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Plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large-scale motions of Earth's
lithosphere. The lithosphere of the earth is divided into a small number of plates which float
on and travel independently over the mantle and much of the earth’s seismic activity occurs
at the boundries of these plates.
As per this theory, 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 (fig. 1.7).
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. Earlier theories (that still have some supporters) proposed gradual
shrinking (contraction) or gradual expansion of the globe.
Fig. 1.7: World Plates
Tectonic plates are able to move because the Earth's lithosphere has a higher strength
than the underlying asthenosphere. Lateral density variations in the mantle result in
convection. Plate movement is thought to be driven by a combination of the motion of the
seafloor away from the spreading ridge (due to variations in topography and density of the
Lecture Delivered by: Dr. Shahnawaz Ah. Baba
Plate tectonics
crust, which result in differences in gravitational forces) and drag, downward, at the
subduction zones. Another explanation lies in the different forces generated by the rotation of
the globe and the tidal forces of the Sun and the Moon. The relative importance of each of
these factors is unclear, and is still subject to debate.
Types of plate boundaries
Three types of plate boundaries exist, with a fourth, mixed type, characterized by the
way the plates move relative to each other. They are associated with different types of surface
phenomena. The different types of plate boundaries (Fig. 1.8.) are:
1. Transform boundaries (Conservative) occur where two lithospheric plates slide, or
perhaps more accurately, grind past each other along transform faults, wherein plates
are neither created nor destroyed. The relative motion of the two plates is either
sinistral (left side toward the observer) or dextral (right side toward the observer).
Transform faults occur across a spreading center. Strong earthquakes can occur along
fault. The San Andreas Fault in California is an example of a transform boundary
exhibiting dextral motion.
Fig. 1.8: Three types of plate boundary
2. Divergent boundaries (Constructive) occur where two plates slide apart from each
other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor
spreading, allowing for the formation of new ocean basin as continent splits: the ridge
forms at spreading center; the ocean basin expands; plate area increases; and causes
many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent
Lecture Delivered by: Dr. Shahnawaz Ah. Baba
Plate tectonics
rifting, divergent boundaries may cause new ocean basin to form as continent splits:
continent spreads; central rift collapses; and ocean fills basin. Active zones of Midocean ridges (e.g., Mid-Atlantic Ridge and East Pacific Rise), and continent-tocontinent rifting (such as Africa's East African Rift and Valley, Red Sea) are both
examples of divergent boundaries.
3. Convergent boundaries (Destructive) (or active margins) occur where two plates
slide towards each other to form either a subduction zone (if one plate moves
underneath the other) or a continental collision. At zones of continent-to-continent
subduction (e.g., Western South America, and Cascade Mountains in western United
States): the dense oceanic lithosphere plunges beneath the less dense continental;
earthquakes trace path of downward-moving plate as it descends into asthenosphere; a
trench forms; the subducted plate partially melts; and magma rises to form continental
volcanoes. At zones of ocean-to-ocean subduction (e.g., the Andes mountain range in
South America, Aleutian islands, Mariana islands, and the Japanese island arc): older,
cooler, denser crust slips beneath less dense crust; strong quakes, deep trench forms in
arc shape; subducted plate heats in upper mantle; magma rises to form curving chains
of volcanic islands. Deep marine trenches are typically associated with subduction
zones, and the basins that develop along the active boundary are often called
"foreland basins". The subductings lab contains many hydrous minerals, which
release their water on heating; this water then causes the mantle to melt, producing
volcanism. Closure of ocean basins can occur at continent-to-continent boundaries
(e.g., Himalayas and Alps): collision between masses of granitic continental
lithosphere; neither mass is subducted; plate edges are compressed, folded, uplifted.
Driving forces of plate motion
Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. The
vectors show direction and magnitude of motion. Plate tectonics is basically a kinematic
phenomenon: Earth scientists agree on the observation and deduction that the plates have
moved with respect to one another, and debate and find agreements as to how and when. But
still, a major question remains as to what the motor behind this movement is - the
geodynamic mechanism - and here science diverges in different theories. Generally, it is
accepted that tectonic plates are able to move because of the relative density of oceanic
lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the
mantle is acknowledged to be the original source of energy driving plate tectonics, through
Lecture Delivered by: Dr. Shahnawaz Ah. Baba
Plate tectonics
convection or large scale upwelling and doming. As a consequence, in the current view,
although it is still a matter of some debate, because of the excess density of the oceanic
lithosphere sinking in subduction zones a powerful source of plate motion is generated. When
the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than
the underlying asthenosphere, but it becomes denser with age, as it conductively cools and
thickens. The greater density of old lithosphere relative to the underlying asthenosphere
allows it to sink into the deep mantle at subduction zones, providing most of the driving force
for plate motions. The weakness of the asthenosphere allows the tectonic plates to move
easily towards a subduction zone. Although subduction is believed to be the strongest force
driving plate motions, it cannot be the only force since there are plates such as the North
American Plate which are moving, yet are nowhere being subducted. The same is true for the
enormous Eurasian Plate.
The main features of plate tectonics are:

The Earth's surface is covered by a series of crustal plates.

The ocean floors are continually moving, spreading from the center, sinking at the
edges, and being regenerated.

Convection currents beneath the plates move the crustal plates in different directions.

The source of heat driving the convection currents is radioactivity deep in the Earth’s
mantle.
Advances in sonic depth recording during World War II and the subsequent development of
the nuclear resonance type magnometer (proton-precession magnometer) led to detailed
mapping of the ocean floor and it came many observation that led scientists like Howard
Hess and R. Deitz to revive Holmes' convection theory. Hess and Deitz modified the theory
considerably and called the new theory "Sea-floor Spreading". Among the seafloor features
that supported the sea-floor spreading hypothesis were: mid-oceanic ridges, deep sea
trenches, island arcs, geomagnetic patterns, and fault patterns.
Mid-Oceanic Ridges: The mid-oceanic ridges rise 3000 meters from the ocean floor and are
more than 2000 kilometers wide surpassing the Himalayas in size. The mapping of the
seafloor also revealed that these huge underwater mountain ranges have a deep trench which
bisects the length of the ridges and in places is more than 2000 meters deep. Research into the
heat flow from the ocean floor during the early 1960s revealed that the greatest heat flow was
Lecture Delivered by: Dr. Shahnawaz Ah. Baba
Plate tectonics
centered at the crests of these mid-oceanic ridges. Seismic studies show that the mid-oceanic
ridges experience an elevated number of earthquakes. All these observations indicate intense
geological activity at the mid-oceanic ridges.
Lecture Delivered by: Dr. Shahnawaz Ah. Baba