Joy Nowak
Laboratory title: Plate Tectonics
Lab Objectives:
Students will understand continental drift.
Students will learn how our current continents were formed.
Students will simulate the three types of plate boundaries.
Students will be able to recognize which boundaries create mountain
ranges, earthquakes, tsunamis, and cause some volcanic activity.
Benchmarks Addressed:
CCG: Matter : Understand chemical and physical changes SC.05.PS.02
Describe the ability of matter to change state by heating and cooling.
SC.05.PS.02.01 Recognize that heating and cooling cause changes in
states of matter.
SC.05.PS.02.02 Identify changes in states of matter seen in the
environment.
CCG: Force : Understand fundamental forces, their forms, and their effects
on motion.
SC.05.PS.03 Describe and compare the motion of objects. SC.05.PS.03.01
Recognize and describe the motion of an object in terms of one or more
forces acting on it.
SC.05.PS.04 Identify examples of magnetism and gravity exerting force on
an object.
CCG: The Dynamic Earth : Understand changes occurring within the
lithosphere, hydrosphere, and atmosphere of the Earth.
SC.05.ES.03 Identify causes of Earth surface changes.
SC.05.ES.03.02 Identify effects of rapid changes on Earth’s surface
features including earthquakes and volcanoes.
Materials and Costs:
Item………………………………………………………………………… $
Plastic knifes or spoons…………………………………………… $1.25
Estimated total, one-time start up……………………………………. $1.25
Graham crackers (2 boxes)…………………………………..……. $5.98
Fruit roll-ups (2 boxes)……………………………………………... $6.58
Frosting (2 cans)….……………………………………………..….. $3.34
Wax paper………………………………………………………….... $1.79
Estimated total cost per year………………………………………….. $17.69
Time:
Initial prep time: Buying snack items for lab. ~ 30 min.
Preparation time: Putting out snack items. ~ 5 min.
Instruction time: 30 min.
Clean-up time: 10 min.
Background information:
Wikipedia
http://en.wikipedia.org/wiki/Plate_tectonics
Plate tectonics (from Greek τέκτων, tektōn "builder" or "mason") is a theory of
geology that has been developed to explain the observed evidence for large
scale motions of the Earth's lithosphere. The theory encompassed and
superseded the older theory of continental drift from the first half of the 20th
century and the concept of seafloor spreading developed during the 1960s.
The outermost part of the Earth's interior is made up of two layers: above is the
lithosphere, comprising the crust and the rigid uppermost part of the mantle.
Below the lithosphere lies the asthenosphere. Although solid, the asthenosphere
has relatively low viscosity and shear strength and can flow like a liquid on
geological time scales. The deeper mantle below the asthenosphere is more rigid
again. This is, however, not because of cooler temperatures but due to high
pressure.
The lithosphere is broken up into what are called tectonic plates — in the case of
Earth, there are seven major and many minor plates (see list below). The
lithospheric plates ride on the asthenosphere. These plates move in relation to
one another at one of three types of plate boundaries: convergent or collision
boundaries, divergent or spreading boundaries, and transform boundaries.
Earthquakes, volcanic activity, mountain-building, and oceanic trench formation
occur along plate boundaries. The lateral movement of the plates is typically at
speeds of 5 - 10 cm/yr.[1]
Synopsis of the development of the theory
Detailed map showing the tectonic plates with their movement vectors.
In the late 19th and early twentieth centuries, geologists assumed that the Earth's
major features were fixed, and that most geologic features such as mountain
ranges could be explained by vertical crustal movement, as explained by
geosynclinal theory. It was observed as early as 1596 that the opposite coasts of
the Atlantic Ocean — or, more precisely, the edges of the continental shelves —
have similar shapes and seem once to have fitted together.[2] Since that time
many theories were proposed to explain this apparent compatibility, but the
assumption of a solid earth made the various proposals difficult to
explain.[citation needed]
The discovery of radium and its associated heating properties in 1896 prompted
a re-examination of the apparent age of the Earth,[3] since this had been
estimated by its cooling rate and assumption the Earth's surface radiated like a
black body.[4] Those calculations implied that, even if it started at red heat, the
Earth would have dropped to its present temperature in a few tens of millions of
years. Armed with the knowledge of a new heat source, scientists reasoned it
was credible that the Earth was much older, and also that its core was still
sufficiently hot to be liquid.
Plate tectonic theory arose out of the hypothesis of continental drift proposed by
Alfred Wegener in 1912[5] and expanded in his 1915 book The Origin of
Continents and Oceans. He suggested that the present continents once formed a
single land mass which had drifted apart thus releasing the continents from the
Earth's core and likening them to "icebergs" of low density granite floating on a
sea of more dense basalt.[6] But without detailed evidence and calculation of the
forces involved, the theory remained sidelined. The Earth might have a solid
crust and a liquid core, but there seemed to be no way that portions of the crust
could move around. Later science proved theories proposed by English geologist
Arthur Holmes in 1920 that their junctions might actually lie beneath the sea and
Holmes' 1928 suggestion of convection currents within the mantle as the driving
force.[7][8]
The first evidence that crust plates did move around came with the discovery of
variable magnetic field direction in rocks of differing ages, first revealed at a
symposium in Tasmania in 1956. Initially theorized as an expansion of the global
crust,[9] later collaborations developed the plate tectonics theory, which
accounted for spreading as the consequence of new rock upwelling, but avoided
the need for an expanding globe by recognizing subduction zones and
conservative translation faults. It was at this point that Wegener's theory moved
from radical to mainstream, and became accepted by the scientific community.
