• Can cause vertical displacement of ground

Earthquake

Release of energy as plates slide past each other, get locked, and then release

Focus vs Epicenter

Focus = point in the ground at which energy is released

Epicenter = point on the ground’s surface directly above focus

Elastic Rebound Theory

Plates become stuck as they move past each other. Over time, the strain associated with the stress begins to build. The release causes the plates to “snap” back into proper position

Richter Scale

Used since 1935, measures amplitude magnitude

• Logarithmic scale from 1.0 to > 8.0

• Amplitude Magnitude = size of seismic wave on seismometer

Logarithmic Details on the Richter Scale

10fold increase in energy from one whole number to the next

• or 31.5fold increase in energy released

Moment Magnitude (M) Scale

Used since 1993 and is more accurate than Richter scale

• Measures fault’s length, slippage, ruptures, nature of materials involved

Earthquake Wave Propogation

P Waves and S Waves

P Waves

• Compressional

• 1.5-8km/s

• Shake ground in the direction they are propagating

• less impact than S waves, can be sent out before S waves

S Waves

• Shear/Secondary Waves

• Ground shakes perpendicularly to direction of propagation (usually vertically)

• Slower than P waves

• Higher impact than P waves

Determinant Variables of Magnitude and Intensity of Earthquake

1. Earthquake Properties - magnitude, type, location of epicentre and depth of focus

2. Local Geological Conditions - Distance from event, path of seismic waves, bedrock type, water saturation

3. Societal conditions - quality of building, preparedness, time of day

Settings for Volcanic Activity

• Subduction Boundaries

  • Continental-Oceanic or Oceanic-Oceanic

• Divergent Plate Boundaries

  • Along seafloor spreading areas or continental rifts

• Hotspots

  • Hawaiian island chain, Yellowstone

Crater

Circular depression near the vent of the volcano

Pyroclastics

Rock and clastics that leave the volcano and form new ground

Cinder Cone

Cone that forms from pyroclastic materials

Caldera

Large Depression

Types of Eruptions

Extrusive and Effusive

Extrusive Volcanos

Typically associated with plate convergence = thicker magma

Cinder cone

• Explodes 1- 20 years, small

Shield Volcano

• Low profile, long-lasting lava flows from the centre

Composite Volcanos

• Small, steep profile. Infrequent eruptions, but pyroclastic lava = low viscosity = fast moving

Dome Complexes

• Small size, domes under pressure which explode

Effusive Eruptions

Gentle, slow, lots of lava

Form shield volcanos and flood/plateau basalts

Associated with plate divergence

Explosive Eruptions

Magma comes from subducted ocean plates

• Magma is thicker than effusive volcanos → blocked pipes = explosion

  • Usually composite volcanos

Earthquakes + Volcanos

Earthquakes can trigger volcanos through movement of tectonic plates

Volcanos can trigger earthquakes through the movement of magma under tectonic plates

Caldera

Crater-shaped depression that forms after a volcano collapses in on itself after an eruption

• Yellowstone

Geomorphology

Study of landform, origin, evolution, and distribution

• deals heavily with denudation

Geomorphologic Sequences

1. Equilibruim, stability

2. Destabilization event (landslide)

3. Period of adjustment

4. Establishment of new equilibrium over time

Differential Weathering

Different materials that weather/break down at different rates

Regolith and Bedrock

Regolith - Loose surface material overlying bedrock

Bedrock - Hard consolidated rock

• weathering of bedrock creates regolith

5 Factors that Influence Weathering

1. Rock Composition and Structure

2. Climate

3. Subsurface Water

4. Slope Orientation

5. Vegetation

Rock Composition and Structure

• The character of the rock determines how easily weathering occurs (hard, soft, soluble, insoluble, etc.)

• Joints (fractures) in the rock increase the surface area of the rock which increases the amount of weathering 

Climate

Precipitation, temperature, freeze and thaw cycles

• Wetter + warmer environments = speeding up of weathering processes

• Colder environments = freeze-thaw cycles

Subsurface Water

Water movement within soil and rock structures

Slope Orientation

The way the slope faces affects how moist, dry, or cool or hot it will be. Slopes facing the sun are warmer and dryer with less vegitation.

