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Seafloor spreading is a process that occurs at midocean ridges, where new
oceanic crust is formed through volcanic activity and then gradually moves away
from the ridge. Seafloor spreading helps explain continental drift in the theory of
Earlier theories (e.g. by Alfred Wegener and Alexander du Toit) of continental
drift were that continents "plowed" through the sea. The idea that the seafloor
itself moves (and carries the continents with it) as it expands from a central axis
was proposed by Harry Hess from Princeton University in the 1960s. The
theory is wellaccepted now, and the phenomenon is known to be caused by
convection currents in the plastic, very weak upper mantle, or asthenosphere.
Plate tectonics (from the Late Latin tectonicus, from the Greek:
"pertaining to building") is a scientific theory that describes the largescale
motions of Earth's lithosphere. The theory builds on the concepts of continental
drift, developed during the first decades of the 20th century. It was accepted by
the geoscientific community after the concepts of seafloor spreading were
developed in the late 1950s and early 1960s.
The lithosphere is broken up into tectonic plates. On Earth, there are seven or
eight major plates (depending on how they are defined) and many minor plates.
Where plates meet, their relative motion determines the type of boundary:
convergent, divergent, or transform. Earthquakes, volcanic activity,
mountainbuilding, and oceanic trench formation occur along these plate
boundaries. The lateral relative movement of the plates typically varies from zero
to 100 mm annually.
Tectonic plates are composed of oceanic lithosphere and thicker continental
lithosphere, each topped by its own kind of crust. Along convergent boundaries,
subduction carries plates into the mantle the material lost is roughly balanced by
the formation of new (oceanic) crust along divergent margins by seafloor
spreading. In this way, the total surface of the globe remains the same. This
prediction of plate tectonics is also referred to as the conveyor belt principle.
Earlier theories (that still have some supporters) proposed gradual shrinking
(contraction) or gradual expansion of the globe.
Tectonic plates are able to move because the Earth's lithosphere has a higher
strength and lower density than the underlying asthenosphere. Lateral density
variations in the mantle result in convection. Their movement is thought to be
driven by a combination of the motion of seafloor away from the spreading ridge
(due to variations in topography and density of the crust that result in differences
in gravitational forces) and drag, downward suction, at the subduction zones. A
different explanation lies in different forces generated by the rotation of the globe
and tidal forces of the Sun and the Moon. The relative importance of each of
these factors is unclear, and is still subject to debate (see also below).
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An earthquake (also known as a quake, tremor or temblor) is the result of a
sudden release of energy in the Earth's crust that creates seismic waves. The
seismicity, seismism or seismic activity of an area refers to the frequency, type
and size of earthquakes experienced over a period of time. Earthquakes are
measured using observations from seismometers. The moment magnitude is the
most common scale on which earthquakes larger than approximately 5 are
reported for the entire globe.…read more