Misplaced Pages

#472527

46-585: The Lord Howe Rise is associated with seafloor spreading which also resulted in the creation of the Tasman Sea. The geology has not yet been characterised as well as other parts of Zealandia but when previous samples are analysed with current geological techniques they fit with the Zealandia hypothesis. The seafloor is known to be dominated by soft sediments and the highest quality recent survey only mapped approximately 25,500 km (9,800 sq mi) of

92-426: A magnetometer towed on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. By identifying a reversal with a known age and measuring the distance of that reversal from the spreading center, the spreading half-rate could be computed. In some locations spreading rates have been found to be asymmetric; the half rates differ on each side of

138-631: A mid-ocean ridge that was active from 80 to 60 million years ago, and now lies 800 kilometres offshore from mainland Australia . The Lord Howe Rise contains a line of seamounts called the Lord Howe Seamount Chain which formed during the Miocene period when this part of Zealandia existed over the Lord Howe hotspot . One rhyolite sample has been dated at 97 million years drilled on the southern Lord Howe Rise. Lord Howe Island

184-642: A rift in a continental land mass , similar to the Red Sea - East Africa Rift System today. The process starts by heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome (see isostasy ). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120-degree angles. These areas are named triple junctions and can be found in several places across

230-405: A base or reference level h b {\displaystyle h_{b}} , the depth of the ocean d ( t ) {\displaystyle d(t)} is of interest. Because d ( t ) + h ( t ) = h b {\displaystyle d(t)+h(t)=h_{b}} (with h b {\displaystyle h_{b}} measured from

276-586: A constant temperature T 1 . Due to its continuous creation, the lithosphere at x > 0 is moving away from the ridge at a constant velocity v , which is assumed large compared to other typical scales in the problem. The temperature at the upper boundary of the lithosphere ( z = 0) is a constant T 0 = 0. Thus at x = 0 the temperature is the Heaviside step function T 1 ⋅ Θ ( − z ) {\displaystyle T_{1}\cdot \Theta (-z)} . The system

322-591: A failed arm that was opening more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment reported that in the Afar region , September 2005, a 60 km fissure opened as wide as eight meters. During this period of initial flooding the new sea is sensitive to changes in climate and eustasy . As a result, the new sea will evaporate (partially or completely) several times before

368-406: A few kilometers to tens of kilometers wide, a crustal accretion zone within the boundary zone where the ocean crust is youngest, and an instantaneous plate boundary – a line within the crustal accretion zone demarcating the two separating plates. Within the crustal accretion zone is a 1–2 km-wide neovolcanic zone where active volcanism occurs. In the general case, seafloor spreading starts as

414-482: A hypothesis of continental drift in 1912, he suggested that continents plowed through the ocean crust. This was impossible: oceanic crust is both more dense and more rigid than continental crust. Accordingly, Wegener's theory wasn't taken very seriously, especially in the United States. At first the driving force for spreading was argued to be convection currents in the mantle. Since then, it has been shown that

460-484: A rate less than 40 mm/year. The highest known rate was over 200 mm/yr during the Miocene on the East Pacific Rise . In the 1960s, the past record of geomagnetic reversals of Earth's magnetic field was noticed by observing magnetic stripe "anomalies" on the ocean floor. This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred from data gathered with

506-400: Is a geophysical mechanism whereby the cooling and subsequent densifying of a subducting tectonic plate produces a downward force along the rest of the plate. In 1975 Forsyth and Uyeda used the inverse theory method to show that, of the many forces likely to be driving plate motion, slab pull was the strongest. Plate motion is partly driven by the weight of cold, dense plates sinking into

SECTION 10

#1732773201473

552-401: Is added to each tectonic plate on either side of a mid-ocean ridge is the spreading half-rate and is equal to half of the spreading rate). Spreading rates determine if the ridge is fast, intermediate, or slow. As a general rule, fast ridges have spreading (opening) rates of more than 90 mm/year. Intermediate ridges have a spreading rate of 40–90 mm/year while slow spreading ridges have

598-549: Is assumed to be at a quasi- steady state , so that the temperature distribution is constant in time, i.e. T = T ( x , z ) . {\displaystyle T=T(x,z).} By calculating in the frame of reference of the moving lithosphere (velocity v ), which has spatial coordinate x ′ = x − v t , {\displaystyle x'=x-vt,} T = T ( x ′ , z , t ) . {\displaystyle T=T(x',z,t).} and

644-509: Is continually formed during seafloor spreading. Seafloor spreading helps explain continental drift in the theory of plate tectonics . When oceanic plates diverge , tensional stress causes fractures to occur in the lithosphere . The motivating force for seafloor spreading ridges is tectonic plate slab pull at subduction zones , rather than magma pressure, although there is typically significant magma activity at spreading ridges. Plates that are not subducting are driven by gravity sliding off

690-641: Is forced under either continental crust or oceanic crust. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge . Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Ring of Fire of

736-477: Is found within the crustal accretion zone. The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced. Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean , sea level

