Final Gel 001 Flashcards
Wegener
Alfred wegener come up with the idea of continental drifts, was a German meteorologist /geologist
Pangea
Alfred wegener: continents were once combined into huge content called Pangea
Continental drift
the movement of continents resulting from the motion of tectonic plates.
Exploration of the oceans
•WWII & cold war
•echo sounding
Bathymetry
•The depth and topography of the sea floor
•Avg depth of world ocean ~4km below sea level
Mid-Ocean ridges
• Occur in the mid-atlantic, East pacific, Indian Ocean, between Australia & Antarctic in the southern Ocean
• roughly symmetrical
Abyssal plains
Flat regions of Ocean between the Mid-Ocean ridges and the continents
Fracture zone
Mid-Ocean ridges are bisected at right angles by steep-walled fracture zones that parallel one another, segmenting and off setting ridges into smaller pieces
Deep Ocean trenches
Regions adjacent to chain of active volcanic islands/ exhibit narrow, deep ocean trenches
• depths 8-11 km
• Marianas trench in the western Pacific) the challenger deep
Continental shelf, slope, rise
• Shelf: gently steepening underwater continuation of the coastal plain along of edge continents
• shelf → slope: abrupt drop off. Covered by sediment few km thick
• slope: marks true edge of continents. Is the drop off of shelf region
• rise: gentle stope covered with sediment derived from the continental shelf hat acts as a transition zone between the steep continental slope and flat abyssal plains
Coastal plain
the continental shelf is just the gently steepening underwater continuation of the coastal plain along the seaward edge of the continents. The coastal plain meets the continental shelf at the shoreline.
Plate
•Most plate boundaries occur on the seafloor and coincide with mid-ocean ridges, deep-ocean trenches, and fracture zones. These bathymetric features of the seafloor are fundamental geologic boundaries that mark the edges of thick slabs of rock called plates.
Tectonics
“tectonics” refers to large scale movement and deformation of Earth’s outer surface (crust plus upper
mantle)
Plate tectonics
•the continual motion, creation, and destruction of parts of the planet’s active surface.
• ~100-150 km thick: based on composition = crust, mantle, and core, strength = lithosphere, asthenosphere,
earthquake distribution
earthquake distribution follow somewhat linear patterns. They tend to be located along the edges of continents or strung out along linear trends down the middle of oceans.
volcano distribution
volcano distribution follows a similar linear trend as the earthquake locations. The distribution of active volcanoes around the Pacific Ocean is called the “Ring of Fire” because they are among the most violent and deadly volcanoes in the world.
locations of earthquakes and volcanoes
The locations of earthquakes and volcanoes, in concert with large bathymetric features on the seafloor,
mark the locations of tectonic plate boundaries - they occur irrespective of the geographic boundaries
of continents and oceans based on the arbitrary position of sea level.
Lithosphere
Lithosphere = consists of the crust and upper mantle down to about 100 to 150 km beneath Earth’s surface
- lithosphere beneath continents is thicker than lithosphere beneath oceans (primarily because
continental crust is thicker than oceanic crust) - lithosphere is a “cool and strong” layer that behaves rigidly (it bends, flexes, and breaks, but does
not flow easily) - plates consist of lithospheric rock
Asthenosphere
Asthenosphere = upper mantle down to ~ 400-600 km
- boundary between lithosphere and asthenosphere defined by a temperature of ~1280°C, the
temperature at which rock (at high pressures) begins to slowly flow when acted upon by a force
- “hot, weak, semi-plastic” strength properties – the asthenosphere is solid, but mobile
- heat moves by convection (more on this later) in the asthenosphere
So, in sum, rigid lithospheric plates move above weak, slightly molten asthenospheric rock.
Divergent plate boundaries
Boundary between two plates that contributes to the growth of ocean basins or the break-up of continents.
- divergent boundaries 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 (continental rifting,
- the primary force at divergent boundaries is extension – ‘stretching’ caused by the motion of the two plates away from each other
- two main types of divergent boundaries: mid-ocean ridges and continental rifts S
Extensional stress
the primary force at divergent boundaries is extension – ‘stretching’ caused by the motion of the two plates away from each other
seafloor spreading
•Mid-ocean ridges are commonly called “spreading ridges.
•Seafloor spreading is the 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
The lava erupts and solidifies along a narrow, central rift valley (aka ‘axial rift’) that occupies the ridge axis. The axial rift valley has typical dimensions of about 500 m deep and 10 km wide, bordered by
steep cliffs.
As plates are pulled apart along the spreading axis by extensional (divergent) forces, the rocks of the
brittle crust break along faults, with blocks of rock sliding downward to create the axial rift valley and
adjacent ridges
Magma vs. Lava
•both magma and lava are molten rock. The only difference is that magma is below ground, whereas lava is above.
