Studied by 0 people
Looks like no one added any tags here yet for you.
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?
composition of the seafloor - volcanic “basalt” overlain by veneer of marine sediment
seafloor seismicity - earthquake distribution along the axes of mid-ocean ridges
heat flow - highest over crest of mid ocean ridge
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
on continental crust there is an upwelling of asthenosphere underneath
heat energy in asthenosphere pushes overlying continental rock causing it to flex which eventually causes it to break
as two faulted parts of continental rift split apart from one another seafloor spreading begins and establish linear sea
as faulted margin pulls away from mid ocean ridge that eventually develops in the middle of a growing ocean basin
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)
as process continues, continents progressively split apart from one another and it becomes elongated ocean basin