782-510: Is small compared to L 2 / A {\displaystyle L^{2}/A} , where L is the ocean width (from mid-ocean ridges to continental shelf ) and A is the age of the ocean basin. The effective thermal expansion coefficient α e f f {\displaystyle \alpha _{\mathrm {eff} }} is different from the usual thermal expansion coefficient α {\displaystyle \alpha } due to isostasic effect of

828-480: Is the density of water. By substituting the parameters by their rough estimates: gives: where the height is in meters and time is in millions of years. To get the dependence on x , one must substitute t = x / v ~ Ax / L , where L is the distance between the ridge to the continental shelf (roughly half the ocean width), and A is the ocean basin age. Rather than height of the ocean floor h ( t ) {\displaystyle h(t)} above

874-775: The Farallon Plate : from the huge slab width with the Nevada , the Sevier and Laramide orogenies ; the Mid-Tertiary ignimbrite flare-up and later left as Juan de Fuca and Cocos plates, the Basin and Range Province under extension, with slab break off , smaller slab width, more edges and mantle return flow. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts . However, most scientists working today believe that

920-540: The West Norfolk Rise . Much of the basin remains unexplored in relation to oil and gas reserves. Seafloor spreading Seafloor spreading , or seafloor spread , is a process that occurs at mid-ocean ridges , where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Earlier theories by Alfred Wegener and Alexander du Toit of continental drift postulated that continents in motion "plowed" through

966-577: The asthenosphere does not directly cause motion by the friction of such basal forces. The North American Plate is nowhere being subducted , yet it is in motion. Likewise the African , Eurasian and Antarctic Plates . Ridge push is thought responsible for the motion of these plates. The subducting slabs around the Pacific Ring of Fire cool down the Earth and its core-mantle boundary . Around

SECTION 20

#1732773201473

1012-424: The heat equation is: where κ {\displaystyle \kappa } is the thermal diffusivity of the mantle lithosphere. Since T depends on x' and t only through the combination x = x ′ + v t , {\displaystyle x=x'+vt,} : Thus: It is assumed that v {\displaystyle v} is large compared to other scales in

1058-458: The mantle at oceanic trenches . This force and slab suction account for almost all of the force driving plate tectonics . The ridge push at rifts contributes only 5 to 10%. Carlson et al. (1983) in Lallemand et al. (2005) defined the slab pull force as: Where: The slab pull force manifests itself between two extreme forms: Between these two examples there is the evolution of

1104-497: The North Pacific): Assuming isostatic equilibrium everywhere beneath the cooling plate yields a revised age depth relationship for older sea floor that is approximately correct for ages as young as 20 million years: Thus older seafloor deepens more slowly than younger and in fact can be assumed almost constant at ~6400 m depth. Parsons and Sclater concluded that some style of mantle convection must apply heat to

1150-618: The Pacific Ocean. The Pacific is also home to one of the world's most active spreading centers (the East Pacific Rise) with spreading rates of up to 145 ± 4 mm/yr between the Pacific and Nazca plates . The Mid-Atlantic Ridge is a slow-spreading center, while the East Pacific Rise is an example of fast spreading. Spreading centers at slow and intermediate rates exhibit a rift valley while at fast rates an axial high

1196-595: The base of the plate everywhere to prevent cooling down below 125 km and lithosphere contraction (seafloor deepening) at older ages. Their plate model also allowed an expression for conductive heat flow, q(t) from the ocean floor, which is approximately constant at 1 ⋅ 10 − 6 c a l c m − 2 s e c − 1 {\displaystyle 1\cdot 10^{-6}\mathrm {cal} \,\mathrm {cm} ^{-2}\mathrm {sec} ^{-1}} beyond 120 million years: Slab pull Slab pull

1242-533: The change in water column height above the lithosphere as it expands or retracts. Both coefficients are related by: where ρ ∼ 3.3   g ⋅ c m − 3 {\displaystyle \rho \sim 3.3\ \mathrm {g} \cdot \mathrm {cm} ^{-3}} is the rock density and ρ 0 = 1   g ⋅ c m − 3 {\displaystyle \rho _{0}=1\ \mathrm {g} \cdot \mathrm {cm} ^{-3}}

1288-472: The continent is completely severed, then a new ocean basin is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the Niger Delta . The Niger River has formed in the failed rift arm of the triple junction . As new seafloor forms and spreads apart from

1334-399: The continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust and upper mantle lithosphere begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The Red Sea is an example of a new arm of the sea. The East African rift was thought to be

1380-488: The elevated mid-ocean ridges a process called ridge push . At a spreading center, basaltic magma rises up the fractures and cools on the ocean floor to form new seabed . Hydrothermal vents are common at spreading centers. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone. Spreading rate is the rate at which an ocean basin widens due to seafloor spreading. (The rate at which new oceanic lithosphere

1426-399: The elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists . Seafloor spreading can stop during the process, but if it continues to the point that