Magma chamber
the underlying rock of the asthenosphere passively rises beneath the thin lithosphere above and begins to melt as the overlying weight of rock is reduced, producing magma (this is called “decompression
melting” and occurs by the release of pressure rather than any increase in temperature)
- magma accumulates in magma chambers beneath the axis of the mid-ocean ridge, sort of a ‘holding pen’ for the magma supply
- most of the magma solidifies in place beneath the surface, while some finds its way to the surface where it pours out as lava, cools and solidifies, forming brand-new seafloor
Pillow basalt
the lava interacts with cold seawater to form bulbous pillow shapes with the lava solidifying to form the main rock of the ocean floor – pillow basalt
Seafloor
Because all seafloor forms at mid-ocean ridges and spreads laterally with time, the youngest seafloor lies along the ridge axis and the oldest seafloor lies along the outer margins of the oceans adjacent to continents
- all ocean floor on the planet is less than 200 million years old (extremely young compared to the continents which have rocks ranging back to 4.0 billion years old)
- of all tectonic settings, underwater mid-oceanic spreading ridges produce the greatest sheer volume of volcanic material (essentially produces all of the seafloor rock)
- 80% of all volcanic activity on Earth takes place under water along the mid-ocean ridge axis
- eruptions on the seafloor are generally benign since they occur under the pressure of >2 km of water
Marine sediment
Sand!
Pelagic rain
- shells of dead plankton float in suspension until gently falling as a ‘pelagic rain’ to the seafloor
seismicity
seismicity (i.e., earthquakes) along mid-ocean ridges is due to the active extensional stress of two plates pulling away from one another. Seafloor spreading breaks seafloor crust by extensional stress, creating faults within the rigid crust and generating earthquakes when the faults rupture. Magma migrates up from below to ‘heal’ the crack and thus create more seafloor within the rift valley
Heat flow
Heat flow, the rate of heat release from the Earth’s interior, is highest over the crest of mid-ocean ridges and decreases away from the ridge axis.
– the high heat flow above mid-ocean ridges is due to the active magma bodies beneath the surface and active volcanism on the seafloor. Temp as high as 780°F (415°C).
- the elevation of the mid-ocean ridges above the adjacent abyssal plains is due to the hot, expanded, buoyant, partially molten rock just beneath the ridge axis, pushing upward.
- i.e., the youngest lithosphere near the ridge axis is hot and buoyant, so its density is lowered and thus mid-ocean ridges are relatively high regions of the ocean floor
Seafloor spreading rates
Through drilling of the seafloor from ship-mounted drilling rigs, geologists determined that the oldest seafloor was only about 200 million years old (very young compared to the age of rock comprising the continents, which may be as old as 4.0 billion years)
- oldest seafloor located in the western Pacific
These maps enabled a determination of spreading rates of the ocean basins.
- Mid-Atlantic Ridge is spreading at a rate of ~2 to 3 cm/yr (~1” per yr) which causes the ridge to build upward. The East Pacific Rise (EPR) (a ‘rise’ is not as rough and jagged as a ‘ridge’) is spreading at a rate of
~10 to 17 cm/yr (4-6”/yr).- the faster spreading rate causes the 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
Continental rifting
Ocean basins are born when a continent splits and separates into two divergent continents in a process called continental rifting.
- 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 are formed along continental rifts
- continental rifts may (or may not) evolve through time into mid-ocean
ontinental rifts are formed within continental rock as it rifts apart. As the rift widens and the asthenosphere rises into the rift valley, volcanism changes to produce oceanic rock. (continental rock
is different from the basaltic rock created by seafloor spreading)
East African rift
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). It’s a hyper-arid land marked by fractures in the surface, common earthquakes and active volcanoes. The crust is sagging and will likely be inundated by the sea in a few million years.
- upwelling (rising) asthenosphere beneath Africa pushes up the overlying lithosphere, stretching it and causing it to break along faults. Collapse of fault blocks creates a rift valley.
- as lithosphere stretches and thins, the underlying asthenosphere rises even further, melts to form magma, and produces volcanism
linear sea
as seafloor spreading continues and the rift valley sags downward, seawater may flood in and an elongate linear sea may develop
- - e.g., the Red Sea is a linear sea, a nascent ocean basin
– in perhaps 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 brand new ocean basin
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
Oceanic crust
oceanic crust is about 7-10 km thick. Oceanic crust is composed of silicate rock dominated by the elements Si and O along with smaller amounts of iron (Fe) and magnesium (Mg).
- the rock composing the oceanic crust is full of iron and magnesium that makes the rock dense, relative to continental rock. Rock composing oceanic crust has an average density of 2.9 g/cc he dominant rock composing both continental and oceanic crust are igneous rocks that form by the solidification of a molten fluid like magma or lava. Both oceanic crust and continental crust “float” above denser rock of the mantle (density of 3.3 g/cc).