Additional work on the association of seafloor spreading and magnetic field
reversals by Harry Hess and Ron G. Mason[10][11][12][13] pinpointed the
precise mechanism which accounted for new rock upwelling.
Following the recognition of magnetic anomalies defined by symmetric, parallel
stripes of similar magnetization on the seafloor on either side of a mid-ocean
ridge, plate tectonics quickly became broadly accepted. Simultaneous advances
in early seismic imaging techniques in and around Wadati-Benioff zones
collectively with numerous other geologic observations soon solidified plate
tectonics as a theory with extraordinary explanatory and predictive power.
Study of the deep ocean floor was critical to development of the theory; the field
of deep sea marine geology accelerated in the 1960s. Correspondingly, plate
tectonic theory was developed during the late 1960s and has since been
accepted all but universally by scientists throughout all geoscientific disciplines.
The theory revolutionized the Earth sciences, explaining a diverse range of
geological phenomena.
Key principles
The division of the outer parts of the Earth's interior into lithosphere and
asthenosphere is based on mechanical differences and in the ways that heat is
transferred. The lithosphere is cooler and more rigid, whilst the asthenosphere is
hotter and mechanically weaker. Also, the lithosphere loses heat by conduction
whereas the asthenosphere also transfers heat by convection and has a nearly
adiabatic temperature gradient. This division should not be confused with the
chemical subdivision of the Earth into (from innermost to outermost) core,
mantle, and crust. The lithosphere contains both crust and some mantle. A given
piece of mantle may be part of the lithosphere or the asthenosphere at different
times, depending on its temperature, pressure and shear strength. The key
principle of plate tectonics is that the lithosphere exists as separate and distinct
tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere.
Plate motions range up to a typical 10-40 mm yr-1 (Mid-Atlantic Ridge; about as
fast as fingernails grow), to about 160 mm yr-1 (Nazca Plate; about as fast as
hair grows).[14][15]
The plates are around 100 km (60 miles) thick and consist of lithospheric mantle
overlain by either of two types of crustal material: oceanic crust (in older texts
called sima from silicon and magnesium) and continental crust (sial from silicon
and aluminium). The two types of crust differ in thickness, with continental crust
considerably thicker than oceanic (50 km vs 5 km).
One plate meets another along a plate boundary, and plate boundaries are
commonly associated with geological events such as earthquakes and the
creation of topographic features like mountains, volcanoes and oceanic trenches.
The majority of the world's active volcanoes occur along plate boundaries, with
the Pacific Plate's Ring of Fire being most active and most widely known. These
boundaries are discussed in further detail below.
Tectonic plates can include continental crust or oceanic crust, and typically, a
single plate carries both. For example, the African Plate includes the continent
and parts of the floor of the Atlantic and Indian Oceans. The distinction between
continental crust and oceanic crust is based on the density of constituent
materials; oceanic crust is denser than continental crust owing to their different
proportions of various elements, particularly, silicon. Oceanic crust is denser
because it has less silicon and more heavier elements ("mafic") than continental
crust ("felsic").[16] As a result, oceanic crust generally lies below sea level (for
example most of the Pacific Plate), while the continental crust projects above sea
level (see isostasy for explanation of this principle).
Types of plate boundaries
Three types of plate boundary.
Three types of plate boundaries exist, 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 are:
1 Transform boundaries occur where plates slide or, perhaps more accurately,
grind past each other along transform faults. The relative motion of the two
plates is either sinistral (left side toward the observer) or dextral (right side
toward the observer). The San Andreas Fault in California is one example.
2 Divergent boundaries occur where two plates slide apart from each other. Midocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as
Africa's Great Rift Valley) are both examples of divergent boundaries.
3 Convergent boundaries (or active margins) occur where two plates slide
towards each other commonly forming either a subduction zone (if one plate
moves underneath the other) or a continental collision (if the two plates contain
continental crust). Deep marine trenches are typically associated with
subduction zones. The subducting slab contains many hydrous minerals,
which release their water on heating; this water then causes the mantle to
melt, producing volcanism. Examples of this are the Andes mountain range in
South America and the Japanese island arc.
Transform (conservative) boundaries
Main article: Transform boundary
John Tuzo Wilson recognized that because of friction, the plates cannot simply
glide past each other. Rather, stress builds up in both plates and when it reaches
a level that exceeds the strain threshold of rocks on either side of the fault the
accumulated potential energy is released as strain. Strain is both accumulative
and/or instantaneous depending on the rheology of the rock; the ductile lower
crust and mantle accumulates deformation gradually via shearing whereas the
brittle upper crust reacts by fracture, or instantaneous stress release to cause
motion along the fault. The ductile surface of the fault can also release
instantaneously when the strain rate is too great. The energy released by
instantaneous strain release is the cause of earthquakes, a common
phenomenon along transform boundaries.
