EESA06 Topic 4: Convergent Margins

What are the three types of convergent boundaries?

• Ocean-ocean

• Ocean-continent

• Continent-continent

Describe ocean-ocean convergent boundaries.

Denser ocean floor (i.e., older) subducts under less dense (i.e., newer) ocean floor

Describe ocean-continent convergent boundaries.

Oceanic crust subducts beneath continental crust

Describe continent-continent convergent boundaries.

• Obduction occurs, with only minimal subduction

• Subducting slab delaminates (i.e., breaks off)

• The end result of oceanic subduction

Describe obduction

• Crumpling

• Nothing gets pushed into the mantle

• Resulting structure tends not to be very volcanic

What is the Pacific Ring/Rim of Fire?

Volcanoes lining Pacific subduction zones

What phase of the Pacific Ocean does the Pacific Rim record?

• Its death

• i.e., the floor is subducting under converging continents

How is the Pacific Ocean closing?

• Zipper-like convergence of Eurasian and Australian plates, closing the Philippine plate

• The North American Plate is converging towards these plates as well

What are the 3 ways plates interact around the Pacific Ring?

• Ocean crust vs ocean crust - subduction

• Ocean crust vs continental crust - subduction

• Oceanic or continental crust sliding past each other - transform

The pacific ring closure is a continuation of  a process that started where?

Alpine-Himalayan Mountain Belt

Describe and give an example of ocean crust vs ocean crust (subduction) plate interactions.

e.g., island arcs like Japan and Indonesia

What are volcanic island arcs?

Where oceanic crust is subducted below other oceanic crust

Island arcs mostly contain what type of volcano?

Stratovolcanoes

Describe the accretionary wedge

• Faulted mounds of sediments

• Sediment scraped into mounds rather than washed into the ocean

• Susceptible to submarine landslides and collapse, leading to tsunamis, etc.

 

How do volcanic islands contribute to sediment in the ocean?

Erosion of land produces sediment

What is the backarc region?

The basin behind the volcanic arc

Describe and give an example of ocean crust vs continental crust (subduction) plate interactions.

e.g., magmatic arcs like west coast North and South America

What is a Benioff zone?

• Seismically active subduction zones

• EQs closest to plate boundary will be shallow

• Deeper EQs as you move towards overriding plate

 

How does magma flow differ in ocean-continent interactions vs. ocean-ocean interactions?

Magma gets stuck in ocean-continent interactions

Describe and give an example of oceanic or continental crust sliding past each other (transform) plate interactions.

e.g., San Andreas Fault

How does ocean floor age impact the steepness of the descending plate (i.e., angle of subduction)?

• Younger, thinner, bouyant = shallow angle

  • Floor rams into overriding crust, deforming overriding crust

    • i.e., overriding plate pulled down a little

  • e.g., image a

• Older, thicker, denser = steeper angle, since it's primed to subducts

  • e.g., image b

How does angle of subduction influence the result of subduction?

• Shallow =  production of mountain ranges

• Steep = pulls overriding plate

 

What are example of shallow versus steep angles of subduction

• Shallow = East Pacific

• Steep = NW Pacific

What are some hazards of Arc Volcanos?

• Sector collapse

• Lateral blast

• Ash clouds

• Pyroclastic flows/Nuee Ardente

• Ignimbrites

 

Describe sector collapse

• the collapse of a large portion of the volcano

• Can lead to landslides

• Results in lateral blasts

• Magma in underground chamber released due to decompression

Describe vertical ash clouds

Ash carried into air when viscous lava traps exsolving gas and explodes

Describe pyroclastic flow (nuee ardente)

• fast-moving, extremely hot avalanche of volcanic gas, ash, and rock fragments that rushes down the slopes of a volcano during an explosive eruption

• Travel longer distances and in river valleys, etc.

• Very hot

• Cool into ignimbrite

 

What is ignimbrite?

• type of volcanic rock formed from the deposition and solidification of pyroclastic flows

• They are very large

 

Describe volcanic tephra (ash)

• Anything blown from a volcano

• Short term effects (e.g., air quality) are worse than long term effects (e.g., nutrients in soil)

 

What can volcanic hazard assessments determine?

• How active volcanoes are

• Especially when looking at earth's history rather than human history

 

Describe pumice

• "frothy magma"

• Easy to date, therefore very telling for when eruptions happened

 

Describe lahars

• mudflow made of volcanic ash

• Concrete-like substance

• Can happen months post-eruption

• Travels far

 

What is a strombolian eruption?

A mild fountaining of lava

What is a Plinian eruption?

A large explosive ashy eruption

Why did Mount Vesuvius' eruption preserve its destruction?

