The
shape of thingsTechnology allows us to produce images of the sea floor making it relatively easy to see the effects of plate tectonics on the surface of the earth. The features that form when plates collide or pull apart include:
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All these features are found on the thumbnail image above - see if you can identify them all
The edges of the continents - the continental margins can be classified according to their tectonic setting. Those located at the edges of diverging plates are called passive margins, because in these areas not much happens in terms of volcanoes or earthquakes. They are sinking areas where thick sequences of sediments accumulate. In contrast, margins at the edges of converging plates are called active margins, because they experience frequent volcanic or earthquake activity. The are generally rising so are not sites of extensive sediment accumulation. Passive margins occur around Australia, on both sides of the Atlantic Ocean, in Europe, Africa, and North and South America. Active margins occur around much of the Pacific Rim, in North and South America, the Alaska and Kamchatka Peninsulas, the Aleutian Islands, and Japan.
The continents are surrounded by a gently sloping (less than 1o) submerged plain called the continental shelf.. The widths of continental shelves vary considerably, in some areas they are very wide (over 1000km along some passive margins) while in other areas a shelf barely exists at all (along some active margins). The edge of the shelf, called the shelf break, is marked by an abrupt increase in slope (average of 4o) occurring at an average depth of 135 m.
The topography of continental shelves is generally the same as the coastal areas they border. In fact, almost all shelves were above water during the maximum extent of the Pleistocene ice ages (approximately 18,000 years ago). Coastal canyons and ridges that extend to the shoreline are often continued underwater to the edge of the shelf where they form submarine canyons funnelling sediment from the shelf to the Abyssal Plain.
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This is a processed image of the sea floor from eastern Bass Strait (70 X 120
km). It shows the continental slope, submarine canyons and abyssal sea
floor of eastern Bass Strait. Depths range from 500m (red) to 4,500m (purple)
- a section is blown up to the left! The main canyon is called Bass Canyon.
The NSW and southern Queensland shelf extends from Bass Strait in the south to Capricorn Channel in the north. The shelf is relatively narrow in comparison with other parts of Australia, having an average width of about 25 km. It is narrowest adjacent to Montague Island (10 km) and widest between Morton and Fraser islands (75 km).
The shelf break occurs at a depth of between 145 and 165 m along the southern to middle parts of the NSW shelf. Northwards however, the shelf break occurs at progressively shallower depths, reaching a minimum of about 70 m adjacent to Fraser Island. In general the shelf gradient is steepest between the shoreline and the 60-100 m isobath. In contrast, between 100-150 m the shelf is almost flat and has been termed the "middle shelf plain" by some researchers. The depth and morphology of the shelf break is thought to be controlled partly by the underlying bedrock geometry and by the prograding sediment wedge and its differential compaction (Jones et al., 1975, Davies, 1979).
Terraces (breaks in slope) occur along all sections of the shelf but are particularly evident between Sugarloaf Point and Jarvis Bay. In this area 3 main terraces have formed, at 40-60 m, 80-100 m and 120-140 m. The terraces formed during periods of lower sea level when the shelf was subject to erosion.
Yellow – Cainozoic, Green – Cretaceous, Purple
– Triassic, Brown – Late Devonian, Pink – Oceanic crust. Vertical
exaggeration = x 14
Along passive margins, sediments eroded from the continent finish up in a pile
at the bottom of the continental slope, called the continental rise (sediment
wedge).
Simplified diagram of a passive margin showing the shelf,
slope, rise and abyssal plain.
Would you sleep more peacefully in Sydney or Los Angeles? Active continental margins, like the west cost of American, can be pretty dangerous places. Volcanoes and earthquakes are caused by the subduction of the oceanic plate under the continental plate (see section on plate tectonics). A trench is formed at the subduction site and sediments eroding from the continent accumulate on the shelf or are transported into the trench (continental rises are generally absent on active margins because the presence of the trench doesn't allow for sediment to accumulate). In addition, sediments lying on the oceanic plate can be scraped off by the continental plate and form what is known as an accretionary wedge. These sediments can be many kilometers thick. The accretionary wedge is composed of deformed sediment and may also contain bits of the oceanic crust (see section on ophiolites). This jumble of rocks is termed a tectonic melange.
A poorly constructed fence. From http://www.muohio.edu/tectonics/at_report/
active_tectonics.html
Above Right. Diagram showing the subduction of oceanic
crust under the continental crust. The descending plate is subjected
to intense heat and pressure which causes parts of the slab to melt. The resulting
magma rises to the surface as volcanoes (the volcanic arc). The diagram also
shows the faulted and folded rocks of the accretionary wedge that forms on the
edge of the overriding plate.
