GEL Topic 10: Divergent Tectonics

two main types of divergent boundaries

mid-ocean ridges and continental rifts

primary stress at divergent boundaries

extensional - “starching” caused by motion of 2 plates pulling away from each other

mid-ocean ridges

new lithosphere is created by seafloor spreading along divergent margins

• known as “spreading ridges”

• contribute to continental growth of older ocean basins

• Atlantic, pacific and Indian oceans

seafloor spreading

magma wells up along fractures in the lithosphere near the mid-ocean ridge axis and pours out as lave onto the seafloor

magma vs. lava

they are both molten rock but magma is below ground and lava is once it reaches the surface

Process of Seafloor Spreading

• as plates are pulled apart by extensional (divergent) forces, the rocks of the brittle crust break along fractures and faults, with blocks of rock sliding downward to create the axial rift valley

• underlying asthenosphere rock rises beneath thin lithosphere and begins to melt as overlying weight of rock is reduced, producing magma

• magma accumulates in magma chambers beneath the axis of the mid-ocean ridge

• most of magma solidified in place beneath the surface while some finds its way to the surface through fractures where it pours out as lava

• lava cools and solidifies and forms new seafloor and pillow basalt

• continued tectonic extension of plates bordering mid-ocean ridge displaces lithospheric rock along faults and fractures to either side of mid-oceanic ridge

• new lava solidifies, the is fractured by extensional tectonic stress, split by new lava welling up with fissures

• rock of oceanic lithosphere formed by seafloor volcanism along mid-ocean ridge system

faults

planes of weakness within the body which abrupt movement occurs in response to tectonic stress

fractures

plana cracks/fissues within a body of rock with no movement along the planar surface

magma chamber

“holding pen” for magma supply located beneath the axis of the mid-ocean ridge

pillow basalt

formed when lava interacts with cold seawater to form bulbous pillow shapes with the lava solidifying; main rock of ocean floor

fissure eruptions

when lava oozes out of the elongate fractures onto the floor of the rift valley

• magma wells up along fracture and oozes out as lava onto surface as linear expression by gas pressure - lava spreads out laterally

rift valley

valley at the crest of the mid-ocean ridge; makes area buoyant because asthenosphere is hot and expansive so lithosphere is buoy upward

why does the mid-atlantic ridge system affect Iceland specifically?

rises about the seafloor in Iceland and cuts through it between North American and Eurasian plates

• where fissure eruptions occur

• can witness seafloor spreading in action

where is the youngest seafloor?

along the ridge axis because all seafloor forms at mid-ocean ridges and spreads laterally with time

where is the oldest seafloor?

along the outer margins of the oceans adjacent to continents

how old is ocean floor?

less than 200 million years old

what is most volcanically active region of the world?

mid-ocean ridges

• most of world’s volcanism likely occurs out of sight within rift valleys on mid-ocean ridge crest 2 or more km beneath sea level

what treats illusion continents are drifting?

when plates move and ocean basins open up between continents because new seafloor is forming in between, widening ocean and increasing the distance between two continents

why has map of earth’s surface changed continiously?

seafloor spreading causes ocean basins to open and close and continental configuration evolves through time

what has contributed to confirmation of seafloor spreading?

1. composition of the seafloor - volcanic “basalt” overlain by veneer of marine sediment

2. seafloor seismicity - earthquake distribution along the axes of mid-ocean ridges

3. heat flow - highest over crest of mid ocean ridge

4. exploration of volcanic rift valleys by manned and remotely operated submerssibles - hydrothermal vents and supported fauna based in chemosynthesis

how did dredging and drilling the ocean floor help confirm seafloor spreading?

• became clear ocean crust is almost entirely composed of pillow basalt

  • solid basalt is covered by thick layer of marine sediment and pelagic rain

  • clay enters oceans through rivers, deltas or winds and slowly falls through water to seafloor

• marine sediment absent over crests of mid-ocean ridges because there has not been enough time for sediment to accumulate

  • thickens away from ridges over abyssal plains

why is there more marine sediment accumulated away from ridge axis?

longer time for marine sediment to accumulate above pillow basalt

• lack of sediment on mid-ocean ridge is due to young age of seafloor there

how can age of sediment on seafloor be determined?

microscopic fossils of marine organisms that once floated in the water before dying

• their shells settle to the seafloor where they are buried by younger sediment and come up with drilled sediment core and are identified by paleontologists to date the sediments

• we can radiometrically date pillow basalts beneath the marine sediments

marine sediment

composed of clay and shells of dead marine plankton

pelagic rain

shells of dead plankton float in suspension until gently falling to seafloor

how did maps of earthquake distribution along seafloor help confirm seafloor spreading?

• exhibit distinct linear patterns that correspond with mid-ocean ridges and cross-cutting fracture zones

• seismicity (earthquakes) along mid-ocean ridges are due to active extension stress of two plates pulling away from one another

what causes earthquakes along mid-ocean ridge?

extensional stress of two plates pulling away from one another

• seafloor spreading breaks seafloor crust by extensional stress, creating faults within the riding crust and generating earthquakes when faults rupture

• magma migrates up from below to “heal” the crack and thus creates more seafloor within rift valley

how does heat flow help confirm seafloor spreading?

