EESA06 Topic 3: Divergent Margins

Lecture 4 - Embryonic and Juvenile Oceans; Lecture 5 - Mature Ocean Basins

What is the Wilson cycle?

• Describes how oceans open and close

• i.e., ocean basin lifecycle

What is the most important group of minerals? Why?

• Silicate minerals

  • i.e., those containing silica and oxygen

• Accounts for 90% of all minerals

How does silica content differ across the lithosphere?

• Continental crust - felsic

  • high levels of silica + feldspar

• Oceanic crust - mafic

  • less than 50% silica; mostly magnesium and iron

• Mantle - ultramafic

  • less than 40% silica; mostly magnesium and iron

How is silica related to magma's properties?

• It controls most of magma's properties

• e.g., velocity

Describe how magma is generated.

• From melting of rock

• i.e., when rocks

  • Undergo decompression

  • Are heated

  • Get wet

Describe decompression melting.

• Rock rises from an area of high pressure to lower pressure

• Change in pressure causes rock to (partially) melt

Describe Flux-Induced melting

• Adding water to rock changes how it behaves (i.e., melts) at different pressures or temperatures

Where does decompression happen? Explain.

• At mantle plumes

• This is where mantle rock is brought up

  • i.e., moves from higher to lower pressure

Is melting usually partial melting or full melting? How does this influence the resulting magma?

• Partial; silica-rich melts before silica-poor

• Magma is more silica-rich than melting rock

Differentiate between wet magma and dry magma

Hint: location, silica, viscosity, colour, density, other

	Wet	Dry
Location	Convergent margins	Divergent margins
Silica content	Higher	Lower
Viscosity	Higher	Lower
Colour	Lighter	Darker
Density	Lower	Higher
Other Notes	Water causes melting where we would usually see solid rock	Produced from melting of the top of the mantle

How do high silica magmas compare to low silica magmas?

• They're more viscous

• They crystalize minerals with more silica tetrahedra in their structure

 

Why is ocean floor mostly basalt (i.e., more silica-rich than mantle)?

• Ultramafic mantle → melts a little through decompression → mafic ocean floor

 

What minerals are abundant in convergent margins?

• Silica

• i.e., felsic

What volcano types are associated with convergent margins? Describe these volcanoes.

• Stratovolcanoes

• High viscosity magma

• Magma doesn't get far → pointy volcano

What minerals are abundant in divergent margins?

• Iron

• i.e., mafic

What volcano types are associated with divergent margins? Describe these volcanoes.

• Shield volcanoes

• Mafic magma

• Lower volcanos, wide-reaching flows (lower viscosity magma)

• e.g., erte ale volcano

How does cooling time and time spend underground influence magma?

• Give examples of each instance.

• does silica amount differ across instances?

• Rapid cooling → low crystal count

  • e.g., obsidian

• Medium cooling → slightly more crystals

  • e.g., rhyolite

• Slow cooling + completely underground → high crystal count

  • e.g., granite

In each instance, silica amount is similar

What are igneous rocks? List the two types.

• The product of cooled magma

• Volcanic rocks: erupt at surface

• Plutonic rocks: cool underground

Where does igneous rock cool? Give examples.

• Earth's surface (extrusive/volcanic)

  • e.g., lava flow

• Underground (intrusive/plutonic)

  • e.g., dikes

What are examples of heterogenous rocks?

• Granites

• Other igneous ricks

• Sedimentary and metamorphic rocks

why do shield and stratovolcanoes produce different magma?

difference in silica content

What is continental rifting?

Pulling apart/extension of land

How does continental rifting relate to the creation of passive margins?

• First 2 major stages of the Wilson Cycle

• Rifting → continent extends, thinning the crust and allowing a valley to form

• Continent then becomes two, separated by oceanic crust formed from basalt eruptions

• The land that meets the water = passive margin

Differentiate between a rock and a mineral.

• Rock = naturally formed, consolidated material

  • composed of grains of one or more minerals

• Mineral = naturally occurring, inorganic, crystalline solid

  • Has a specific chemical composition

  • Building blocks of rocks

How do new oceans form?

• Triple junctions

• Passing over mantle plumes gradually weakens continental crust

• Eventually, crust becomes so weak, a rift is created

• With the expansion of the rift, water fills in an oceanic crust is created

What is an embryonic ocean?

• Early-stage ocean basin

• Forms when a continental rift begins to separate a landmass

• Allows water to accumulate in the widening gap

What is a triple junction?

a point on Earth where the boundaries of three tectonic plates meet

Discuss thermal doming above plume heads.

