Part II – Analyzing the ocean depth and age adjacent to the Midocean ridge.

In Part I of this exercise, we examined the depth of the Atlantic Ocean basin and observed the prominent mid-ocean ridge (MOR) near the center of the ocean basin. The MOR is also known to be an area of shallow earthquake and volcanic activity and is interpreted as a spreading center where new ocean lithosphere is formed. The newly formed oceanic crust and uppermost mantle then moves away from the ridge as part of plate tectonic processes. The uplift of the MOR is interpreted to be due to rising hot material that forms the new ocean lithosphere. As observed in the ocean profile, the ocean depth increases away from the MOR. The subsidence of the oceanic lithosphere is caused by cooling of the lithosphere as it moves away from the ridge and area of rising hot material. We can examine this process using ocean crust age and depth data. In the figure below, the age of the oceanic crust for a part of the North Atlantic Ocean is shown by the colors. The bold line is the Atlantic Ocean profile used in Part I. The numbers above the profile to the east of the MOR are interpreted age boundaries in millions of years. The ocean age information is derived from radiometric dating of ocean crust samples from deep drilling, dating of index fossils in sediments overlying the newly-formed crust, and the paleomagnetic reversals time scale.

Ocean crust ages for a portion of the North Atlantic Ocean (from

http://www.ngdc.noaa.gov/mgg/image/crustageposter.gif).

In the Table on the next page, the first two columns show distance from the ridge and ocean depth. The ocean depth data are the average of the depths on the two sides (west and east of the ridge) of the MOR. Using the distance scale on the OceanAge.pdf image above, estimate by interpolation the ocean crust age at 100 km increments out to 1200 km from the ridge and record the results in the Table. Then, take the square root of the age data and record the results in the last column. The first two age and square root of age data points, for 0 and 100 km distance, have already been entered in the Table. You should produce an Excel plot, or use other software, to attach to these pages for submission.

Part II Questions:

★Q2.1: Plot the ocean crust age and depth data. How does the depth change with increasing age?

★Q2.2: Plot the square root of age and depth data. How does the depth change with increasing SQRT (age)?

A theoretical cooling model of the oceanic lithosphere can be derived from the theory of the flow of heat through solids. The theory indicates that the ocean depth should increase away from the ridge approximately following the equation: Depth = slope • SQRT (age) + y-intercept.

The theory is further explained by the following:

Newly formed oceanic lithosphere moves away from the mid-ocean ridge and cools as it is removed from underlying sources of heat. Cooling has two effects: 1) lithosphere contracts and increases in density; 2) the depth of the lithosphere/asthenosphere boundary is controlled by temperature and cooling causes the lithosphere to increase in thickness away from the mid-ocean ridge. Cooling and contraction of the lithosphere cause a progressive increase in the depth to the top of the lithosphere away from the ridge. This is accompanied by a decrease in heat flow.

Parsons & Sclater (1977) determined the nature of age-depth relationships of oceanic lithosphere, and have shown that the depth, d (metres) is related to age t (million years) by:

This relationship only holds for oceanic lithosphere younger than 80 Ma. For older lithosphere, the relationship indicates a more gradual increase of depth with age. This relationship explains the observed bathymetry in most areas. Thus, if one knows the depth of the seafloor, it is possible to estimate the age of the seafloor. The depth of the seafloor depends upon its age rather than distance from the ridge.

★Q2.3: Estimate a “best fit” straight line through the data points in the SQRT (age) vs. Depth graph (above) using Excel (linear trend line). Calculate the slope and intercept coefficients of the line and record them here (the form of this equation is y = bx + a, where y is depth, b is the slope, x is SQRT(age), and a is the y-intercept Depth = ______ • SQRT (age) + ______ (Depth is in m, age is in m.y.; put b in the 1st space, a in the 2nd)

The SQRT (age) vs. depth data should display a straight line relationship that is consistent (in shape and values of the coefficients) with the ocean lithosphere cooling model (and the physics of heat conduction), and thus provides strong evidence supporting the sea floor spreading process and plate tectonics theory.

Data Link