• high heat flow above mid-ocean ridges is due to active magma bodies beneath surface and active volcanism on seafloor

• elevation of mid-ocean ridges above the adjacent abyssal plains is due to hot buoyant, partially molten rock just beneath ridge axis, pushing upward

  • youngest lithosphere near ridge axis is hot and buoyant s its density is lowered and this mid-ocean ridges are relatively high regions of ocean floor

heat flow

the rate of heat release from earth’s interior

• highest over the crest of mid-ocean ridges and decrease away from ridge axis

what is density like for older lithosphere with large distance from mid-ocean ridge?

has cooled significantly as it slowly travels laterally so its density increases with age causing it to subside very gradually, depending with distance from mid-ocean ridge

• low area of dense volcanic rick is progressively covered with marine sediment, smoothing it out and creating extensive abyssal plain that reside between mid-ocean ridges and continental margins

what creates elevation of mid-ocean ridges?

underlying heat creating buoyancy in rocks

how did exploring axial rift valleys help confirm seafloor spread?

geoscientist and biologists use manned submersibles and remotely operate vehicles

• find hydrothermal vents

hydrothermal vents

located along fractures in mid-ocean rift valleys; gush superheated water laden with chemical elements into seawater

• form tall chimneys and towers of minerals that precipitate from hot water

• influence ocean chemistry (hydrosphere) and deep-sea chemosynthesizing fauna

where does water from hydrothermal vents come from?

seawater penetrating through fractures in riff system which comes into contact with hot rock and the water heats and becomes acidic and the buoyant

hydrothermal waters

• acidic and dissolve minerals from rocks they pass through on their way up to the seafloor

  • when superheated waters surge outward they release massive amount of elements that influence chemistry of seawater in the world

• support weird fauna of tubeworms and other bizarre creatures that thrive by chemosynthesis

elements in hydrothermal waters

includes silicon, iron, magnesium, lead, zinc, copper, cobalt, gold and silver which all precipitate out to form the chimney-like vents

chemosynthesis

biological process where bacteria produces organic matter by using energy derived from chemical reactions involving inorganic compounds like hydrogen sulfide or methane instead of sunlight as in photosynthesis

where is oldest seafloor located?

in the water pacific

how do scientist map the age of the seafloor?

combining all of the seafloor data (bathymetry, mapping, drilling, seismicity, heat flow, magnetic studies, age-dating of seafloor rocks)

How does knowing ages of seafloor tell us when an ocean basin first originated?

the oldest Atlantic Ocean seafloor is about 180m.y in age, telling us that the Atlantic began to open as Eastern North American and northwestern Africa began to separate at that time

what did maps of sea floor help determine?

spreading rates of ocean basins

how are ocean basins born?

continental rifting

continental rifts

linear features where continental lithosphere actively stretches and pulls apart, typically driven by upwelling of hot asthenosphere beneath continent

• new divergent plate boundaries are formed along continental rifts

• may evolve into mid-ocean ridges

afar triangle

in African countries of Djibouti and Eritrea, marks the location where the three rift arms meet

• hyper-arid land marked by fractures in the surface, common earthquakes and active volcanoes

• crust is sagging downward and will likely be inundated by sea in a few million years

• upwelling asthenosphere beneath Africa pushes up overlying lithosphere, stretching it and causing it to break along faults → underlying asthenosphere rises even further, melts to form magma and produces volcanism

• as faults and fractures develop due to extensional stress, the underlying asthenosphere supplies magma that rises along faults and fractures, feeding volcanoes along rift valley

• constant tectonic extension within rift valley creates fractures and faults that generate earthquakes

• if continental rifting continues until rift valley subsides and intersects a coastline, seawater floods in and rift valley becomes a shallow arm of ocean

East African Rift Valley

• divergent plates diverge with extensional stress and faults develop within that gap (some blocks lifted to create mountains)

• magma rises through fractures and into rift valley because weight of overlying rock decreases → magma melts

  • feeds volcanoes and builds them upward

• evolves to form continental margins

what causes earthquakes in East African rift?

as plates pull apart and crack/fracture they release seismic energy that is expressed as an earthquake

can continental rifting evolve into seafloor spreading?

yes - when continental rifting ends, asthenosphere will feed different type of lava into gap

• seafloor volcanism will form a new ocean crust

• continental rifts are formed within continental rock as it breaks apart - as the rift widens and the asthenosphere rises into rift valley, volcanism changes to produce oceanic rock

• as seafloor spreading continues and rift valley sags downward, seawater may flood in and develop elongate linear sea

Red Sea as a linear sea

• continental rock broke apart so magma fills that gap by seafloor spreading with pillow basalt

• as they separate sediments are washed off continents and cover them and as they subside they become continental margins

• linear rifts evolve into seafloor spreading and linear sea evolves into larger wider ocean basin

formation of new ocean after continental rifting

1. on continental crust there is an upwelling of asthenosphere underneath

2. heat energy in asthenosphere pushes overlying continental rock causing it to flex which eventually causes it to break

3. as two faulted parts of continental rift split apart from one another seafloor spreading begins and establish linear sea

4. as faulted margin pulls away from mid ocean ridge that eventually develops in the middle of a growing ocean basin

5. gets covered by layer of sediment derived from land (continents erode → rivers transport sediment to margins of shoreline → sediment distributed and covered up faulted edge of former rift margin)

6. as process continues, continents progressively split apart from one another and it becomes elongated ocean basin