Geography test 4

Bathymetry

A measure of the underwater ocean floor topography. Once they were able to map this, scientists discovered interconnected underwater mountain chains called mid-ocean ridges.

Understanding of continental drift needed understanding of sea floor bathymetry

Harry Hess

Geologist → spreading ocean crust makes way for oozing magma forming mid-oceanic ridges

Seafloor spreading

• Upwelling creates sea floor; movement of ocean crust apart
• Moves continents apart

But is the Earth Getting Larger?

No. The areas that are sinking do so because this old part of the Lithosphere (oceanic crust) is the densest because it is oldest and furthest from the ridge and so it moves downwards into the upper mantle and sinks with gravity into the trenches. This requires the force needed to pull apart the ocean crust in the first place

Hess and Robert Dietz

→ Old sea floor sinks along subduction zones: Deep trenches • Oldest parts of ocean crust are dense; sink due to gravity.

Seafloor spreading and subduction

The upper mantle is slowly moving due to convection, causing magma to rise through fractures and small volcanoes along the ridge. As the material rises to form elevated ridges, where gravity forces it downhill - called ridge push. This forms the ocean crust as it moves and cools, and as it cools, it becomes denser. Slab pull is associated with the diverging plate separation and cooling, causing it to increase in density. Eventually the heavy weight of the crustal formation forces the oceanic crust down under the lighter continental crust through the process of subduction. The material is then melted down and becomes magma in the asthenosphere, where over millions of years, it rises up again to become oceanic crust or extrusive igneous rock (recall extrusive igneous rock is magma that eventually builds mountains). The trenches are very deep, with the deepest being the Mariana Trench.

3 types of plate movement

1. Subduction
2. Divergence
3. Transform

Subduction

Where plates are moving together.

Divergence

Where plates are moving apart.

Transform

Where plates slide past each other.

convergent plate boundaries

A tectonic plate boundary where two plates collide, come together, or crash into each other. Occur when plates collide and subduct.

Convergent oceanic plates

occur in the ocean.

Convergent continental plates

occur on land.

Convergent oceanic and continental plates

occur between ocean and land.

divergent plate boundaries

Areas where plates move away from each other, forming either mid oceanic ridges or rift valleys.

While most divergent oceanic boundaries occur in the ocean, a few of them also occur on land as divergent continental plate boundaries. These pull apart and create a rift valley sometimes with water in it.

transform plate boundaries

Areas where two plates grind past each other resulting in faults such as the San Andreas Fault. Earthquakes often occur at fault lines.

Earthquakes and volcanoes at plate boundaries

"Ring of Fire" along Pacific edge of ocean

More generally...
• Volcanic activity located at plate boundaries

Hot spots...

Upwelling magma along the plate boundaries often creates active volcanoes, especially along the outside rim of the pacific ocean. This ring runs through many of the most earthquake and volcanic parts, including New Zealand, Japan, Alaska, west coast of north and south America.

Hpt Spots

Areas of upwelling magma (volcanoes) NOT associated with plate boundaries

• Some _______ _______ are fixed to lower mantle and don't move → creates a chain of volcanoes as plates move over it
• Others move with plate motion

For example, the Islands of Hawaii are formed from slowly moving plate that moves over the ___ ____, creating shallow vents, which move magma to the surface, creating the volcanic islands.

Emperor Seamounts (Hawaiian Islands):

Oldest island in Hawaii is ~5 million years old.
• Bend in the chain at islands that are ~40 million years old → possible movement of plate and movement of plume
• Now know that plumes can also move

The oldest islands in this chain are 40 million years old. At that point, there is a bend in the chain

geological cycle

Includes:
• Rock cycle (last lecture)
• Tectonic cycle (this lecture)
• Hydrologic cycle (covered in 2nd, 3rd year)

influence the formation of the Earth's crust as we know it today, based on past history (uniformitarianism). The hydrologic cycle wears down the Earth's crust via exogenic processes, including weathering, erosion, transportation of materials and deposition driven by the energy and atmosphere, and water and weather systems (water, ice, and wind) The rock cycle forms the three rock types in the crust: igneous, metamorphic and sedimentary Tectonic cycle is endogenic, moving new material to the surface and recycling the old rock material via subduction

Measurement of Relief

RADAR-based methods: Shuttle Radar Topography Mission (spaceborne).

Lidar-based methods: Laser pulse emission (airborne).

Orders of Relief

Classification of relief based on scale (area):
1. First Order: Coarse detail, vast oceans and continents.
2. Second Order: Intermediate within. oceans and continents.
3. Third Order: Most detailed; individual landforms.