A good example of this type of plate boundary is the San Andreas Fault which is
found in the western coast of North America and is one part of a highly complex
system of faults in this area. At this location, the Pacific and North American
plates move relative to each other such that the Pacific plate is moving northwest
with respect to North America. Other examples of transform faults include the
Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. Transform
faults are also found offsetting the crests of mid-ocean ridges (for example, the
Mendocino Fracture Zone offshore northern California).
Divergent (constructive) boundaries
Bridge across the Álfagjá rift valley in southwest Iceland, the boundary between
the Eurasian and North American continental tectonic plates.
Main article: Divergent boundary
At divergent boundaries, two plates move apart from each other and the space
that this creates is filled with new crustal material sourced from molten magma
that forms below. The origin of new divergent boundaries at triple junctions is
sometimes thought to be associated with the phenomenon known as hotspots.
Here, exceedingly large convective cells bring very large quantities of hot
asthenospheric material near the surface and the kinetic energy is thought to be
sufficient to break apart the lithosphere. The hot spot which may have initiated
the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a
rate of a few centimeters per year.
Divergent boundaries are typified in the oceanic lithosphere by the rifts of the
oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise,
and in the continental lithosphere by rift valleys such as the famous East African
Great Rift Valley. Divergent boundaries can create massive fault zones in the
oceanic ridge system. Spreading is generally not uniform, so where spreading
rates of adjacent ridge blocks are different, massive transform faults occur.
These are the fracture zones, many bearing names, that are a major source of
submarine earthquakes. A sea floor map will show a rather strange pattern of
blocky structures that are separated by linear features perpendicular to the ridge
axis. If one views the sea floor between the fracture zones as conveyor belts
carrying the ridge on each side of the rift away from the spreading center the
action becomes clear. Crest depths of the old ridges, parallel to the current
spreading center, will be older and deeper (from thermal contraction and
subsidence).[citation needed]
It is at mid-ocean ridges that one of the key pieces of evidence forcing
acceptance of the sea-floor spreading hypothesis was found. Airborne
geomagnetic surveys showed a strange pattern of symmetrical magnetic
reversals on opposite sides of ridge centers. The pattern was far too regular to
be coincidental as the widths of the opposing bands were too closely matched.
Scientists had been studying polar reversals and the link was made by Lawrence
W. Morley, Frederick John Vine and Drummond Hoyle Matthews in the MorleyVine-Matthews hypothesis. The magnetic banding directly corresponds with the
Earth's polar reversals. This was confirmed by measuring the ages of the rocks
within each band. The banding furnishes a map in time and space of both
spreading rate and polar reversals.
Convergent (destructive) boundaries
Main article: Convergent boundary
The nature of a convergent boundary depends on the type of lithosphere in the
plates that are colliding. Where a dense oceanic plate collides with a less-dense
continental plate, the oceanic plate is typically thrust underneath because of the
greater buoyancy of the continental lithosphere, forming a subduction zone. At
the surface, the topographic expression is commonly an oceanic trench on the
ocean side and a mountain range on the continental side. An example of a
continental-oceanic subduction zone is the area along the western coast of South
America where the oceanic Nazca Plate is being subducted beneath the
continental South American Plate.
While the processes directly associated with the production of melts directly
above downgoing plates producing surface volcanism is the subject of some
debate in the geologic community, the general consensus from ongoing research
suggests that the release of volatiles is the primary contributor. As the subducting
plate descends, its temperature rises driving off volatiles (most importantly water)
encased in the porous oceanic crust. As this water rises into the mantle of the
overriding plate, it lowers the melting temperature of surrounding mantle,
producing melts (magma) with large amounts of dissolved gases. These melts
rise to the surface and are the source of some of the most explosive volcanism
on Earth because of their high volumes of extremely pressurized gases (consider
Mount St. Helens). The melts rise to the surface and cool forming long chains of
volcanoes inland from the continental shelf and parallel to it.[citation needed] The
continental spine of western South America is dense with this type of volcanic
mountain building from the subduction of the Nazca plate. In North America the
Cascade mountain range, extending north from California's Sierra Nevada, is
also of this type. Such volcanoes are characterized by alternating periods of quiet
and episodic eruptions that start with explosive gas expulsion with fine particles
of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with
hot magma. The entire Pacific Ocean boundary is surrounded by long stretches
of volcanoes and is known collectively as The Ring of Fire.
Where two continental plates collide the plates either buckle and compress or
one plate delves under or (in some cases) overrides the other. Either action will
create extensive mountain ranges. The most dramatic effect seen is where the
northern margin of the Indian Plate is being thrust under a portion of the Eurasian
plate, lifting it and creating the Himalayas and the Tibetan Plateau beyond. It may
have also pushed nearby parts of the Asian continent aside to the east.[17]
When two plates with oceanic crust converge they typically create an island arc
as one plate is subducted below the other. The arc is formed from volcanoes
which erupt through the overriding plate as the descending plate melts below it.
The arc shape occurs because of the spherical surface of the earth (nick the peel
of an orange with a knife and note the arc formed by the straight-edge of the
knife). A deep undersea trench is located in front of such arcs where the
descending slab dips downward. Good examples of this type of plate
convergence would be Japan and the Aleutian Islands in Alaska.