Passing over mantle plumes → doming/uplifting of the continental crust above

What are failed rifts (Aulocogens)?

a rift that begins to split a continent but does not fully develop into an ocean basin

What are flood basalts?

subset of LIPs, consisting of thick, widespread lava flows that cover large areas

what is a juvenile ocean?

• an early-stage ocean basin that forms as a result of continental rifting and the initial separation of tectonic plates

• represents the second stage in the Wilson Cycle

describe juvenile ocean shelves

• Sediment from continent collects where land meets water

• shallow, submerged edges of newly formed continental margins

• Form on the transition from continental to oceanic crust

• Sediments wash into basins where they are buried and lithified

what are incipient oceaninc crust?

newly forming or embryonic oceanic crust that develops in an area where continental rifting is leading to the creation of a new ocean basin

What causes the uplift of the East African Rift?

Thermal doming

Where are most sediments today?

• Continental shelves

• i.e., passive continental margins

Describe the tectonic activity of continental shelves.

• Tend to be stable

• Shelves on active margins get destroyed consistently

Continental shelves are home to what?

• A lot of biological activity

• Most of the world's

  • Oil

  • Gas

  • Fisheries

Why is there such a large cliff in the Grand Banks, Newfoundland?

• High part = mostly continental crust

• Low part = all ocean crust

Why is the Red Sea considered a juvenile ocean?

• Has started to accumulate a shelf

• There is creation of ocean crust at the centre

What plates form the triple junction at the East African Rift?

• African

• Arabian

• Somalian

What are horsts and grabens?

• H = highs

• G = lows

• Occur at a rift

What are large igneous provinces (LIPs)?

• New ocean crust (basalt) along the rift

• Include flood basalts

Differentiate between Pahoehoe lava, Ropey lava, and AA lava

How does the Red Sea's opening influence surrounding plates? (Plate reconfiguration)

• Pushes Arabian plate north into Europe

• This is happening

  • Along the Dead Sea Fault Zone

  • Faster than the African Plate is moving NW

what is the Dead Sea Fault?

transform plate boundary between master moving Arabian and African plates

What is an endorheic basin?

• closed drainage basin where water has no outlet to the ocean

• e.g., Dead Sea

What are salt diapirs?

• upward movements of salt within a layer of overlying rock

• typically caused by buoyancy and pressure differences in the Earth’s crust

How do salt diapirs relate to the Atlantic Ocean's creation?

• Salt accumulated in rifts during Pangea

• They broke up, forming early Atlantic

 

What can we learn from EQs of antiquity?

• How often did they happen?

  • i.e., fault's activity

• Are there no EQs where there should be?

• When might the next EQ be?

How does the Red Sea's opening influence Turkey?

T's moving westward along the North Anatolian Fault (NAT) and the East Anatolian Fault (EAF)

Why is NAT Fault one of the world's most dangerous faults?

Escape tectonics

• i.e., a tectonic block or region is forced out or moved laterally due to the compressive forces exerted by surrounding tectonic plates

• Cascading EQs propagate along here

How does ocean floor change with age? What does this do in terms of the ocean floor's behaviour?

• Older

  • Denser

  • higher mineral content

  • thicker lithosphere

  • More sediment

  • Farther from MOR

• Eventually, floor will subduct due to density

In this image, what does the width of each band represent?

• The rate of spreading

• e.g., during the Miocene, Atlantic spread slower than Pacific

Does the ocean spread at the same rate at all points across the global mid-ocean ridge system?

No

What is the total length of the global mid-ocean ridge system?

80 000km

what are the two controls of seafloor spreading rate?

• slab pull

• ridge push

describe slab pull

(seafloor spreading)

subducting older oceanic lithosphere exerts greater tension to mid-ocean ridge relative to younger oceanic lithosphere

describe ridge push

thermal uplift of the midocean ridge causes gravitational sliding of the plates odd the ridge

How does the rate of spreading influence ridge morphology?

• Centre axial magma chamber is

  • Big when there's fast rate

  • Small when there's slow rate

• Fast flows lead to

  • Broad, more diffuse volcanism

  • Broad shape

  • Pointy tops

  • Far lava spread

How can you study mid-ocean ridges and sea floor spreading?

• Direct observation with submersibles (mostly autonomous vehicles now)

  • e.g., smokers, seafloor minerals

• Studying ancient MORS

  • i.e., Ophiolites

• Iceland

  • i.e., Mid-Atlantic Ridge is on land

What are smokers?