First order are at the coarsest level of detail, including the relief found in oceans and continents, in general
Second order: These are at an intermediate level of detail and include, for example, continental and oceanic features such as mountain masses, lowlands, plains, shields (large rock areas), mid-ocean ridges, etc.
Third order: most detailed description of relief. Local landscapes such as individual mountains, valleys, etc.

Formation of Crust Grouped into 3 Categories:

1. Residual Mountains/stable continental cratons → inactive remnants of ancient activity. E.g. Canadian Shield

2. Tectonic mountains, landforms → Active folding, faulting and movements

3. Volcanic landforms → accumulation of molten rock (will discuss after earthquakes)

Craton

An ancient crystalline rock that allows the continent to grow by building on landmass via additional crustal material.

Mountain Building (Orogenesis) at Plate Boundaries

3 Types of tectonic activity:
1. Oceanic plate and continental plate collision
• Subduction zone = igneous intrusions, volcanoes; buckling of rock
2. Oceanic plate and oceanic plate collision
• Creates an ocean trench
• Magma rises, creating volcanoes and additional material
3. Continental plate and continental plate collision
• Folding, thrusting and warping of rocks creating uplift

Orogenesis

mountain building and tectonic activity at convergent plate boundaries

1. Oceanic-continental collision = subduction zone as oceanic plate goes beneath the continental plate - Creates magma that is forced upwards and creates igneous intrusions, sometimes erupting volcanoes. Compression also causes rocks to buckle. Example western Canada and US cordilleran mountain system

2. Oceanic plate and oceanic plate collision - forms curving island arcs/mountains in the ocean. Also creates an ocean trench when a denser oceanic plate subducts beneath a lighter one. Magma is then forced upwards creating volcanoes. E.g. Philippines, Aleutian islands near Alaska

3. Continental plate and continental plate collision - creates intense folding and thrusting of the lithosphere to form mountains. Alps of Europe, Himalayas

Cratons as Nuclei for Continent-Building

Cratons form solid surface for which lithospheric plates, materials and crustal pieces crush up against over time:

• Material, ocean floor, islands, etc. forced against craton margins
• Terranes: Pieces attached to plates
• Different histories, compositions, etc.

Terranes and Crustal Deformation

Pushing against craton creates deformation; warping of Earth surface

Stress and Strain

Stress = Force that affects an object (Force/Unit Area)
- Stress applies pressure on the rocks, changing their structure

Strain = The resulting landforms that occur
- Strain - a measure of stretching, shortening and twisting of rock

3 Types of Stress = Tension, Compression and Shear

2 Types of Strain = Folding (bending) and Faulting (breaking)

Folding and faulting of rock depend on the composition of the rock, the quality of the rock (ductile vs. brittle) and the pressure applied

Stress and strain resulting in crustal deformation

Rocks are strained by powerful tectonic forces, gravity, weight of overlying rocks.

The stress is determined in units of pressure, similar to what you have in the atmosphere.

3 types of stress/strain

stress --> resulting strain --> surface expression

1. Tension > Stretching > Thinning crust/normal fault.
- Tension causes stretching of the rocks, which results in a thinning of the crust and development of a "normal fault"

2. Compression > Shortening > Folding/reverse fault
- Compression causes shortening of the rocks, which cause them to fold, sometimes resulting in a "reverse fault"

3. Shear > Shearing, twisting laterally > Bending horizontally/strike-slip fault
Shear causes twisting and tearing of rocks, creating faulting and horizontal bending

Faulting

shifts in the earths crust. The stress on rocks is so great that it causes them to fracture.

- Normal Fault
- Reverse or Thrust Fault
- Strike-slip Fault

Normal Fault

Crust is pulled apart: Rocks on either side of the fracture shift relative to the other side → Hanging wall side drops down relative to the remaining crust (Footwall).

caused by forces pulling rocks apart. Here one side of the rock moves vertically. The part that shifted down relative to the other part is called the hanging wall. It drops relative to the footwall

Reverse or Thrust Fault

Crust is pushed together: Rocks move upwards along fault plane with compression (hanging wall) relative to remaining Footwall → Movement = major earthquake!

caused when rocks are pushed together (e.g. convergence of plates). This causes rocks to become compressed. Here rocks move upwards along the fault plane. In this case, the hanging wall indicates movement upwards of rocks.

Thrust fault is a reverse fault that occurs when the fault creates a low angle so that the overlying block shifts over top of the underlying block. In some places in California, these occur without a fault being visible, so they appear to be invisible under the surface. However, they can create major earthquakes when they move.

