Do students need an overview of the earth as a whole before diving into details ? Postulate: Earth is a gigantic machine. a) Incredibly complex and active internally. b) Myriad surficial processes create constant major change. Areas: India into Asia, rift expansions Sediment and oceanic volcanics displace water Erosion takes a continent in 10 ma (with no isostasy) c) Constant readjustment but is there a master plan ? Introducing a geology course to students

Freeboard: the hidden hand behind earth’s master equilibrium Freeboard At least through Phanerozoic (and before ?) Sea level has oscillated back and forth onto continents by about 250 m (+/-) Requires some process that maintains volume of ocean basins to volume of ocean waters within about 5% (250m / 5 km) Maintained despite: Changing areas of continents: contiental shelves, deltas, Basin and Range, etc Changing areas of ocean basins: India into Asia, Nevada into Pacific, volcanic arcs Changing av. depths of oceans: rapidly spreading wide ridges displace water

The key Isostasy : load (at depth of compensation loads are equal) volume oceans = area x depth = volume of continents = area x thickness Hess: ratio exists between depth of oceans & thickness of contients (about 1 : 5)

Higher sea level: less freeboard less relief = slower erosion rate less area to erode High sea level = slower erosion of continents Low sea level = accelerated erosion of continents

“The maintenance of a volume-area-isostasy equilibrium to within a few percent by volume is the unifying theme behind an endless number of seemingly random, seemingly purposeless geologic changes.” Constant freeboard: Wise, D. U., 1974, Continental margins, freeboard, and the volumes of continents and oceans through time, Geology of Continental Margins, C. E. Burke and C. L. Drake, eds., Springer-Verlag, p

Early earth: a) As ocean waters collected, what was effect on evolving ocean basins and continental crusts ? b) Vice versa ? c) What controlled these conflicting processes ? Present earth: a) Volcanoes in ocean are constantly displacing ocean water. b) Sediments from land constantly filling ocean basins c) Erosion constantly attacking continents (N. America to sea level in 10 ma if no isostasy) d) Why should continents continue to exist ?