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9.8 Oceanography: 2. Plate tectonics and the current ocean
Extract from Earth and Environmental Science Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 28 July 09]
Prior Learning: Stage 5- 5.9.2a) Theory of plate tectonics and Additional Content- Theory of plate tectonics
Preliminary Course 8.5 Dynamic Earth
Background: Recall that the Earth’s crust is composed of a series of lithospheric plates ‘floating’ on the Earth’s mantle. Oceanic crust, due to its composition, is more dense than continental crust resulting in the continental crust floating higher on the denser upper mantle. This results in continental crust forming the land masses of continents.
gather, process, and analyse information, and use available evidence, to assess the impact of improved technological developments on understanding of the age of the sea floor
- Gather information from your teacher, the Internet, popular and scientific journals, text books and electronic or paper encyclopaedias. Check that the text books are recent: otherwise they may not have up to date information. To trace technologies used you could put the name Glomar Challenger into your search engine and for later technologies search for JOIDES Resolution.
A web site to start with
could be University of California , Berkeley , Museum of Paleontology , USA
Earth and Environmental Science, the HSC Course by Hubble, Huxley and Imlay-Gillespie is a very good reference.
- Process the information by extracting the part of the text that refers specifically to improved technological developments on understanding of the age of the sea floor.
- Analyse the information you have gathered from various sources to determine which is the most reliable and trustworthy and which answers the dot point best. Is there conflicting material or do all the sources generally agree about the technologies?
- Use available evidence gleaned from the information you have gathered and processed to assess the impact of these technological developments on our understanding of the age of the sea floor.
assess the reliability of information used to estimate the age of ocean beds
- Use the information that you gathered in the activity above. The best way to assess the reliability of the information is to have information from at least three sources. If the sources have very similar information they are probably reliable. If one has different information check a fourth or even a fifth source.
- Another way to be more certain of reliability is to see if the source is reputable, for example the US Geological Survey website or the GSA (Geological Society of Australia) would both be trusted sources.
identify the regions of the crust where new ocean basins are forming and where ocean floors are subducting
- New ocean basins are forming at Mid Ocean Ridges (MOR) (not always in the middle of oceans). The location of these ridges is illustrated by a map produced by the US Geological Survey .
- If the rate of production of material at the mid ocean ridges and and the
rate of subduction were not in equilibrium the Earth would be either getting
larger or getting smaller. Subduction occurs at trenches or subduction zones.
Here is a map
of the Earth showing where subduction is occurring Extreme Science, USA. Scroll across to the right and click on the map to enlarge it.
outline the types of evidence used to date ocean floors
- Relative dating of ocean floors indicates that the further from the ridge
the rocks are the older they are. This can be done by comparing the age of
rocks that are close to the MOR to those that are further away. The rocks
can be obtained for dating by drill bits that are lowered to the ocean floor
from special ships designed for scientific research.
- The first strong evidence that the ocean floor was spreading from the MORs
was obtained by British geologists Fred Vine and Drummond Matthewsand the
Canadian Lawrence Morley in 1963 who analysed the paleomagnetism preserve
in the basalts of the oceanic crust using magnetometers.
- Fossils can be taken from sedimentary rocks in the ocean floor and these can be dated using carbon dating.
outline the reasons why the oldest sea floor present on the Earth today is generally less than 250 million years old
- The processes of sea-floor spreading is constantly forming new ocean floor.
The new sea floor forms at mid ocean ridges where magma comes to the surface
and forms new basalt.
- The process of subduction destroys old ocean floor as it is subducted under
continental plates.
- The ocean floor acts like a huge conveyor belt, transporting sea floor crust and accumulated sediment to subduction zones.
More information can be obtained at this website.
Discover Our Earth, Cornell University , New York , USA . (Last accessed 7 Aug 08)
identify the role of plate tectonics in maintaining the equilibrium between the area of sea floor and area of continental land present on the Earth
- As discussed in the point above, seafloor is constantly being generated
and destroyed. This ongoing process has not significantly altered the total
area of seafloor that exists as part of the Earth’s crust over time.
The processes of seafloor spreading and subduction have maintained a constant
area of sea floor.
- The Earth's crust is divided into tectonic plates which move away from
the MOR and towards subduction zones. While there is a balance between seafloor
spreading and subduction, some oceans are growing while others are shrinking.
This process and forces generated by these variations give rise to continental
drift and ongoing changes in the distribution of the Earth’s oceans.
- Erosion of continental crust is a constant process. As mountains are eroded there is a significant change in the overlaying weight of crustal material. In response to these changing forces the underlying crustal material will gradually rise to bring the system back into balance. This phenomenon is known as isostatic adjustment and can be modelled by cutting the top off a wooden block floating in water.
- Isostatic rebound is responsible for maintaining the balance of continental crust on the Earth’s surface.
discuss the reasons for, and impacts of, possible shifts in the equilibrium between the area of sea floor and the area of continental land
- The area of seafloor and continental land has remained fairly stable over the period of the Earth’s history.
- What has varied dramatically during this time is the distribution of land masses and oceans.
- 200 million years ago the continents were combined into one large land mass called Pangea (pan = all, gaea = Earth) surrounded by one huge ocean called Panthalassa (pan = all, thalassa = sea)
- Over time this single land mass split apart forming Laurasia and Gondwana
and gradually reformed into the continental land masses we see today. This
has resulted in the formation of new ocean basins through the formation, subduction
and redistribution of ocean floor.
- Changing sea levels have impacted significantly on the area of exposed
continental crust.
- A good explanation and a diagram are found here.
Scroll down to Isostatic Rebound. Wikipedia, the free encyclopedia, USA (Last
accessed 7 Aug 2008 )
- On land mountains are eroding and the material is carried downstream by rivers. Some is then deposited in river deltas as the water slows. This can cause significant loading on areas of the crustal plate and result in the slow sinking of continental plate regions.
describe evidence for the closing of former ocean basins in terms of the presence of deep marine sedimentary rocks in present-day continental mountain belts
- One of the best examples of the presence of deep marine sedimentary rocks present in continental mountain belts is the Himalayas.
- The Himalayas have been formed as a result of the collision between the
Eurasian and Indo-Australian crustal plates. The resultant folding and faulting
has produce the mountain belt.
- There are a number of visual clues to the origin of the crustal material
making up the Himalayas . These include the layering of sedimentary rocks
which would have formed long ago as horizontal sediments on a sea bed.
- Ammonite fossils are found in large numbers high up in the Himalayas and in Tibetan sandstone that previously was on the bottom of the Tethys Sea . The Himalayas are still forming today.
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