Strike-Slip Fault

Movement of one plate past another: Crust associated with one plate moves laterally along another plate.

created by lateral shear and horizontal movement along a transform boundary. The movement can be described as right lateral or left lateral, depending on the motion. They can also cause vertical displacement of the ground.

3 ways in which earths' crust can form and develop

The first is through the collision of terranes onto very old cratons (known as shield rock), causing mountains to form.

The second is through crustal deformation - folding and faulting associated with plate movement

The third is through volcanic eruptions

Earthquakes

Friction between plate boundaries
• The stress of motion creates strain in the rocks
• Occur when friction is released, sides suddenly break loose
→Sharp release of energy at time of fracture = seismic or shock waves
→Lurching into new position
Elastic rebound theory is used to explain these processes

Epicenter

The location on the ground surface directly above the focus.

Populated areas closer to the epicenter also tend to experience more devastation than those further away from the epicenter.

Foreshock and aftershock

Sometimes earthquakes release a foreshock or an earthquake that comes before the main earthquake, while an aftershock occurs after the main earthquake and within the same general area. Measuring these foreshocks and aftershocks help seismologists predict when a major earthquake might occur. Interestingly some foreshocks can be the same or a similar magnitude as the main shock.

elastic rebound theory

It is assumed that two sides of the fault are locked together due to friction. Over time, they resist the movement and stress associated with forces moving the plates. The stress continues to build strain, and the two sides store up elastic energy like a spring. Eventually the strain is greater than the frictional lock and both sides move abruptly to a new condition with less strain, thereby releasing the energy

Earthquake terminology

Focus → Initiation of motion

Epicenter → Directly above the focus Shock waves

Foreshock → Before main shock

Aftershock → After main shock

Determining Magnitude and Frequency

- Richter scale
- Moment magnitude (M) scale
- Seismometer or seismograph

Richter Scale

• Used since 1935 measures amplitude and magnitude
• Logarithmic scale from 1.0 to >8.0

Seismometer or seismograph:

Measures movement of ground during an earthquake

Moment magnitude (M) scale

• Used since 1993 and is more accurate than Richter Scale
• Measures fault's length, amount of fault slippage, size of surface area that has ruptured, nature of materials
• Logarithmic scale from 1.0 to >8.0

Modified Mercalli Scale

an older method designed in 1902 to determine the damage to terrain and structures based on a Roman-numeral scale from I to XII. This was developed before there were instruments to measure earthquake seismic waves. - Scientists are still not yet able to accurately forecast earthquakes, by studying them, they can determine the probability or chance of an earthquake occurring over periods of decades.

Energy Propagation

P Waves (compressional)
→ Travel at speeds of 1.5 to 8 kms per second.
→ Shake the ground in the direction they are propagating
→ Small, low impact; can be detected with warnings sent out before destructive S waves

S Waves (shear/secondary waves)
→ Shakes ground perpendicularly/transverse to direction of propagation
→ Slower travel speeds than P waves.
→ Very high impact

Effects of Earthquakes

Magnitude and Intensity depends on:
1. Earthquake properties → magnitude, type, location (of epicenter) and depth (of focus)
2. Local geological conditions → distance from event, path of seismic waves, bedrock type, amount of water saturation
3. Societal conditions near earthquake → quality of building construction, preparedness, time of day

logarithmic scale

A method of displaying data in multiples of 10.

a 7 means Major earthquake. Serious damage.

Can we predict earthquakes?

No. Neither the USGS nor any other scientists have ever predicted a major earthquake. We do not know how, and we do not expect to know how any time in the foreseeable future.

what kind of fault occurs in California earthquakes?

The San Andreas fault is the primary feature of the system and the longest fault in California, slicing through Los Angeles County along the north side of the San Gabriel Mountains. It can cause powerful earthquakes—as big as magnitude 8.

How are sensors used to detect the potential for an earthquake deep in the ground?

Allow us to detect and measure earthquakes by converting vibrations due to seismic waves into electrical signals, which we can then display as seismograms on a computer screen. Seismologists study earthquakes and can use this data to determine where and how big a particular earthquake is.

What are mega-thrusts?

A very large earthquake that occurs in a subduction zone, a region where one of the earth's tectonic plates is thrust under another. The Cascadia subduction zone is located off the west coast of North America.

Cascadia Earthquake

The undersea Cascadia thrust fault ruptured along a 1000 km length, from mid Vancouver Island to northern California in a great earthquake, producing tremendous shaking and a huge tsunami that swept across the Pacific.

Slow-slip silent quakes

long lived shear slip events at subduction interfaces and the physical processes responsible for the generation of slow earthquakes. They are slow thrust-sense displacement episodes that can have durations up to several weeks, and are thus termed "slow".