Oceanic / Continental
Continental / Continental
Oceanic / Oceanic
Plates may collide at an oblique angle rather than head-on to each other (e.g.
one plate moving north, the other moving south-east), and this may cause strikeslip faulting along the collision zone, in addition to subduction or compression.
Not all plate boundaries are easily defined. Some are broad belts whose
movements are unclear to scientists. One example would be the MediterraneanAlpine boundary, which involves two major plates and several micro plates. The
boundaries of the plates do not necessarily coincide with those of the continents.
For instance, the North American Plate covers not only North America, but also
far northeastern Siberia, plus a substantial portion of the Atlantic Ocean.
Driving forces of plate motion
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. The current view, although it is still a matter of some debate, is that
excess density of the oceanic lithosphere sinking in subduction zones is the most
powerful source of plate motion. When it forms at mid-ocean ridges, the oceanic
lithosphere is initially less dense than the underlying asthenosphere, but it
becomes more dense 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.[18] 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 sources of plate motion are a matter of intensive research and
discussion among earth scientists.
Two and three-dimensional imaging of the Earth's interior (seismic tomography)
shows that there is a laterally heterogeneous density distribution throughout the
mantle. Such density variations can be material (from rock chemistry), mineral
(from variations in mineral structures), or thermal (through thermal expansion and
contraction from heat energy). The manifestation of this lateral density
heterogeneity is mantle convection from buoyancy forces.[19] How mantle
convection relates directly and indirectly to the motion of the plates is a matter of
ongoing study and discussion in geodynamics. Somehow, this energy must be
transferred to the lithosphere in order for tectonic plates to move. There are
essentially two types of forces that are thought to influence plate motion: friction
and gravity.
Friction
Basal drag
Large scale convection currents in the upper mantle are transmitted through the
asthenosphere; motion is driven by friction between the asthenosphere and the
lithosphere.
Slab suction
Local convection currents exert a downward frictional pull on plates in subduction
zones at ocean trenches. Slab suction may occur in a geodynamic setting
wherein basal tractions continue to act on the plate as it dives into the mantle
(although perhaps to a greater extent acting on both the under and upper side of
the slab).
Gravitation
Gravitational sliding: Plate motion is driven by the higher elevation of plates at
ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot
mantle material it gradually cools and thickens with age (and thus distance from
the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle
material from which it is derived and so with increasing thickness it gradually
subsides into the mantle to compensate the greater load. The result is a slight
lateral incline with distance from the ridge axis.
Casually in the geophysical community and more typically in the geological
literature in lower education this process is often referred to as "ridge-push". This
is, in fact, a misnomer as nothing is "pushing" and tensional features are
dominant along ridges. It is more accurate to refer to this mechanism as
gravitational sliding as variable topography across the totality of the plate can
vary considerably and the topography of spreading ridges is only the most
prominent feature. For example:
1. Flexural bulging of the lithosphere before it dives underneath an adjacent
plate, for instance, produces a clear topographical feature that can offset or at
least affect the influence of topographical ocean ridges.
2. Mantle plumes impinging on the underside of tectonic plates can drastically
alter the topography of the ocean floor.
Slab-pull
Plate motion is partly driven by the weight of cold, dense plates sinking into the
mantle at trenches.[20] There is considerable evidence that convection is
occurring in the mantle at some scale. The upwelling of material at mid-ocean
ridges is almost certainly part of this convection. Some early models of plate
tectonics envisioned the plates riding on top of convection cells like conveyor
belts. However, most scientists working today believe that the asthenosphere is
not strong enough to directly cause motion by the friction of such basal forces.
Slab pull is most widely thought to be the greatest force acting on the plates.
Recent models indicate that trench suction plays an important role as well.
However, it should be noted that the North American Plate, for instance, is
nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and
Antarctic Plates. The overall driving force for plate motion and its energy source
remain subjects of ongoing research.
External forces
In a study published in the January-February 2006 issue of the Geological
Society of America Bulletin, a team of Italian and U.S. scientists argued that the
westward component of plates is from Earth's rotation and consequent tidal
friction of the moon. As the Earth spins eastward beneath the moon, they say,
the moon's gravity ever so slightly pulls the Earth's surface layer back westward.
It has also been suggested (albeit, controversially) that this observation may also
explain why Venus and Mars have no plate tectonics since Venus has no moon,
and Mars' moons are too small to have significant tidal effects on Mars.[21] This
is not, however, a new argument.
It was originally raised by the "father" of the plate tectonics hypothesis, Alfred
Wegener. It was challenged by the physicist Harold Jeffreys who calculated that
the magnitude of tidal friction required would have quickly brought the Earth's
rotation to a halt long ago. Many plates are moving north and eastward, and the
dominantly westward motion of the Pacific ocean basins is simply from the
eastward bias of the Pacific spreading center (which is not a predicted
manifestation of such lunar forces). It is argued, however, that relative to the
lower mantle, there is a slight westward component in the motions of all the
plates.
Relative significance of each mechanism
Plate motion based on Global Positioning System (GPS) satellite data from
NASA JPL. Vectors show direction and magnitude of motion.
The actual vector of a plate's motion must necessarily be a function of all the
forces acting upon the plate. However, therein remains the problem of to what
degree each process contributes to the motion of each tectonic plate.
