GEL 001: Divergent Plate Boundaries

divergent plate boundary

• boundary between two plates that contributes to the growth of ocean basins or the break-up of continents

• commonly occur along mid-oceanic ridges and contribute to the continual growth of older ocean basins (e.g. Atlantic, Pacific, Indian Oceans) or they may occur within continents where they act to open new ocean basins (e.g., continental rifting)

• two main types: mid-ocean ridges and continental rifts

extensional stress

stretching caused by the motion of the plates away from each other

seafloor spreading

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

axial rift / rift valley

• lava erupts and solidifies along a narrow, central rift valley (aka ‘axial rift’) that occupies the ridge axis

• axial rift valley has typical dimensions of ~500 m deep and 10 km wide, bordered by steep cliffs

magma vs. lava

• magma and lava are molten rock

• magma is below ground, while lava is above ground

magma chamber

where magma accumulates beneath the axis of the mid-ocean ridge, sort of a ‘holding pen’ for the magma supply

faults

fissure eruptions

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

pillow basalt

• the main rock of the ocean floor and is formed when lava interacts with cold seawater to form bulbous pillow shapes and solidifies

• oceanic crust is composed almost entirely of pillow basalt

pelagic rain

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

marine sediment

• composed of clay and shells of dead marine plankton

• covers solid pillow basalt

• absent over crests of mid-ocean ridges because there hasn’t been enough time for sediment to accumulate to any appreciable thickness

• thickens away from the ridges, reaching ~a km in thickness over abyssal plains

heat flow

• rate of heat release from the Earth’s interior

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

• active magma bodies beneath the surface and active volcanism on the seafloor = high heat flow above mid-ocean ridges

hydrothermal vents

• located along fractures in mid-ocean rift valleys

• gushes superheated water filled with chemical elements into the seawater

• form tall chimneys and towers of minerals that precipitate from the hot waters (one in the Pacific is 15 stories high)

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

  • contain silicon, iron, magnesium, lead, zinc, copper, cobalt, gold and silver

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

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

chemosynthesis

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

seafloor spreading rates

• maps of seafloor age enabled determination of spreading rates

• Mid-Atlantic Ridge is spreading at a rate of ~2-3 cm/year (~1” per yr) and causes ridge to build upward

• East Pacific Rise (EPR) —rise is not as rough or jagged as a ridge—is spreading at a rate of ~10-17 cm/yr (4-6”/yr)

  • faster spreading rate causes EPR to grow as a broad, low feature

• at a typical rate of ~2.5 cm/yr, seafloor spreading produced the 5000 km wide Atlantic Ocean in ~200 million years

age of the seafloor

• oldest seafloor was ~200 million years old

• oldest seafloor located in the western Pacific

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

• knowing the ages of the seafloor tells us when an ocean basin first originated

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

continental rifting

when a continent splits and separates into two divergent continents; ocean basins are born

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

• new divergent plate boundaries formed along continental rifts

• continental rifts may (or may not) evolve through time into mid-ocean ridges

East African Rift

one of the best-studied examples of a continental rift on Earth

The Afar Triangle

• in the African countries of Djibouti and Eritrea

• marks the location where the three rift arms meet (Gulf of Aden, Red Sea, EA Rift)

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

• crust sags downward and will likely be inundated by the sea in a few million years

linear seas (e.g., Red Sea)

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

• the Red Sea is a linear sea, a nascent ocean basin

• in ~10 m.y., East Africa may pull away from the rest of Africa, opening up a linear seaway

• a narrow linear sea may widen through time, forming a new ocean basin

• ex) the Gulf of California between mainland Mexico and the Baja peninsula

origin of continental margins

with time, the diverging, faulted edges of the continental rift develop into continental margins (shelf-slope-rise) on either side of the growing ocean basin

• continental margins are the actual margins of the old continental rift, only now separated by a few thousand kilometers of an intervening ocean basin

  • sediment eroded from continents and transported by rivers is deposited on the faulted margin, covering it with a thick blanket of sediment and smoothing the upper surface of the continental shelf and slope

• we see them today as the broad continental shelf, steep continental slope, and gentle ramp of the continental rise