Adapted from IODP Initial Science Plan http://www.iodp.org/isp.html
This is the place where new oceanic lithosphere is made and how fast this is happening effects the topography of the ridge. The East Pacific Rise (that runs down the west coast of America) has one of the fastest spreading rates – as high as 15 cm per year in some segments. In contrast the poor old Mid-Atlantic Ridge is only spreading at about 1 to 2 cm per year. It takes about 50 my for new ocean lithosphere to cool (from > 1000°C) to an equilibrium state and sink to its maximum depth below sea-level.
Depth
to the top of the ocean crust versus age – averaged for the Pacific, Indian
and Atlantic Oceans. The increase in depth with age is due to cooling
and consequent contraction of the crust as it moves away from the mid ocean
ridge.
Slow spreading ridges (< 4 cm/yr) are the most common type of ridge. They develop a deep rift valley along the main ridge axis – up to 2000 m deep and 8-20 km wide. Erupted pillow lavas form ridges or circular seamounts in what is termed the neovolcanic zone (ie. spot where active volcanism is occurring or has recently occurred). The floor of the valley is a mass of faults and fissures and large earthquakes are common. Eruptions are thought to be very infrequent, happening at 1000 to 10,000 year intervals.
Bathymetric
profile across an idealised slow spreading ocean ridge, showing the rift valley.
Fast spreading ridges (8 -15 cm/yr) have no axial valley but instead develop
a narrow trough, generally 5-40 m deep and up to 250 m wide. Volcanism appears
to be concentrated in a line below the ridge, forming an elongate shield volcano
1 to 2 km wide, up to 100 krn long. Fast spreading ridges. have a more constant
magma supply than slow spreading ridges, which translates into more frequent
eruptions (on a scale of 1 – 10 years). Lavas are dominated by fluid sheet
flows, in contrast to the pillow lavas of the slow spreading ridges (high temperature
lavas are less viscous than lower temperature ones, so erupt as sheet flows
rather than as pillows).
Bathymetric
profile across an idealised fast spreading ocean ridge.
Intermediate spreading ridges (4 - 8 cm/yr) have characteristics in common with
both fast and slow spreading ridges. They are morphologically variable but generally
have a small axial valley 1-5 km wide and 50 –200 m deep. The neovolcanic
zone is located in the floor of the valley. Eruptions occur on a scale of 10
– 100 year intervals. Both sheet and pillow lavas are common. Examples
of intermediate spreading ridges are the Galapagos Ridge and the Southeast Indian
Ridge.
As we
noted in the section on plate tectonics, transform faults or transform boundaries
as they are sometimes called, occur when two plates slide horizontally past
each other. Transform faults commonly connect two spreading centers (divergent
plate boundaries) or, sometimes trenches (convergent plate boundaries, eg the
Alpine fault zone on the South Island of New Zealand). When they offset active
spreading centers they produce a characteristic zig-zag plate boundary pattern.
Because transform faults are associated with spreading centers most are found
on the ocean floor, however a few occur on land, for example the San Andreas
fault zone in California.
The San Andreas fault photographed from the air above California's Carrizo Plain, east of San Luis Obispo. A linear valley has been eroded along the main trace of the fault. The black line at left is a fence line. Photo courtesy of the USGS.
Transform faults occur only between the offset ends of the ridge segments. Anything beyond this is called a fracture zone. Unlike transform faults, where lithosphere on either side of the fault is moving in opposite directions, the lithosphere on either side of the fracture zone is moving in the same direction. Because of this transform faults are associated with shallow earthquakes (as the plates slip past each other friction builds up and is released as earthquakes), whereas fracture zones are seismically inactive. Transform faults and fracture zones are visible on the sea floor as cliffs – the height difference is due to the differences in age of the lithosphere on opposite sides of the fault or fracture.
Transform
fault offsetting the spreading ridge axis. As the faults move away from the
area of active spreading they become aseismic fracture zones.
Abyssal plains are those deep parts of the ocean - 4000 to 6000 meters deep - that begin at the edge of the continental margin and continue to the ocean ridges. They represent nearly 30% of the earth’s surface (30% of the Atlantic and nearly 75% of the Pacific ocean floors) and are generally extremely flat. The original rough basalt topography is covered by a layer of fine sediment that can be up to 5 km thick (but is usually less than 1 km). There is much more exposure of the original topography in the Pacific, because the trenches stop sediment from blanketing the sea floor. Occasional oval-shaped abyssal hills and seamounts protrude from the plain. Abyssal hills are less than 1 km high and form as part of the spreading process. They are often located parallel to mid-ocean ridges and may be found alone or in groups. Seamounts are extinct volcanoes formed by stationary hot spots in the mantle (see section on seafloor volcanism). If the volcano extends above the sea level, it is called an "island", if not it is a "seamount". A guyot is a former island that has had it’s top eroded off by waves. Seamounts are most common in the Pacific Ocean.