Vegitation

Can shield rock from weathering and stabilize soils

Can increase weathering via organic acids from decaying plants

Roots break rocks apart

Types of Weathering

Physical (Mechanical) and Chemical

Physical Weathering

Disintegration of rock without chemical alteration

• Breaking up rock increases SA for weathering

Chemical Weathering

Chemical breakdown of rock always in the presence of water

Types of Physical Weathering

Frost Wedges

• Water pools in rocks, expands when freezing and splits rocks

Salt Crystal Growth

• Ocean water evaporates from rocks, forms salt crystals which break apart the rock

Exfoliation

• Rock peels or slips off in sheets as pressure is released via the erosion of overlaying material

Movement of Material Down Slopes

If the slope is steep enough, gravity via shear stress will overcome friction and material will move

• Angle of Repose - maximum slope angle allowing stability and equilibrium

• Shear strength - Resisting force created by cohesion and internal friction

• Shear Stress - Driving force caused by gravity

• Water - saturated soil moves faster than partially saturated

Driving Forces

depend on

• Weight, size, and shape of material

• Slope exceeding the angle of repose

• Moisture availability (partial saturation binds as air bubbles create vacuums. Full saturation allows for flow as material is not bound together)

Resisting Forces

Resisting forces depend on

• Cohesiveness

• Shear Strength

• Material

Shear Strength vs Shear Stress

Shear Strength = Force resisting movement created by cohesion and friction

Shear Stress = Force that causes movement due to gravity

How mass movement is triggered

If shear stress is greater than shear strength, there is disequilibrium and materials move down slope

• can be triggered by heavy rain, wildfire, earthquake

Slope Shape

Waxing Slope - materials here have high PE, and can move due to weathering

Waning Slope - materials move down waxing slope and collect on the waning slope (deposition)

Mass Movements / Mass Wasting

Movements of rock due to imbalance of shear stress and shear strength.

• Often a result of weathering which weakens the material

• The greater the slope angle, the higher likelihood of a mass movement

What is Required for Slope Failure?

1. Slope becomes saturated with water

2. Slope becomes over-steepened - greater angle of repose

3. Volcanic eruption melts snow and ice - movement of debris downslope

4. Earthquakes shakes debris or fractures rock

Causes of Human-Induced Mass Movements

1. Deforestation

2. Building on steep gradients

3. Road Construction

4. Undercutting, mining

Types of Mass Movements

Fall - Material falls through air

Slide - Rapid movement of non-saturated material

Flow - Movement of saturated material

Creep - Persistent, gradual movement caused by freeze/thaw cycles

Types of Slides

Traditional - Along a plane

Rotational - Along a concave surface

Types of Creep

Continuous - slow deformation of soil, produced by gravity acting downslope

Discontinuous - Movement caused by freeze-thaw cycle which creates ridges

Fluvial Geomorphology

Movement of sediment in water down streams. Shapes landscapes.

Deposition

Landforms that are shaped by the placement of eroded material

• As wind and water lose energy, they stop carrying materials

• Creates floodplains, terraces, and deltas

Hydraulic Action

Erosion caused by flowing water only. Loosens and lifts rocks

• Greatest at high elevations due to turbulence and small sediment load

Abrasion

Debris movement, which grinds down the streambed

Stream Competence

Stream’s ability to move particles of a different size

Capacity

Total possible load a stream can support

Sediment Movement Types in Streams

Solution - Dissolved materials 

Suspension - Fine particles suspended and moved by water turbulence

Saltation - Bouncing in short hops into flow (Bed and Suspended Loads)

Traction - Rolling or sliding along the bed

Types of River Forms

Straight Streams, Braided Streams, and Meandering Streams

Straight Streams

Control is rock type, short lengths, rare in nature

• High gradient

Braided Streams

Occurs with reduced discharge and excess sediment

• Steeper, wider, shallower, and straighter than meandering channels

Meandering streams

Control is slope

• Gradual slope

• Outer parts = max velocity, greatest erosion

• Inner parts = min velocity, sediment deposition

Alluvium

Mineral and silt deposits formed by running water

• Depositions of alluvium creates sandbars and floodplains

Floodplains

Flat areas on sides of stream that floods frequently

• Natural levees form as sediments accumulate in thickness

Point Bar Deposits

Deposits that form on the inside of a river bend

Sequence of Deposition Land Formations

Weathering → Mass Movement → Erosion → Transportation → Deposition