Earth's Restless Surface

Earth is a geologically active planet.

The continents, composed of less dense rock, move slowly upon the denser mantle, driven by upwelling ‘plumes” of hot rock. Under tremendous pressures and temperatures, rock will “flow” like stiff plastic.

These upwelling plumes of rock are generated by heat withinthe Earth’s core, which drives convection cells in the mantle.

Plate tectonics is the term used for the study of surface motions on the Earth. “Plates” are pieces of crust, on which the continents and oceans basins reside.

Earth’s crust is composed of about a dozen large plates, with many smaller plates wedged between. Plate motion is responsible for a significant part of the Earth’s active surface geology.

Volcanoes and earthquakes, expressions of Earth’s restless crust, are observed to be clustered along plate boundaries.

Earthquakes occur when pieces of the crust or upper mantle suddenly release accumulated strain caused by plate tectonics. Energy propagates from the quake site via seismic waves.

Volcanoes occur when compressional motions of the crust heat and melt subsurface rocks. Gases (including water vapor) mixed with the molten rock, rise through cracks in the crust, driven by the intense pressure of the heated gases. Magma may be released at the surface explosively (e.g., Mount St Helens) or as flowing rock (e.g., Kilauea Volcano in Hawaii).

New crust is created at spreading plate boundaries, as molten material is uplifted from the mantle. Most of these boundaries are located along mid-ocean ridges.

Where plates collide, often one plate is subducted(or pushed)beneath the other. The subducted plate “returns” to the mantle; compressive forces cause melting, which fuels volcanic activity on the surface.

Another consequence of colliding plates is mountain-building. Earth’s major mountain chains are created when one plate butts into another, forcing one or both plates to be buckled upward.

A consequence of plate motion is that the relative positions of the continents have evolved with geologic time.

Earth’s surface is constantly changing - "re-surfaced" due to the cumulative actions of:

Plate motions

Volcanic activity

Erosion processes due to wind, water and ice.

Erosion is caused by the Earth’s substantial atmosphere, and active hydrological cycle. Flowing water, sand driven by the wind, and ice grinding over the surface gradually remove massive amounts of continental material and deposit much of that material into the ocean basins.

Due to this re-surfacing, the Earth’s surface is geologically young. Ancient mountain ranges, ocean basins, and other structures have long since eroded away. Rocks exposed on the surface exhibit a variety of ages, from fresh magma to the oldest rocks, which are about 3.8 billion years old.

Oceanic tides are another global process which affects the Earth.

Tides are caused by the differential gravitational forces of the Moon and Sun acting on the Earth’s liquid oceans. Water responds to the changing gravitational pulls of the Sun and Moon much faster and with more effect than the solid body of the Earth.

Frictionproduced by the ocean tides is gradually slowing the Earth's rotation rate. The year contained many more days a billion years ago compared to today.