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Shifting Ground


Click on the illustration to see how land rises and falls as the plates that support continents jostle against each other. Animation by Tanya Atwater

Earth’s continents sit atop huge plates floating on the planet’s viscous mantle. The movement of these plates explains many geological phenomena that long perplexed scientists. For instance, earthquakes occur when adjacent plates suddenly slip past each other, causing land on top to lurch forward. In some places where these tectonic plates meet, one surface descends beneath the other. Gradually, Earth swallows, or recycles, the land, pulverizing and melting its constituents, which will eventually resurface elsewhere in millions of years. In places where tectonic plates collide head on (such as where India is colliding with the subcontinent of Asia) the continents on either side of the plate boundary crinkle up like the crumple zones of two colliding cars. The Himalayas and the Rocky Mountains are the uplifted manifestations of such collisions of tectonic plates.

Recently scientists have discovered new forces that cause some regions of Earth to rise and others to fall.  Researchers believe that the mantle on which the planet’s crust floats is not perfectly smooth and symmetrical. It’s “lumpy.” They say viscous blobs of rock lazily circulating below the surface, akin to, but much slower-moving than, roiling water in a hot tea kettle, cause zones of different density and bouyancy inside the planet. Tectonic plates gradually rise then fall as they travel sluggishly over zones of greater bouyancy in the slowly-convecting mantle underneath. The Pliomax team believes that in some places this effect, called dynamic topography, might have substantially changed the elevation of Pliocene beaches and they’re working to adjust their measurements for such effects.

Learn about other adjustments the Pliomax team makes to its field measurements:

  • At Deformable Planet, vertical movements of land caused advance and retreat of glaciers.
  • At Attractive Ice Sheets, variations in the gravitational attraction of ice sheets.
  • At Axis Upheaval, shifts in Earth’s axis caused by changes in the size of polar glaciers.