Ocean basins exist because denser, Fe-rich oceanic crust sinks deeper into the underlying mantle, relative to continental crust. R
Continental crust
the dominant rock composing both continental and oceanic crust ‘ are igneous rocks that form by the solidification of a molten fluid like magma or lava. continental crust is about 30-70 km thick. Continental crust is formed by a variety of different processes that tend to incorporate lighter elements into the rock such as potassium, sodium and aluminum. (remember that almost all rocks are silicates
– it’s the additional elements that determine the density and other characteristics of the rock.)
Rock composing continental crust has an average density of 2.7 g/cc.
Both oceanic crust and continental crust “float” above denser rock of the mantle (density of 3.3 g/cc). Rock composing lighter continental crust ‘floats’ higher above the underlying mantle (analogous to a buoyant iceberg)
Convergent Plate Boundaries
Plate boundary where two plates converge to produce linear mountain belts.
- the primary force at convergent boundaries is compression – squeezing and ‘deformation’ caused by the squeezing motion of the two plates toward each other
- ‘deformation’ refers to the bending and breaking of rock, commonly associated with tectonic compression and the uplift of mountains
- whereas lithosphere is created along mid-ocean ridges, lithosphere is commonly destroyed along convergent margins by burial back into the mantle
- three types of convergent plate boundaries – oceanic/continental convergence, oceanic/oceanic convergence, and continent/continent convergence
compression stress
the primary force at convergent boundaries is compression – squeezing and ‘deformation’ caused by the squeezing motion of the two plates toward each other
oceanic-continental convergence
Along convergent boundaries where a plate composed of oceanic lithosphere dives beneath a continental part of a plate, lithospheric material is returned to the mantle. The process where oceanic lithosphere descends down into the mantle is called subduction
subduction
The process where oceanic lithosphere descends down into the ‘ mantle is called subduction. oceanic lithosphere is denser than continental lithosphere and thus subducts into the asthenosphere beneath the continental plate at rates of 10-15 cm/yr
- along oceanic-continental convergent boundaries, the process of subduction creates a deep oceanic trench that forms along the arcuate contact between the two plates (e.g., Peru-Chile trench, Middle American trench)
- deep-ocean trenches are the bathymetric expression of a subduction zone
magma chambers
As the subducting plate reaches depths of about 100 to 150 km where the temperatures and pressures are just right, the water is released into the overlying rock of the asthenosphere. The infusion of water lowers the melting point of the rock of the mantle just above the subducting plate, causing it to melt, creating magma.
- this buoyant molten rock (magma) rises toward the surface (melting its way upward) where it pools within bodies called magma chambers. The chamber supplies magma to volcanoes on the surface within the mountain belt
volcanic arc
The chamber supplies magma to volcanoes on the surface
within the mountain belt.
- the volcanoes align roughly parallel to the convergent margin, forming a linear belt called a continental
volcanic arc
- compression of the rocks along the edge of the continental plate causes them to deform and warp upward into a high mountain chain. The linear belt of volcanoes pierces through the deformed rocks and forms high peaks.
inclined zone of seismicity
Earthquakes, ranging in depth from near-surface to ~670 km, are common along the dense subducting plate as it grinds downward against the over-riding plate (earthquake activity is commonly called ‘seismicity’) – this inclined zone of seismicity is characteristic of subduction zones
- of all plate boundaries, subduction-zone earthquakes are commonly the largest in magnitude
Oceanic-oceanic convergence
Many deep-ocean trenches occur adjacent to linear chains of islands called volcanic island arcs. (as opposed to continental volcanic arcs) island arcs are formed by convergence of two plates composed of oceanic lithosphere, with the older, ‘colder’ (and thus denser) plate subducting beneath the younger, ‘warmer’ less dense plate
- island arc volcanism results when magmas generated along the subduction zone buoyantly reach the overlying seafloor and erupt, with the underwater volcano eventually building up above sea level
through time as a volcanic island
Continental-continental convergence
A third type of convergent margin exists where continental plates converge with other continental plates (e.g., Himalayas/Tibetan Plateau) rocks along the continent-continent convergent margin are squeezed and lifted upward to heights of 8+ km along the spine of the Himalayas (Everest is 29,035 ft high)
- since both plates along the convergent margin are composed of relatively low-density continental rock, neither will subduct. (Because there is no subduction involved, continent-continent convergent
boundaries are called collision zones.)
Ex. Convergence
Currently the Indian part of the plate is being pushed beneath
the south Asian plate, but both plates are jammed, permitting the accumulation of tremendous amounts of tectonic stress
