Tectonic Plates and Plate tectonics Theory
In 1977, after decades of tediously collecting and mapping ocean sonar data, scientists
began to see a fairly accurate picture of the seafloor emerge. The Tharp-Heezen map
illustrated the geological features that characterize the seafloor and became a crucial
factor in the acceptance of the theories of plate tectonics and continental drift. Today,
these theories serve as the foundation upon which we understand the geologic processes
that shape the Earth.
In much the same way that geographic borders have separated, collided, and been redrawn
throughout human history, tectonic plate boundaries have diverged, converged, and reshaped
the Earth throughout its geologic history. Today, science has shown that the surface of the
Earth is in a constant state of change. We are able to observe and measure mountains rising
and eroding, oceans expanding and shrinking, volcanoes erupting and earthquakes striking.
Before the Tharp-Heezen map of the seafloor was published in 1977, scientists had little
understanding of the geological features that characterized the seafloor, especially on a
global scale. The data and observations represented by the Tharp-Heezen map became
crucial factors in the acceptance of the theories of plate tectonics and continental drift.
The theory of plate tectonics states that the Earth’s solid outer crust, the lithosphere, is
separated into plates that move over the asthenosphere, the molten upper portion of the
mantle. Oceanic and continental plates come together, spread apart, and interact at
boundaries all over the planet.
Each type of plate boundary generates distinct geologic processes and landforms. At
divergent boundaries, plates separate, forming a narrow rift valley. Here, geysers spurt
super-heated water, and magma, or molten rock, rises from the mantle and solidifies into
basalt, forming new crust. Thus, at divergent boundaries, oceanic crust is created. The
mid-ocean ridge, the Earth’s longest mountain range, is a 65,000 kilometres (40,390
miles) long and 1,500 kilometres (932 miles) wide divergent boundary. In Iceland, one of
the most geologically active locations on Earth, the divergence of the North American
and Eurasian plates along the Mid-Atlantic Ridge can be observed as the ridge rises
above sea level.
At convergent boundaries, plates collide with one another. The collision buckles the edge
of one or both plates, creating a mountain range or subducting one of the plates under
the other, creating a deep seafloor trench. At convergent boundaries, continental crust
is created and oceanic crust is destroyed as it subducts, melts, and becomes magma.
Convergent plate movement also creates earthquakes and often forms chains of
volcanoes. The highest mountain range above sea level, the Himalayas, was formed 55
million years ago when the Eurasian and Indo-Australian continental plates converged.
The Mediterranean island of Cyprus formed at a convergent boundary between the
African and Eurasian plates. Hardened mounds of lava, called pillow lavas, were once on
the bottom of the ocean where this convergence occurred, but have been pushed up and
are now visible at the surface.
•Scientists are able to calculate average rates of tectonic plate movement for
a given time period. These rates of movement range widely. For example, the
rate of spreading at the Mid-Atlantic Ridge near Iceland is relatively slow,
about 2.5 centimetres (1 inch) per year. This is similar to the rate at which
fingernails grow. The fastest known rate of plate movement, 15 centimetres (6
inches) per year, occurs on the East Pacific Rise in the South Pacific.
•The Mid-Atlantic Ridge runs down the centre of the Atlantic Ocean. Along its
crest, the ridge has a deep rift valley that, on average, is similar to the depth
and width of the Grand Canyon: 1 to 3 kilometres (0.6 -1.8 miles) deep and 6.5
to 29 kilometres (4-18 miles) wide.
•The highest mountain range above sea level, the Himalayas, was formed 55
million years ago when the Eurasian and Indo-Australian continental plates
converged. Due to ongoing convergence, the Himalayas, including Mount
Everest, continue to rise by approximately 2 centimetres (≈1 inch) each year.
Questions
1. At which type of tectonic boundary is oceanic crust created? At which type of tectonic boundary is
oceanic crust destroyed? Provide at least one example where each type of boundary is found on the Earth.
2. If an oceanic and continental plate converged, which one (if any) would subduct?
3. What happens at each type of plate boundary?
TECTONIC PLATES
INTRODUCTION
The modern theory of the motions of Earth’s outer layers is called plate tectonics. It provides a framework for
understanding many of Earth’s features, such as mountains, earthquakes, and volcanoes, as well as the
distribution of fossils and the ages of rocks. It also helps scientists reconstruct ancient climates and continental
configurations.
The concept of plate tectonics was formulated in the
1960s. According to the theory, Earth has a rigid outer
layer, known as the lithosphere, which is typically about 60
miles (100 kilometers) thick. The lithosphere overlies a
softer layer of molten rock called the asthenosphere. The
lithosphere is broken up into about a dozen large plates
and several small ones. These plates move relative to
each other, typically at rates of 2 to 4 inches (5 to 10
centimeters) per year. As the plates move, they interact
along their boundaries; these interactions depend upon
the type of plate movement, as well.