Lord Howe Rise - Misplaced Pages Continue

1472-495: The fixed and immovable seafloor. The idea that the seafloor itself moves and also carries the continents with it as it spreads from a central rift axis was proposed by Harold Hammond Hess from Princeton University and Robert Dietz of the U.S. Naval Electronics Laboratory in San Diego in the 1960s. The phenomenon is known today as plate tectonics . In locations where two plates move apart, at mid-ocean ridges, new seafloor

1518-414: The mantle half-space model, the seabed height is determined by the oceanic lithosphere and mantle temperature, due to thermal expansion. The simple result is that the ridge height or ocean depth is proportional to the square root of its age. Oceanic lithosphere is continuously formed at a constant rate at the mid-ocean ridges . The source of the lithosphere has a half-plane shape ( x = 0, z < 0) and

1564-406: The mid-ocean ridge it slowly cools over time. Older seafloor is, therefore, colder than new seafloor, and older oceanic basins deeper than new oceanic basins due to isostasy. If the diameter of the earth remains relatively constant despite the production of new crust, a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust

1610-405: The motion of the continents is linked to seafloor spreading by the theory of plate tectonics, which is driven by convection that includes the crust itself as well. The driver for seafloor spreading in plates with active margins is the weight of the cool, dense, subducting slabs that pull them along, or slab pull. The magmatism at the ridge is considered to be passive upwelling, which is caused by

1656-454: The ocean surface): The depth predicted by the square root of seafloor age derived above is too deep for seafloor older than 80 million years. Depth is better explained by a cooling lithosphere plate model rather than the cooling mantle half-space. The plate has a constant temperature at its base and spreading edge. Analysis of depth versus age and depth versus square root of age data allowed Parsons and Sclater to estimate model parameters (for

1702-418: The plates are sliding apart over the mantle upwelling in the process of ridge push. The depth of the seafloor (or the height of a location on a mid-ocean ridge above a base-level) is closely correlated with its age (age of the lithosphere where depth is measured). The age-depth relation can be modeled by the cooling of a lithosphere plate or mantle half-space in areas without significant subduction . In

1748-558: The plates being pulled apart under the weight of their own slabs. This can be thought of as analogous to a rug on a table with little friction: when part of the rug is off of the table, its weight pulls the rest of the rug down with it. However, the Mid-Atlantic ridge itself is not bordered by plates that are being pulled into subduction zones, except the minor subduction in the Lesser Antilles and Scotia Arc . In this case

1794-452: The problem; therefore the last term in the equation is neglected, giving a 1-dimensional diffusion equation: with the initial conditions The solution for z ≤ 0 {\displaystyle z\leq 0} is given by the error function : Due to the large velocity, the temperature dependence on the horizontal direction is negligible, and the height at time t (i.e. of sea floor of age t ) can be calculated by integrating

1840-414: The ridge crest by about five percent. This is thought due to temperature gradients in the asthenosphere from mantle plumes near the spreading center. Seafloor spreading occurs at spreading centers, distributed along the crests of mid-ocean ridges. Spreading centers end in transform faults or in overlapping spreading center offsets. A spreading center includes a seismically active plate boundary zone

1886-436: The thermal expansion over z : where α e f f {\displaystyle \alpha _{\mathrm {eff} }} is the effective volumetric thermal expansion coefficient, and h 0 is the mid-ocean ridge height (compared to some reference). The assumption that v is relatively large is equivalent to the assumption that the thermal diffusivity κ {\displaystyle \kappa }

Lord Howe Rise - Misplaced Pages Continue

1932-590: The total area of Lord Howe Rise. Further north is the Elizabeth and Middleton part of the Coral Sea Islands , which together with reefs around Lord Howe Island are the most southern coral reefs on Earth. The seabed on the rise is mostly sandy mud with some volcanic outcrops, gravel and boulders. The doubleheader fish, Coris bulbifrons , is endemic to reefs on the Lord Howe Rise and

1978-441: The western flank of the rise which is less than 1% of the total area of the rise. In this area about 0.1% of the rises seafloor was classed as hard substrata based on a combined area of 31 km (12 sq mi) for 16 volcanic peaks. Sandstone rocks dredged from the central Lord Howe Rise contained granite pebbles that were in the range 216–183 million years old. It was rifted away from Eastern Australia in association with

2024-435: The world today. The separated margins of the continents evolve to form passive margins . Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge. If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift' or aulacogen . As the two active rifts continue to open, eventually

2070-593: Was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean . At the Mid-Atlantic Ridge (and in other mid-ocean ridges), material from the upper mantle rises through the faults between oceanic plates to form new crust as the plates move away from each other, a phenomenon first observed as continental drift. When Alfred Wegener first presented

2116-474: Was the last volcano to erupt on the rise 6.5 million years ago. Lord Howe Island and Ball's Pyramid cap a seamount towards the central east of the rise in an area known as the Lord Howe platform. The Lord Howe Seamount Chain extends northwards along the rise. The seamounts provide habitat to a diverse range of marine species which attracts commercial fishers, but cover a very small area, less than 1% of

#472527