The diversity of geodynamic settings and properties of each plate must clearly
result in differences in the degree to which such processes are actively driving
the plates. One method of dealing with this problem is to consider the relative
rate at which each plate is moving and to consider the available evidence of each
driving force upon the plate as far as possible.[citation needed]
One of the most significant correlations found is that lithospheric plates attached
to downgoing (subducting) plates move much faster than plates not attached to
subducting plates. The Pacific plate, for instance, is essentially surrounded by
zones of subduction (the so-called Ring of Fire) and moves much faster than the
plates of the Atlantic basin, which are attached (perhaps one could say 'welded')
to adjacent continents instead of subducting plates. It is thus thought that forces
associated with the downgoing plate (slab pull and slab suction) are the driving
forces which determine the motion of plates, except for those plates which are
not being subducted.[citation needed]
The driving forces of plate motion are, nevertheless, still very active subjects of
on-going discussion and research in the geophysical community.
Major plates
The main plates are
▪ African Plate, covering Africa - Continental plate
▪ Antarctic Plate, covering Antarctica - Continental plate
▪ Australian Plate, covering Australia (fused with Indian Plate between 50 and
55 million years ago) - Continental plate
▪ Eurasian Plate covering Asia and Europe - Continental plate
▪ North American Plate covering North America and north-east Siberia Continental plate
▪ South American Plate covering South America - Continental plate
▪ Pacific Plate, covering the Pacific Ocean - Oceanic plate
Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean
Plate, the Juan de Fuca Plate, the Cocos Plate, the Nazca Plate, the Philippine
Plate and the Scotia Plate.
The movement of plates has caused the formation and break-up of continents
over time, including occasional formation of a supercontinent that contains most
or all of the continents. The supercontinent Rodinia is thought to have formed
about 1 billion years ago and to have embodied most or all of Earth's continents,
and broken up into eight continents around 600 million years ago. The eight
continents later re-assembled into another supercontinent called Pangaea;
Pangaea eventually broke up into Laurasia (which became North America and
Eurasia) and Gondwana (which became the remaining continents).
Related article
▪ List of tectonic plates
Historical development of the theory
Continental drift
For more details on this topic, see Continental drift.
Continental drift was one of many ideas about tectonics proposed in the late 19th
and early 20th centuries. The theory has been superseded and the concepts and
data have been incorporated within plate tectonics.
By 1915, Alfred Wegener was making serious arguments for the idea in the first
edition of The Origin of Continents and Oceans. In that book, he noted how the
east coast of South America and the west coast of Africa looked as if they were
once attached. Wegener wasn't the first to note this (Abraham Ortelius, Francis
Bacon, Benjamin Franklin, Snider-Pellegrini, Roberto Mantovani and Frank
Bursley Taylor preceded him), but he was the first to marshal significant fossil
and paleo-topographical and climatological evidence to support this simple
observation (and was supported in this by researchers such as Alex du Toit).
However, his ideas were not taken seriously by many geologists, who pointed out
that there was no apparent mechanism for continental drift. Specifically, they did
not see how continental rock could plow through the much denser rock that
makes up oceanic crust. Wegener could not explain the force that propelled
continental drift.
Wegener's vindication did not come until after his death in 1930. In 1947, a team
of scientists led by Maurice Ewing utilizing the Woods Hole Oceanographic
Institution’s research vessel Atlantis and an array of instruments, confirmed the
existence of a rise in the central Atlantic Ocean, and found that the floor of the
seabed beneath the layer of sediments consisted of basalt, not the granite which
is the main constituent of continents. They also found that the oceanic crust was
much thinner than continental crust. All these new findings raised important and
intriguing questions.[22]
Beginning in the 1950s, scientists including Harry Hess, using magnetic
instruments (magnetometers) adapted from airborne devices developed during
World War II to detect submarines, began recognizing odd magnetic variations
across the ocean floor. This finding, though unexpected, was not entirely
surprising because it was known that basalt—the iron-rich, volcanic rock making
up the ocean floor—contains a strongly magnetic mineral (magnetite) and can
locally distort compass readings. This distortion was recognized by Icelandic
mariners as early as the late 18th century. More important, because the
presence of magnetite gives the basalt measurable magnetic properties, these
newly discovered magnetic variations provided another means to study the deep
ocean floor. When newly formed rock cools, such magnetic materials recorded
the Earth's magnetic field at the time.
As more and more of the seafloor was mapped during the 1950s, the magnetic
variations turned out not to be random or isolated occurrences, but instead
revealed recognizable patterns. When these magnetic patterns were mapped
over a wide region, the ocean floor showed a zebra-like pattern. Alternating
stripes of magnetically different rock were laid out in rows on either side of the
mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with
reversed polarity. The overall pattern, defined by these alternating bands of
normally and reversely polarized rock, became known as magnetic striping.
When the rock strata of the tips of separate continents are very similar it
suggests that these rocks were formed in the same way implying that they were
joined initially. For instance, some parts of Scotland and Ireland contain rocks
very similar to those found in Newfoundland and New Brunswick. Furthermore,
the Caledonian Mountains of Europe and parts of the Appalachian Mountains of
North America are very similar in structure and lithology.
Floating continents
The prevailing concept was that there were static shells of strata under the
continents. It was observed early that although granite existed on continents,
seafloor seemed to be composed of denser basalt. It was apparent that a layer of
basalt underlies continental rocks.
