Laboratory Manual for Introductory Geology 4e

FIGURE 17.3 Tidal range at Mont-Saint-Michel, France.
(a) At low tide.
(b) At high tide.
and low tides. Mont-Saint-Michel in France ranks “only” fourth, at 46 feet, but as
FIGURE 17.3 shows, the result is dramatic.
The tides move enormous amounts of sediment all along the shoreline, and fluvial
processes extend across exposed tidal flats at low tide. As we shall see later, a
major factor in determining how much damage coastal storms will cause when they
come onshore is whether they arrive at high tide or low tide.
17.2.4 Tectonic Activity
Tectonic activity creates ocean basins, enlarges or shrinks them, and may uplift or
lower the land along the coast. When an ocean widens by seafloor spreading or
closes by subduction, its water must occupy the greater or smaller volume of the
ocean basin, lowering or raising sea level along its shorelines. In addition, when submarine
volcanoes build their cones above sea level, they create new lands (FIG. 17.4);
such lands include heavily populated island nations such as Japan, Indonesia, and
the Philippines as well as the state of Hawaii. These islands are in a constant state of
change, as waves erode the land that the new lavas build.
FIGURE 17.4 Growth of the island of Hawaii by addition of lava from Kilauea to the shoreline.
(a) Red-hot lava from the volcano entering the ocean. (b) New land created by lava flows in the 1940s.
436 CHAPTER 17 SHORELINE LANDSCAPES