BASICS OF THE THEORY
In 1912 the German meteorologist Alfred Wegener proposed that the continents had once been joined as a
gigantic landmass called Pangea (also spelled Pangaea). According to this theory, called continental drift, the
supercontinent long ago broke into pieces (the present continents), which have since drifted to their current
positions. At the time, many geologists rejected Wegener’s ideas, partly because he had no convincing
explanation why continents would move.
EARTH’S LAYERS
Since Wegener’s time, geologists have developed a better
picture of Earth’s internal structure. Chemically, Earth can
be divided into three layers: a relatively thin surface crust,
a thick middle layer called the mantle, and an innermost
layer called the core.
CRUST, MANTLE, AND CORE
Earth’s surface layer is a crust of solid rock, the thickness of which ranges from just 3 miles (5 kilometers) in parts
of the ocean bottom to about 45 miles (75 kilometers) in some continental areas. Below the crust is the denser
rock of the mantle, on which the crust effectively floats. The mantle extends to a depth of about 1,800 miles (2,900
kilometers). The rock comprising the upper mantle is cool and rigid. In the lower mantle, where temperatures
reach roughly 6,700 °F (3,700 °C), the rock is soft and can move slowly. Below the mantle is Earth’s core, which
extends to the center of the Earth, some 4,000 miles (6,400 kilometers) below the surface. The core is made
largely of iron and nickel. The outer part of the core is liquid, the motions of which are believed to create
Earth’s magnetic field. The innermost core, though extremely hot, is solid because of the extreme pressure of the
upper layers.
LITHOSPHERE AND ASTHENOSPHERE
Much of what is known about Earth’s internal structure comes from the study of vibrations called seismic waves,
which result from earthquakes. The waves travel through Earth’s interior and reflect or bend upon encountering
changes in the density of the rock. One type of seismic wave is absorbed if it enters liquid. From this, scientists
discovered the asthenosphere, a partially molten zone in the mantle, between about 50 and 200 miles (80 and
350 kilometers) below the surface.
The tectonic plates comprising the lithosphere (the crust and rigid upper mantle) slide across the top of the
asthenosphere. The edges of these plates do not necessarily correspond to the boundaries of continents or
oceans. For example, the North American Plate includes the western half of the North Atlantic Ocean’s seafloor.
PANGEA AND CONTINENTAL DRIFT
On the basis of plate tectonic theory, most
scientists now accept Wegener’s ideas about
continental drift and Pangea. It probably started
to break apart from about 240 million to 200
million years ago after having been assembled
from earlier continents only a few tens of millions
of years earlier. It now seems that earlier
incarnations of Pangea may have occurred, as
part of a roughly 400-million-year-cycle of the
breakup and reassembly of supercontinents.
PLATE MOVEMENTS
The way tectonic plates interact at their
margins depends on whether the crust
forming the top of the plate (at the point of
contact) is oceanic or continental.
Continental crust—made largely
of granite—is less dense than oceanic
crust—made largely of basalt. As the plates
move, they may converge, or come
together; diverge, or spread apart; or slide
past each other along fractures called
transform faults.
CONVERGENT BOUNDARIES
Plates move toward each other at convergent boundaries. Generally, if two plates are converging, the denser
plate will be forced under, or subducted beneath, the less dense one. The subducted crust is eventually
destroyed. The area around converging plates at which subduction occurs is called a subduction zone.
Subduction occurs when oceanic crust meets continental crust and when oceanic crust meets oceanic crust.
For example, the eastward-moving Nazca Plate, under the southeastern Pacific Ocean, is being subducted
under the westward-moving South American Plate. The denser oceanic crust of the Nazca Plate is thrust
down into the mantle, causing it to melt and produce magma (molten rock). The magma and gases rise to the
surface and are vented through the western crust of South America, forming the volcanoes of the Andes
Mountains. Sometimes pieces of the descending plate break off and stick in place deep underground for a
time before suddenly moving, causing powerful earthquakes. A deep ocean trench forms just offshore, where
the oceanic crust is being forced downward.
If two oceanic plates converge, subduction of one under the other may produce an arc of volcanic islands.
The Mariana Islands of the western Pacific Ocean were formed this way.