• Pyroclastic flow + Plinian eruption

• i.e., ash covered people and object

What are some hazards of megathrust earthquakes?

• Tsunamis

• Liquefaction

 

Why were sea defenses not effective in the 2011 Japan earthquake and tsunami?

Seafloor moved eastwards and upwards, meaning the sea level the defenses were built for were lower than the sea levels post EQ

What might trigger tsunamis?

• Fault movement/crustal flexure

• Collapse of accretionary wedges

• Volcanic eruptions

 

How can major EQs be predicted across the Pacific Rim?

• Identification of seismic gaps

• i.e., places that have been "stuck" and have had not recent EQs

• Result in many megathrust EQs when energy finally releases

What can trenching faults help identify?

• History of movement

• Recurrence interval of EQs

 

What are ghost forests and what do they measure?

• Once forest, but the roots are inundated by sea water due to changing water levels

• Record massive EQs and collapse of the crust 

How do you get the reoccurrence interval using ghost forests?

• Date each horizon of the ghost forest

• Tells frequency of EQs

Describe the Cascadia Subduction Zone

Point at which 3 young microplates are forced under North America

What is Project Neptune an example of?

• Ocean floor monitors to measure precursor EQ activity

• i.e., Web-enables Awareness Research Network (WARN) sensors in BC

 

What is Mount Rainier an example of and what hazards does it pose?

• Cascade range stratovolcano

• Due to ice cap, can expect

  • Floods

  • Explosion

  • Etc.

 

What are the major sources of Cascadia EQs?

• Subduction

• Deep Juan de Fuca plate movement

• Crustal faults

What does the effects of an earthquake depend on?

• Local geology

• e.g., bedrock vs. soft sediment

 

When does liquefaction happen?

• Soft sediment starts to behave like liquid

• Sediment grains become "supported" by water

What is a creeping segment?

A fault segment that continuously moves (or "creeps") at a slow, steady rate without significant seismic activity

What is a locked segment?

• A fault segment that is stuck due to friction

• No movement until enough stress builds up to cause a sudden slip, resulting in an earthquake

 

What is the Mercalli Scale?

• Subjective EQ scale

• Based on damage survey

What is the main hazard of transform plate boundaries?

• EQs

• Not volcanoes

What is a magmatic arc volcano?

a volcano that forms above a subduction zone

What is Chile's most active volcano? List its outputs.

• Lascar Volcano

• 30000 tonnes of sulphur dioxide / day

• High silica and highly viscous lava

What is the Tethys Ocean?

• Dead ocean

• Closed during the breakup of Pangea

What does the Alpine-Himalayan Orogeny tell us about the Tethys Ocean?

• Records the closure of the Tethys

• i.e., the collision of African and European plates

Define orogenesis

Process of building mountains

 What is an orogen?

A mountain range

What is a suture zone?

• a geological boundary where two distinct tectonic plates or terranes have collided and fused together

• i.e., oceans close until continents come together at a suture

What is a foreland basin?

• Land next to the thick part of crust

• Lowers beneath the thickest part (elevation)

Define obduction

a geological process in which a piece of oceanic lithosphere is thrust onto a continental plate, rather than subducting beneath it

What material are susceptible to obduction (i.e., don't subduct well)?

• Island arcs

• Continental shelves

• Ophiolites

• Intraplate volcanoes

• Accretionary wedges

What is crustal shortening?

• "crumpling" of the crust

• Earth's crust is compressed due to tectonic forces, causing it to thicken, fold, and fault

What is a fold and thrust belt?

• a region of the Earth's crust that has been deformed by compressional tectonic forces

• Leads to the formation of folds (bent rock layers) and thrust faults (low-angle reverse faults)

 

What is the African Prong?

Point where African plate went over the Eurasian plate

How is the subduction of the mediterranean sea influenced by the Anatolian plate?

Subducts as Anatolian plate "escapes" west

What is an indenter?

a rigid, relatively undeformable landmass or tectonic block that moves into and deforms a surrounding region, typically during a continental collision

What is the Arabian plate an indenter to?

• Eurasia

• Lifts Zagros and Caucasus mountains

 

Why does the Persian Gulf exist?

• Zagros' substantial crustal thickening is pulling down adjacent crust

• Forms foreland basin

 

Where has India been indenting and what has this cause?

• Into Eurasia for 60million years

• Causes obduction and escape tectonics

 

What is the issue with the physiography of Kathmandu?

• Bowl-shaped

• Collects sediment

• EQs will lead to liquefaction

 

Why may the concept of escape tectonic be especially problematic for China?

• They have really high mountains

• Too much pressure at the base may cause the mountain to move laterally