However, based upon abnormalities in plumb line deflection by the Andes in
Peru, Pierre Bouguer deduced that less-dense mountains must have a
downward projection into the denser layer underneath. The concept that
mountains had "roots" was confirmed by George B. Airy a hundred years later
during study of Himalayan gravitation, and seismic studies detected
corresponding density variations.
By the mid-1950s the question remained unresolved of whether mountain roots
were clenched in surrounding basalt or were floating like an iceberg.
In 1958 the Tasmanian geologist Samuel Warren Carey published an essay The
tectonic approach to continental drift in support of the expanding earth model.
Plate tectonic theory
Significant progress was made in the 1960s, and was prompted by a number of
discoveries, most notably the Mid-Atlantic ridge. The most notable was the 1962
publication of a paper by American geologist Harry Hess (Robert S. Dietz
published the same idea one year earlier in Nature. However, priority belongs to
Hess, since he distributed an unpublished manuscript of his 1962 article already
in 1960). Hess suggested that instead of continents moving through oceanic
crust (as was suggested by continental drift) that an ocean basin and its
adjoining continent moved together on the same crustal unit, or plate. In the
same year, Robert R. Coats of the U.S. Geological Survey described the main
features of island arc subduction in the Aleutian Islands. His paper, though littlenoted (and even ridiculed) at the time, has since been called "seminal" and
"prescient". In 1967, W. Jason Morgan proposed that the Earth's surface consists
of 12 rigid plates that move relative to each other. Two months later, in 1968,
Xavier Le Pichon published a complete model based on 6 major plates with their
relative motions.
Explanation of magnetic striping
Seafloor magnetic striping.
The discovery of magnetic striping and the stripes being symmetrical around the
crests of the mid-ocean ridges suggested a relationship. In 1961, scientists
began to theorise that mid-ocean ridges mark structurally weak zones where the
ocean floor was being ripped in two lengthwise along the ridge crest. New
magma from deep within the Earth rises easily through these weak zones and
eventually erupts along the crest of the ridges to create new oceanic crust. This
process, later called seafloor spreading, operating over many millions of years
continues to form new ocean floor all across the 50,000 km-long system of midocean ridges. This hypothesis was supported by several lines of evidence:
1 at or near the crest of the ridge, the rocks are very young, and they become
progressively older away from the ridge crest;
2 the youngest rocks at the ridge crest always have present-day (normal)
polarity;
3 stripes of rock parallel to the ridge crest alternated in magnetic polarity
(normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has
reversed many times.
By explaining both the zebralike magnetic striping and the construction of the
mid-ocean ridge system, the seafloor spreading hypothesis quickly gained
converts and represented another major advance in the development of the
plate-tectonics theory. Furthermore, the oceanic crust now came to be
appreciated as a natural "tape recording" of the history of the reversals in the
Earth's magnetic field.
Subduction discovered
A profound consequence of seafloor spreading is that new crust was, and is now,
being continually created along the oceanic ridges. This idea found great favor
with some scientists, most notably S. Warren Carey, who claimed that the
shifting of the continents can be simply explained by a large increase in size of
the Earth since its formation. However, this so-called "Expanding earth theory"
hypothesis was unsatisfactory because its supporters could offer no convincing
mechanism to produce a significant expansion of the Earth. Certainly there is no
evidence that the moon has expanded in the past 3 billion years. Still, the
question remained: how can new crust be continuously added along the oceanic
ridges without increasing the size of the Earth?
This question particularly intrigued Harry Hess, a Princeton University geologist
and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S.
Coast and Geodetic Survey who first coined the term seafloor spreading. Dietz
and Hess were among the small handful who really understood the broad
implications of sea floor spreading. If the Earth's crust was expanding along the
oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested
that new oceanic crust continuously spreads away from the ridges in a conveyor
belt-like motion. Many millions of years later, the oceanic crust eventually
descends into the oceanic trenches — very deep, narrow canyons along the rim
of the Pacific Ocean basin. According to Hess, the Atlantic Ocean was expanding
while the Pacific Ocean was shrinking. As old oceanic crust is consumed in the
trenches, new magma rises and erupts along the spreading ridges to form new
crust. In effect, the ocean basins are perpetually being "recycled," with the
creation of new crust and the destruction of old oceanic lithosphere occurring
simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get
bigger with sea floor spreading, why there is so little sediment accumulation on
the ocean floor, and why oceanic rocks are much younger than continental rocks.
Mapping with earthquakes
During the 20th century, improvements in and greater use of seismic instruments
such as seismographs enabled scientists to learn that earthquakes tend to be
concentrated in certain areas, most notably along the oceanic trenches and
spreading ridges. By the late 1920s, seismologists were beginning to identify
several prominent earthquake zones parallel to the trenches that typically were
inclined 40–60° from the horizontal and extended several hundred kilometers into
the Earth. These zones later became known as Wadati-Benioff zones, or simply
Benioff zones, in honor of the seismologists who first recognized them, Kiyoo
Wadati of Japan and Hugo Benioff of the United States. The study of global
seismicity greatly advanced in the 1960s with the establishment of the Worldwide
Standardized Seismograph Network (WWSSN) to monitor the compliance of the
1963 treaty banning above-ground testing of nuclear weapons. The muchimproved data from the WWSSN instruments allowed seismologists to map
precisely the zones of earthquake concentration world wide.