We can see a relationship between mantle slip and crust slip. The slip at depth most likely triggers the big earthquakes. The big ones are preceded by foreshocks associated with creep.

Do we have volcanoes in Canada?

More than 100 active volcanos over the last 2 million years

10 volcanos are currently active

Where do volcanic eruptions occur?

- Rift valleys as plates pull apart exposing fissures and openings allowing magma from the asthenosphere to move into the lithosphere
- Mountain chains adjacent to subduction zones
- Mid-Oceanic rifts where the sea floor spreads
- In hot spots not associated with plate boundaries, associated with upwelling magma (e.g. Hawaii)
- these can create island arc volcanoes

Volcanic features and terms

volcano
crater
lava
Pyroclastics
cinder cone
caldera

Volcano

Forms at the end of a pipe or conduit

Crater

A circular depression near the vent (the vent is at the top of the volcano)

Lava

Molten rock that comes out of the volcano

Pyroclastic

rock and clastic (rock fragments) that spew out of the volcano and forms new ground

- these are fragments of rock that have been erupted by the volcano

Cinder cone

A cone that forms from cinders (pyroclastic material)

Caldera

Large depression

Cinder cone

Small, made mainly of pyroclastics. Eruptive phases 1 to 20 years

Shield volcano

vary in size, made of thin flows of balsaltic lava. Long-lived eruptions, low profile

Composite volcano

modest size, steep profile. Basalt to rhyolitic lavas interbedded with pyroclastics. Infrequent eruptions

Dome complexes

modest size, overlapping domes, dacitic to rhyolitic lavas

Balsaltic lava

most volcanic rock is made of basalt. This has low viscosity and results in fast moving lava flows that can spread over large areas before they cool

Rhyolitic lava

This is highly viscous lava that cools once it comes into contact with the air. Viscosity is the resistance to flow, so if it has high viscosity then it is thick flow with high resistance (slow flowing) whereas if it has low viscosity then it has low resistance to flow and flows more quickly. E.g. water has low viscosity.

Dacitic lava has high silica content and solidifies quickly. Silica contains a lot of quartz and makes up a high proportion of sand.

Effusive eruptions

Gentle flow, slow but lots of lava produced

how do effusive eruptions occur?

Typically from:
• Cinder cone volcanos
• Shield volcanos
• Plateau basalts

Explosive eruptions

Magma comes from subducted ocean plates

how do explosive eruptions occur?

Typically from:
• Composite volcanos (e.g. Mount St. Helens, Washington)
• Magma is thicker than from effusive
• Can block the magma pipe → compresses the gases = explosion!

Hazards

• Hot ash from pyroclastics
• Steam and gas explosions
• Poisonous gases → carbon dioxide, sulfuric acid
• Landslides
• Tree destruction
• Lahars → steam that melts ice and snow, causing flooding and mud flows
• Increased atmospheric dust

... And lava flows

Benefits

• New fertile soils
• Geothermal energy
• New 'real estate'
• New rocks and minerals

Where do volcanoes occur?

eruptions occur in areas where plates are either pulling apart or coming together. Where they come together, we often see more explosive volcanic eruptions associated with denser magma and the trapping of gas within the pipe

Geomorphology

Science of landform origin, evolution and distribution.

- describes the science of landscape denudation, which is an exogenic process of weathering or the break down of rock through disintegration, erosion, the transport of weathered materials to different locations, and mass movement, the downslope movement of soils, rocks, etc. due to gravity.

Dynamic equilibrium of landforms

Landscape undergoes constant balancing act between:
Endogenic Processes → Uplift above sea level: Is in disequilibrium.
Exogenic Processes → Sun (heat energy), water (kinetic energy), atmosphere (chemical energy, reactions)

Changes to land surface = compensating reactions

** Surface constantly trying to reach equilibrium following destabilisation event.

Steady-state

Fluctuation around a stable average.

- Small changes in the land surface tend to be small and fluctuate around a stable average. Small events tend to be frequent as the landscape constantly adjusts to changes in the environment. These shape the landscape through many frequently occurring adjustments

Dynamic equilibrium

changing trend over time.

- A changing trend over time, for example the widening of a river

Threshold

Abrupt change, destabilization of the landscape.