Two blocks of continental crust may converge as well. The resulting pressure can deform the crust. If one
block is denser, that block may slide under the other, mainly lifting the other plate rather than sinking too much
itself. The convergence of two continental plates often results in mountain building. A good example of this is
the convergence of the northward-moving Indo-Australian Plate with the Eurasian Plate. As India meets Asia,
the collision produces the highest mountain range on Earth—the Himalayas
DIVERGENT BOUNDARIES
The destruction of crust that occurs with subduction must
be compensated by the creation of crust elsewhere. This
occurs where plates are diverging, or moving away from
each other, at the undersea mountain chains
called oceanic ridges, such as the Mid-Oceanic Ridge that
runs along the middle of the Atlantic Ocean. At these
ridges, rising magma fills in what would otherwise be a
widening crack, producing new seafloor. Undersea
volcanoes and crust expanded by heating produce a long
undersea ridge, but with a rift in the middle where the
plates are separating. This process is known as seafloor
spreading. Shallow earthquakes are common at oceanic
ridges, as the seafloor fractures, fills in, and fractures
again.
Divergence can also happen on continents, producing
fractures called rift valleys. A modern example is the East
African Rift System, also called the Great Rift Valley. Over
millions of years the continental crust may separate
completely, with the area between flooding with water to
become a new ocean
TRANSFORM FAULTS
In some places, plates slide past each other laterally along
fractures in the crust. These places are called transform
faults. The plates stick and then occasionally slip,
producing earthquakes. Most transform faults are found on
the seafloor along oceanic ridges, but the faults also occur on
continents, such as the San Andreas Fault in California. As
plates move past each other along transform faults, crust is
neither destroyed nor created
CAUSES OF PLATE MOTION
The causes of plate motions are not completely
understood, but a major factor appears to be
giant convection cells (churning motions) in the
mantle. Currents of hot material rise while
currents of colder material sink, forming roughly
circular cells—a common pattern of heat
circulation that can also be seen in a pot of
boiling water. In this way, the planet’s interior
redistributes heat resulting from the decay of
long-lived radioactive elements such as uranium.
Plates may also be pulled along by a subducting margin as the dense crust sinks into the mantle. There is
evidence that some old plate fragments may have sunk almost to Earth’s core.
EVIDENCE FOR PLATE TECTONICS
In addition to accounting for earthquakes, volcanoes, mountains, and oceanic ridges as well as the geographic fit
of continental margins, plate tectonics and continental drift have explained many curious facts. Very similar groups
of fossils have been found far apart, probably carried away from each other when continents split. Coal in
Antarctica formed from forests that grew there when that land was much closer to the Equator. Evidence
of glaciers in Africa points to a time when Africa was part of an ancient continent near the South Pole.
PAST AND PRESENT DISTRIBUTION OF ORGANISMS
Similarities in fossils found on distant continents are strong evidence for plate tectonics. For example, fossil
remains of the ancient plant Glossopteris have been found in all of the Southern Hemisphere landmasses—an
unlikely occurrence unless these landmasses were once connected. Similarly, fossils of the ancient freshwater
reptile Mesosaurus have been found only in southern Africa and South America. Because mesosaurs would have
been unable to swim across the open ocean, their geographic distribution strongly suggests that the southern
continents were once joined.
The movements of Earth’s tectonic plates over time have affected the present distribution of living things. For
example, side-necked turtles have not changed very much since the Jurassic period (about 201 to 145 million
years ago). Scientists presume the turtles dispersed throughout the supercontinent of Gondwana before it broke
apart. Today the turtles are found in parts of South America, Africa, Madagascar, Australia, and on a few islands in
the Indian Ocean—but in no other parts of the world. As another example, Australian marsupials, such as
kangaroos and koalas, are distinct from the mammals of other continents. This is because the Australian
marsupials evolved under different ecological conditions than other marsupials after Australia separated from the
other continents.
GEOLOGIC EVIDENCE
Many rocks retain a record of how the planet’s magnetic field was
oriented when they formed. Earth’s magnetic field periodically
reverses polarity, so that the north and south magnetic poles
essentially switch. Bands of rocks with alternating magnetic polarity
on the western Atlantic seafloor have corresponding bands on the
eastern Atlantic seafloor. These matching bands are neatly
explained by seafloor spreading carrying fresh rock, magnetized by
Earth’s magnetic field at the time, off in both directions.
Continental crust, which is rarely subducted, contains rocks up to 4
billion years old. On the other hand, no oceanic crust has been
found over 200 million years old—because, according to plate
tectonics, the denser oceanic crust is subducted, destroyed, and
recycled.
DIRECT MEASUREMENT
Advances in technology have provided direct evidence of plate movement from
measurements. Using methods including radio telescopes stationed on different continents
and Global Positioning System (GPS) satellites, scientists have measured Earth’s plate
motions directly. Typical speeds are 2 to 4 inches (5 to 10 centimetres) per year, matching
well with long-term trends expected from the theory.
Book Task
Describe the 5 pieces of evidence for plate tectonics
Book Task
Describe the 5 pieces of evidence for plate tectonics