Sea-Floor Spreading
- 2. TOPIC OBJECTIVES FOR GROUP REPORTERS: C. Sea-floor Spreading Enumerate and explain the initial and unusual observations that led to the formulation of the sea-floor spreading hypothesis. Enumerate and explain the evidences that support the Sea-Floor Spreading Process. Differentiate continental crust from oceanic crust and explain why the continental crust is older than the oceanic crust.
- 5. • Seafloor spreading is the movement of two oceanic plates away from each other (at a divergent plate boundary), which results in the formation of new oceanic crust (from magma that comes from within the Earth's mantle) along a mid- ocean ridge. Where the oceanic plates are moving away from each other is called a zone of divergence.
- 6. • Proposed by Harry H. Hess in 1960, he hypothesized that the formation of underwater mountains and new ocean floors were caused by the Earth’s mantle rising up between two plates.
- 9. The boundaries where the plates move apart are 'constructive' because new crust is being formed and added to the ocean floor. The ocean floor gradually extends and thus the size of these plates increases.
- 12. 1. Molten material 2. Drilling samples 3. Magnetic stripes
- 16. Evidence from Magnetic Stripes By the mid-1960s studies of the earth's magnetic field showed a history of periodic reversals in polarity. The magnetic history of the earth is thus recorded in the spreading ocean floors as in a very slow magnetic tape recording, forming a continuous record of the movement of the ocean floors.
- 17. • Magnetic surveys conducted near the mid-ocean ridge showed elongated patterns of normal and reversed polarity of the ocean floor in bands paralleling the rift and symmetrically distributed as mirror images on either side of it.
- 19. • "A dramatic proof of sea-floor spreading was discovered in the mid 1960s when data revealed alternating stripes of magnetic orientation on the sea floor, parallel to the mid-ocean ridges and symmetric across them -- that is, a thick or thin stripe on one side of the ridge is always matched by a similar stripe at a similar distance on the other side. This mirror-image magnetic orientation pattern is created by steady sea-floor spreading combined with recurrent reversals of Earth's magnetic field. Iron atoms in liquid rock welling up along a mid-ocean ridge align with Earth's magnetic field. "
- 20. Oceanic Crust Continental Crust • The oceanic crust is • The continental crust that part of the Earth’s accounts for 40% of the crust that covers the surface of the Earth. It ocean basins. It consists is made up of granite rock. This rock is rich in of dark-colored rocks constituents like silicon, made up of basalt. aluminum, and oxygen. • The oceanic crust • The continental crust is scarcely floats on the much thicker when mantle. compared to the oceanic crust. • The continental crust floats much more freely on the magma.
- 21. • The density of the continental crust is much less as compared to the oceanic crust. It has an approximate value of 2.6 g/cm3. Due to this difference in densities in magma between the oceanic crust and the continental crust, the continents stay in their places, and both crusts are able to float on the magma. • With age, the oceanic crust gathers a layer of cooled mantle on the underside. This causes the two-layered structure to sink into the hot, molten mantle. Once in the mantle, the oceanic crust melts and is thus recycled. Due to this process there is an absence of aged oceanic crust.
- 22. • The best theory fitting all this evidence together is a mechanism known as "sea-floor spreading," an idea formalized in the 1960s by North Americans Harry Hess, Robert Dietz, and Tuzo Wilson. • This developing theory received further confirmation by the mapping of 20th-century seismic activity and recognition that it is concentrated at shallow depths of the lithosphere along the "axial rift" of mid-ocean ridges and at deeper depths around subduction zones, just as the sea-floor spreading theory would predict. • Once again, the military played a central role in these scientific discoveries, for it was the 1963 treaty banning above-ground nuclear-weapons testing that created the need for a Worldwide Standardized Seismograph Network (WWSSN), the very instruments used to detect this faint earthquake activity all around the planet.
- 23. References • http://library.thinkquest.org/17457/platetectonics/4.php • http://www.britannica.com/EBchecked/topic/530828/seafloor-spreading- hypothesis • http://mail.colonial.net/~hkaiter/MCASreview.html • http://library.thinkquest.org/17175/current.html • http://www.ucmp.berkeley.edu/fosrec/Metzger3.html • http://www.sciencephoto.com/image/167059/530wm/E3500088- Sea_floor_spreading-SPL.jpg • http://www.aoi.com.au/bcw/Seafloor/index.htm • http://www.ucmp.berkeley.edu/fosrec/Metzger3.html • http://www.aoi.com.au/bcw/Seafloor/index.htm • http://www.geosociety.org/educate/LessonPlans/SeaFloorSpreading.pdf • http://home.comcast.net/~steveriddle2/6th/documents/!workbooks/ch4sec4 KEY.pdf • http://www.encyclopedia.com/topic/seafloor_spreading.aspx • http://library.thinkquest.org/17457/platetectonics/4.php
Editor's Notes
- Other supportive evidence has emerged from study of the fracture zones that offset the sections of the ridge.