- An abrupt change when a threshold is reached. This causes the system to adjust to a new system state and a new set of equilibrium relationships. Major catastrophic changes occur much less frequently than those smaller magnitude changes fluctuating around an average. Large magnitude changes can shape the landscape in a single event

pattern of geomorphic thresholds

1. Equilibrium, stability → remains approx. the same
2. Destabilization event (e.g. landslide)
3. Period of adjustment
4. Development of new equilibrium over time

High potential energy

means that the rock at the top of the slope is in disequilibrium. As rock falls down slope from near the top of the mountain to a resting place, it loses kinetic energy as it rolls down slope. When it reaches the resting place where it can no longer easily move, it will have reached a new equilibrium and will have low potential energy because it is not able to move as easily.

Weathering

breaks down rock → disintegrating or dissolving it. Differential weathering → Not all materials break down at the same rate.

weakens the surface of the rock, increasing the potential for it to break down and pull apart from the rock under the force of gravity

Regolith

Loose surface material overlying Bedrock.

- Is the starting point for soil development

Bedrock

Hard consolidated rock
Upper surface = continued weathering to create regolith

- constant weathering = regolith, which is transported and deposited.
- called the parent rock from which the weathering occurs, developing regolith and soils.

5 factors that influence weathering

1. Rock composition and structure (jointing)
2. Climate
3. Subsurface water
4. Slope orientation
5. Vegetation

Rock composition and structure (jointing)

→The character of the bedrock: hard, soft, soluble, insoluble, broken, unbroken
→Joints: Fractures in rock = increased surface area

Climate

→Precipitation, temperature, freeze and thaw cycles

Wetter and warmer environments = speeding up of weathering processes
Colder environments = freeze-thaw cycles

Rocks don't always behave the same way in different climates: Rocks that break down easily in warm and humid climates, such as limestone, is resistant to weathering if it exists in a dry climate

Subsurface Water

• Position of the water between rocks and in soil and movement of water through rock

• How is water moving through the rock
• Why is water flowing out from the rock near the centre?
• What are the hardness characteristics of the rock?

Water is able to dissolve most elements on Earth. Water combined with elements and various acids created from organic material and pollutants is able to dissolve a variety of rocks when the water moves through cracks

Slope orientation

→Orientation of the slope (facing north, south, east or west) = exposure to sun and wind

Vegetation can both maintain slopes (by holding regolith and soils between roots), and also break apart bedrock through mechanically prying apart rocks when roots are able to penetrate into cracks (next slide). However, sun, wind and rain exposure will likely have a noticeable influence on weathering, especially in middle and higher latitudes.

Vegetation

• Can protect rock: shields from precipitation, roots stabilize soils
• Can weather rock: organic acids from decaying matter = chemical weathering
→Plant roots break apart rock

- also create organic acids that contribute to 'chemical' weathering of the rock or parent material. Once plant roots go into cracks in the rock, they can further pry the rock apart, increasing the surface area and the susceptibility to more weathering.

Physical (mechanical) weathering

Disintegration of rock by breaking it up
• Increases the surface area for more weathering

caused by frost action, salt crystal growth and exfoliation

Chemical weathering

• Chemical breakdown of rock always in the presence of water
• → Increases with increased temperature and moisture

varies depending on different minerals found in the rocks, where some rocks are able to dissolve more easily than others

Examples of physical weathering

Frost Action
Salt crystal growth
Exfoliation

Frost action

• Expansion of water by up to 9% during freezing
• Freeze-thaw action (cold winters and hot summers) exceeds strength of rock

Freezing causes expansion of water, whereas thawing causes water to contract. This expansion and contraction every year eventually breaks rocks apart, especially when frost wedges develop.
- Occurs in humid continental, sub-arctic and polar climates as well as in areas of high elevation.

Salt crystal growth

• Dry weather moves moisture to rock surfaces
• water evaporates, leaving salt
• Crystals enlarge, break apart rock

occurs in arid climates with intense heating
- Moisture is moved via evaporation at the surface of rocks, leaving previously dissolved minerals (salt)
- As crystals grow, they exert force on the rock, breaking it apart (known as salt crystal growth)

Exfoliation

• Rock peels or slips off in sheets - removal of an outer layer of rock
• Likely due to pressure released as overlaying rock is removed
• Often occurs with intrusive igneous rock
• Causes pressure release jointing

Recall that intrusive igneous rock is made up of magma that slowly cools under ground (also known as plutons). As this rock cools and is uplifted, the overlaying material is removed via erosion, releasing the pressure on the rock. This causes it to heave (over millions of years) causing what is known as pressure release jointing. This means that the rocks cracks into joints. Exfoliation then separates the joints into the layers

How Does Material Move Down Slope? Slope and Angle of Repose

Slope: Curved surfaces; represent equilibrium
Movement = forces of erosion overcomes forces of inertia, cohesion and friction
Determined by angle of repose