Abyssal hills are less than 1 km high and form as part of the spreading process. They are circular or elliptical and range from 1 to 8 km in width at the base. They are often located parallel to mid-ocean ridges and may be found alone or in groups. In the Pacific they cover more than 80% of the ocean floor.
Seamounts on the other hand, are extinct volcanoes formed by stationary
hot spots in the mantle (see section on seafloor volcanism) or less commonly
by volcanism at mid-ocean ridges. Active volcanoes in the oceans are generally
less than 30 million years old (because they move off the hot spot or away from
the ridge volcanism). If the volcano is active for a reasonable length of time,
eg 10 million years, it may develop into an island. If it doesn’t make
the grade it is called a seamount. Volcanoes that do extend above sea level,
will eventually become dormant, cool, and sink below the waves. As they sink,
the top is eroded flat by the waves and they become guyots. They are less common
than seamounts, but can be found in linear trends in the Pacific Ocean. They
illustrate how the sea floor subsides as it moves away from the mid-ocean ridges.
The five
volcanoes of the Traney seamount chain located off the coast of San Francisco.
< GLORIA image is from the USGS.
| An underwater landslide on a seamount in the Pacific
Ocean is now thought to be the cause of the tsunami that killed 2100 people
in coastal villages of Papua New Guinea in 1998. Scientists believe the
waves were up to 14 metres high The tsunami deposited sand up to 650 m inland. Scientists from around the world went to PNG a few weeks after the disaster to examine the area. The photo shows a typical tsunami deposit from Arop (one of the hardest hit villages). The tsunami deposited a grey-coloured sand, here about 5 cm thick, on a brown soil containing roots (USGS). |
We have mentioned trenches in the section above on active margins so what else is there to say? Ocean trenches are narrow, steep-sided depressions. The deepest trench, the Mariana Trench is almost 11 km deep. There are a total of 26 oceanic trenches in the world- 22 of which are found in the Pacific Ocean (3 in the Atlantic Ocean and only 1 in the Indian Ocean).
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| Location of the major ocean trenches | Hjort Trench (Geoscience Australia) |
Australia’s
deepest trench (well it’s almost ours) is the Hjort Trench. It is located
south of Macquarie Island and was recently surveyed as part of Australia’s
EEZ claim. The trench is 6.8km deep and in places just 10 km wide. It is located
at the boundary of the Pacific and Australian plates and is the site of some
of the world’s strongest recorded earthquakes. A 1989 quake measured 8.2
on the Richter scale.
< Bathymetric map of the area around Macquarie Island showing the northern part of the Hjort Trench (from Tasmanian National Parks)
Island arcs are curving chains of islands that occur around the margins of ocean basins (see section on New Britian Trench). In a typical island-arc environment, volcanoes lie along the crest of a ridge that is bounded on its convex side by a deep oceanic trench They are analogous to the volcanic arcs that form on land when an oceanic plate descends below a continental plate. However, the formation of island arcs requires the collision of two oceanic plates. The plate edge that is furthest away from the mid ocean ridge that created it (ie colder and denser one) will be the one to sink.
The idealized
island arc- back arc system has the following features:
For example we can look at the components of the Mariana Arc system.
Mariana trench (subduction zone)
Mariana fore-arc
Active Mariana arc
Mariana Trough (marginal basin, active spreading stopped about 6 Ma ago)
West Mariana Ridge (a remnant arc)
Shikoku and Parece-Vela Basins (inactive marginal basin
Satellite image of the Izu-Mariana margin – red dots
are the ODP Leg 185 sites
It is not entirely clear what causes spreading to start behind the volcanic
arc. It has been argued that the dip of the subducting slab determines whether
spreading occurs in the back arc region. A steeper dip seems to be more conducive
to the development of back arc spreading. Back arc basins are most common in
the Western Pacific (eg Woodlark, Fiji and Lau Basins) where old oceanic crust
is steeply subducted (cf the Eastern pacific where much younger less dense oceanic
crust subducts at a shallow angle).
This exercise is simply designed to test you understanding of the terminology required to describe and explain Basin topography.
Interpreting seafloor features is a three dimensional task. Often, it is deep processes which ultimately control the features that we see at the surface. This exercise looks at a seismic section of the seafloor and presents an opportunity to interpret the section presented.
There is a whole world of drilling and geophysical data for spreading and subsidence of the ocean floor! Some relevant examples can be downloaded here and interpreted by you! Pretty exciting stuff!!
We realise that not everyone has had experience with the intrinsic concepts of Marine Science. This 'computer' has numerous examples which will help you understand these fundamental facets! Very helpful, very colourful!