# The mid-ocean ridges define one type of plate tectonic boundary. The rocks on the ocean floor on either side of a mid-ocean ridge are magnetized in a…

The mid-ocean ridges define one type of plate tectonic boundary. The rocks on the ocean floor on either side of a mid-ocean ridge are magnetized in a striped pattern. The variations in the widths of the magnetic stripes are the same on either side of the mid-ocean ridge. How does this tell us about motion at this type of plate boundary?
2. The oldest ocean crust is about one hundred and seventy million years old. Given the present day configuration of the continents, when would you expect the next supercontinent to form?
3. What are the general surface features of convection deep in Earth’s interior?
4. There is about 15,000,000 cubic kilometers (km3) of salt (NaCl) in the ocean. Rivers carry about 0.15 cubic kilometers (km3) of salt (NaCl) into the ocean per year. This ‘salt flux’ is 0.15 cubic kilometers (km3) per year.
(a) Use these figures to calculate an ‘age’ for the Earth, assuming that rivers have been continuously delivering salt to the oceans for all of Earth’s history.
(b) Compare this to the true age of the Earth.
(c) Discuss why these ages may be different.
5. On the handout is a plot of the percentage of rocks on Earth older than a given age. The percentages are plotted on the vertical axis and the rock ages on the horizontal axis. The distribution of rocks older than a certain age is represented by a red curve on this plot. The red curve starts from one hundred percent at an age of 0 million years (now), meaning that all rocks on Earth are older than this age. The red curve ends at zero percent at an age of 3,800 million years, meaning that all rocks are younger than this age. The curve connecting these two points is not a straight line but it is concave upward, passing through fifty percent at about 800 million years, meaning that half of the rocks on Earth are older than this age and half of them are younger than this age.
(a) Propose two different hypotheses to explain this pattern of rock abundance versus age.

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