USGS
http://pubs.usgs.gov/gip/dynamic/understanding.html#anchor6715825
Scientists now have a fairly good understanding of how the plates move and how
such movements relate to earthquake activity. Most movement occurs along
narrow zones between plates where the results of plate-tectonic forces are most
evident.
There are four types of plate boundaries:
• Divergent boundaries -- where new crust is generated as the plates pull away
from each other.
• Convergent boundaries -- where crust is destroyed as one plate dives under
another.
• Transform boundaries -- where crust is neither produced nor destroyed as the
plates slide horizontally past each other.
• Plate boundary zones -- broad belts in which boundaries are not well defined
and the effects of plate interaction are unclear.
Illustration of the Main Types of Plate Boundaries [55 k]
Divergent boundaries
Divergent boundaries occur along spreading centers where plates are moving
apart and new crust is created by magma pushing up from the mantle. Picture
two giant conveyor belts, facing each other but slowly moving in opposite
directions as they transport newly formed oceanic crust away from the ridge
crest.
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge.
This submerged mountain range, which extends from the Arctic Ocean to beyond
the southern tip of Africa, is but one segment of the global mid-ocean ridge
system that encircles the Earth. The rate of spreading along the Mid-Atlantic
Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million
years. This rate may seem slow by human standards, but because this process
has been going on for millions of years, it has resulted in plate movement of
thousands of kilometers. Seafloor spreading over the past 100 to 200 million
years has caused the Atlantic Ocean to grow from a tiny inlet of water between
the continents of Europe, Africa, and the Americas into the vast ocean that exists
today.
Mid-Atlantic Ridge [26 k]
The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers
scientists a natural laboratory for studying on land the processes also occurring
along the submerged parts of a spreading ridge. Iceland is splitting along the
spreading center between the North American and Eurasian Plates, as North
America moves westward relative to Eurasia.
Map showing the Mid-Atlantic Ridge splitting Iceland and separating the North
American and Eurasian Plates. The map also shows Reykjavik, the capital of
Iceland, the Thingvellir area, and the locations of some of Iceland's active
volcanoes (red triangles), including Krafla.
The consequences of plate movement are easy to see around Krafla Volcano, in
the northeastern part of Iceland. Here, existing ground cracks have widened and
new ones appear every few months. From 1975 to 1984, numerous episodes of
rifting (surface cracking) took place along the Krafla fissure zone. Some of these
rifting events were accompanied by volcanic activity; the ground would gradually
rise 1-2 m before abruptly dropping, signalling an impending eruption. Between
1975 and 1984, the displacements caused by rifting totalled about 7 m.
Lava Fountains, Krafla Volcano [35 k]
Thingvellir Fissure Zone, Iceland [80 k]
In East Africa, spreading processes have already torn Saudi Arabia away from
the rest of the African continent, forming the Red Sea. The actively splitting
African Plate and the Arabian Plate meet in what geologists call a triple junction,
where the Red Sea meets the Gulf of Aden. A new spreading center may be
developing under Africa along the East African Rift Zone. When the continental
crust stretches beyond its limits, tension cracks begin to appear on the Earth's
surface. Magma rises and squeezes through the widening cracks, sometimes to
erupt and form volcanoes. The rising magma, whether or not it erupts, puts more
pressure on the crust to produce additional fractures and, ultimately, the rift zone.
Historically Active Volcanoes, East Africa [38 k]
East Africa may be the site of the Earth's next major ocean. Plate interactions in
the region provide scientists an opportunity to study first hand how the Atlantic
may have begun to form about 200 million years ago. Geologists believe that, if
spreading continues, the three plates that meet at the edge of the present-day
African continent will separate completely, allowing the Indian Ocean to flood the
area and making the easternmost corner of Africa (the Horn of Africa) a large
island.
Summit Crater of 'Erta 'Ale [55 k]
Oldoinyo Lengai, East African Rift Zone [38 k]
Convergent boundaries
The size of the Earth has not changed significantly during the past 600 million
years, and very likely not since shortly after its formation 4.6 billion years ago.
The Earth's unchanging size implies that the crust must be destroyed at about
the same rate as it is being created, as Harry Hess surmised. Such destruction
(recycling) of crust takes place along convergent boundaries where plates are
moving toward each other, and sometimes one plate sinks (is subducted) under
another. The location where sinking of a plate occurs is called a subduction zone.
The type of convergence -- called by some a very slow "collision" -- that takes
place between plates depends on the kind of lithosphere involved. Convergence
can occur between an oceanic and a largely continental plate, or between two
largely oceanic plates, or between two largely continental plates.
Oceanic-continental convergence
If by magic we could pull a plug and drain the Pacific Ocean, we would see a
most amazing sight -- a number of long narrow, curving trenches thousands of
kilometers long and 8 to 10 km deep cutting into the ocean floor. Trenches are
the deepest parts of the ocean floor and are created by subduction.