• Active hydrothermal vents

• Cold water filters through fractures and heats up, absorbing minerals

• Deposits formed when hot mineral-filled water meets cold water

• Water cools and minerals rain out

What do the voids in smokers allow?

Water to move up

What does the colour of the smoke from smokers/hydrothermal vents indicate?

• Black = metal-filled, hot water

• White = calcium carbonate filled, warm water

  • i.e., fluid isn't metal-rich

What are Ophiolites?

• Accretion of obducted ocean crust

• Remnants of ancient crust and mid-ocean ridges

• Exposed to land because of ocean closure

When does precipitation of minerals happen?

When mineral-rich water cools quickly

What is rock alteration?

• When minerals move in or out of a rock

• Depends on the chemical composition (e.g., pH) and temperature of the water

Where do Pillow Lavas form?

• a.k.a., pillow basalts

• From where lava erupts under water

 

What are sheeted dikes and where do they form?

• Lava trails through a linear conduit

• Form where basalts are repeatedly injected

Where do layered Gabbros form?

Where basaltic magma cools very slowly in an axial magma chamber

Describe the onion model of building oceanic crust at mid-ocean ridges

• Pillow basalts erupt at surface

• Intrusion of sheeted sikes at depth

• Settling of gabbro from magma chambers

Why is the mid-Atlantic Ridge above sea level in Iceland?

The Iceland Plume has lifted it

How does the Afar Triangle relate to the Mid-Atlantic Ridge in Iceland?

• Similar creation

• Mantle plume under continental crust caused release of flood basalts to form LIP

 

How does the Icelandic Plume relate to the separation of the British Isles and Greenland?

• It's responsible for the separation and releasing flood basalts

• 40 MA

How is the mid-Atlantic Ridge moving relative to the Iceland Plume?

• Northwest away from the plume

• i.e., moving off the plume

 

Differentiate between relative and absolute motion

• Relative = rate of motion of one plate relative to another

• Absolute = entire plate movement

What is ridge jumping?

Ridge is moving away from plume

Describe fissures.

• Cracks in the crust

• Can go down several kilometres

What happens when magma exits a fissure?

• Fissure eruption + magma flow

• Usually low viscosity, not too dangerous

 

What are protective berms?

• Same material as lava used to redirect lava flow

• Must predict where the fissure + flow will occur

Describe the elements seen in this picture

1. ash

2. fountain magma

3. protective berm

4. lava flow

Can effusive eruptions have a global impact? Give an example.

• Yes

• e.g., Lakigigar large fissure eruption (1783)

• 15 cubed km of magma and hydrofluoric acid + sulfur dioxide

• Led to

  • Death of 50% Iceland's livestock

  • Famine

  • Destroyed 21 villages

  • Aerosols that influenced Europe and led to crop failure and, consequently, the French Revolution

 

Describe columnar basalt

• Pooled magma that cools from top and bottom

• Forms columns

 

How do successive lava flows impact oceanic crust?

Thicken crust

Describe the Eyjafjallajokull volcano.

• Sits underneath a small ice cap

• More dangerous due to lava and water interactions

 

Describe phreatomagmatic eruptions

• Eruptions involving steam and magma

• They become explosive and can lead to floods

 

How many times has Katla erupted over the past 1100 year and how far of an impact has it had?

• 20 times

• Ash as far as Scotland

 

How do phreatomagmatic eruptions influence the volcanic ash released?

• Magma melts ice

• Ice interacts with magma

• "shattered rock" explodes out of the volcano

 

What is tephra?

• Anything thrown out of a volcano

• e.g., ash, bombs

 

What is Fjallsjokull?

An outlet glacier of the Vatnajokull Ice Cap

How are flat-topped ("Moberg") mountains formed?

Volcanic eruptions under ice caps

What causes Hyaloclastite breccia formation?

"glassy fragments" formed by fast cooling

Does ocean floor temperature change depending on distance from MOR?

• Yes

• Colder = farther

 

Why is the ocean floor draped in sediment?

• Once off the MOR, it sediment starts to settle over it

• Due to

  • Primary productivity

    • i.e., biological activity in ocean (e.g., photosynthesis from phytoplankton)

  • Terrigenous sediment

    • Sediment moves from continents into ocean

 

Other than sediment, what is on the ocean floor?

• Ocean islands

• Guyots

• Seamounts

• Atolls