Part B2: EXPLANATION: New ocean crust is formed at mid-oceanic ridges as the ris
ID: 152785 • Letter: P
Question
Part B2: EXPLANATION: New ocean crust is formed at mid-oceanic ridges as the rising asthenosphere partially melts and magma flows into the rift, forcing plates to diverge and the sea floor to spread. As magma cools it records the magnetic field of the earth. Periodic eruptions and spreading continues through time causing rocks formed at different times to record different magnetic polarities as the magnetic field of Earth periodically reverses. This results in a magnetic striping of the sea floor as illustrated in Figure 3 Figure 4 depicts magnetic data collected in 1965 by W. C. Pittman Ill and J. R. Heirtzler that was originally used to test this hypothesis of seafloor spreading. These data were published in Science in 1966. The figure shows magnetic data from four traverses (A through D) over a mid- ocean ridge in the South Pacific, collected by pulling a magnetometer behind a research ship The crest of the ridge is shown as a dashed line. Peaks in the magnetic data correspond to ocean crust that was formed during times of normal polarity, whereas the troughs (low spots) correspond to ocean crust that was formed during times of reverse polarity. Four peaks were found by the researchers to be distinctive enough to correlate from transect to transect. They have been labeled 1, 2, 3, and 4 on Figure 4. When answering the questions, remember that these are real data, and there are many ambiguities in nature 1 2 34 Age before present Nomal magneti podartymilions of years Normal magnetic polarity Observed magnetic profile from oceanographic survey (conceptually similar curves A through D in Figure Reversed magnetic polarity Mid-oceanic ridge Lithosphere Zone of magma injection, cooling and "locking in" of the magnetic polarity Figure 3. Schematic diagram illustrating the development of magnetic striping on the ocean floor associated with seafloor spreading. Figure retrieved from http://pubs.usgs.govExplanation / Answer
From the scale given in the above figure it can be seen that the distance from the transects 1 to 4 is measured as 500 kms on the either side of the figure. We are asked to find the spreading rate of the ridge system represented by the above figure.
For that we need to know about the maximum calculated age of the transect 4 on one side. Since it is a symmetrically spreading ridge, therefore it can be said that the spreading rate happening in one side of the ridge is same as that of the other side. Therefore from this concept we can calculate the half spreading rate to be 500/7 km per million year which gives us the value of 71.42 km/million years. Now the full spreading rate can be calculated by multiplying the value with the factor of 2 which ultimately leads us to the value of 142.85 km/million year.