Off the coast of South America along the Peru-Chile trench, the oceanic Nazca
Plate is pushing into and being subducted under the continental part of the South
American Plate. In turn, the overriding South American Plate is being lifted up,
creating the towering Andes mountains, the backbone of the continent. Strong,
destructive earthquakes and the rapid uplift of mountain ranges are common in
this region. Even though the Nazca Plate as a whole is sinking smoothly and
continuously into the trench, the deepest part of the subducting plate breaks into
smaller pieces that become locked in place for long periods of time before
suddenly moving to generate large earthquakes. Such earthquakes are often
accompanied by uplift of the land by as much as a few meters.
Convergence of the Nazca and South American Plates [65 k]
On 9 June 1994, a magnitude-8.3 earthquake struck about 320 km northeast of
La Paz, Bolivia, at a depth of 636 km. This earthquake, within the subduction
zone between the Nazca Plate and the South American Plate, was one of
deepest and largest subduction earthquakes recorded in South America.
Fortunately, even though this powerful earthquake was felt as far away as
Minnesota and Toronto, Canada, it caused no major damage because of its great
depth.
Ring of Fire [76 k]
Oceanic-continental convergence also sustains many of the Earth's active
volcanoes, such as those in the Andes and the Cascade Range in the Pacific
Northwest. The eruptive activity is clearly associated with subduction, but
scientists vigorously debate the possible sources of magma: Is magma
generated by the partial melting of the subducted oceanic slab, or the overlying
continental lithosphere, or both?
Oceanic-oceanic convergence
As with oceanic-continental convergence, when two oceanic plates converge,
one is usually subducted under the other, and in the process a trench is formed.
The Marianas Trench (paralleling the Mariana Islands), for example, marks
where the fast-moving Pacific Plate converges against the slower moving
Philippine Plate. The Challenger Deep, at the southern end of the Marianas
Trench, plunges deeper into the Earth's interior (nearly 11,000 m) than Mount
Everest, the world's tallest mountain, rises above sea level (about 8,854 m).
Subduction processes in oceanic-oceanic plate convergence also result in the
formation of volcanoes. Over millions of years, the erupted lava and volcanic
debris pile up on the ocean floor until a submarine volcano rises above sea level
to form an island volcano. Such volcanoes are typically strung out in chains
called island arcs. As the name implies, volcanic island arcs, which closely
parallel the trenches, are generally curved. The trenches are the key to
understanding how island arcs such as the Marianas and the Aleutian Islands
have formed and why they experience numerous strong earthquakes. Magmas
that form island arcs are produced by the partial melting of the descending plate
and/or the overlying oceanic lithosphere. The descending plate also provides a
source of stress as the two plates interact, leading to frequent moderate to strong
earthquakes.
Continental-continental convergence
The Himalayan mountain range dramatically demonstrates one of the most
visible and spectacular consequences of plate tectonics. When two continents
meet head-on, neither is subducted because the continental rocks are relatively
light and, like two colliding icebergs, resist downward motion. Instead, the crust
tends to buckle and be pushed upward or sideways. The collision of India into
Asia 50 million years ago caused the Eurasian Plate to crumple up and override
the Indian Plate. After the collision, the slow continuous convergence of the two
plates over millions of years pushed up the Himalayas and the Tibetan Plateau to
their present heights. Most of this growth occurred during the past 10 million
years. The Himalayas, towering as high as 8,854 m above sea level, form the
highest continental mountains in the world. Moreover, the neighboring Tibetan
Plateau, at an average elevation of about 4,600 m, is higher than all the peaks in
the Alps except for Mont Blanc and Monte Rosa, and is well above the summits
of most mountains in the United States.
Above: The collision between the Indian and Eurasian plates has pushed up the
Himalayas and the Tibetan Plateau. Below: Cartoon cross sections showing the
meeting of these two plates before and after their collision. The reference points
(small squares) show the amount of uplift of an imaginary point in the Earth's
crust during this mountain-building process.
| The Himalayas: Two Continents Collide |
Transform boundaries
The zone between two plates sliding horizontally past one another is called a
transform-fault boundary, or simply a transform boundary. The concept of
transform faults originated with Canadian geophysicist J. Tuzo Wilson, who
proposed that these large faults or fracture zones connect two spreading centers
(divergent plate boundaries) or, less commonly, trenches (convergent plate
boundaries). Most transform faults are found on the ocean floor. They commonly
offset the active spreading ridges, producing zig-zag plate margins, and are
generally defined by shallow earthquakes. However, a few occur on land, for
example the San Andreas fault zone in California. This transform fault connects
the East Pacific Rise, a divergent boundary to the south, with the South Gorda -Juan de Fuca -- Explorer Ridge, another divergent boundary to the north.
The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the
many fracture zones (transform faults) that scar the ocean floor and offset ridges
(see text). The San Andreas is one of the few transform faults exposed on land.
The San Andreas fault zone, which is about 1,300 km long and in places tens of
kilometers wide, slices through two thirds of the length of California. Along it, the
Pacific Plate has been grinding horizontally past the North American Plate for 10
million years, at an average rate of about 5 cm/yr. Land on the west side of the
fault zone (on the Pacific Plate) is moving in a northwesterly direction relative to
the land on the east side of the fault zone (on the North American Plate).
San Andreas fault [52 k]
Oceanic fracture zones are ocean-floor valleys that horizontally offset spreading
ridges; some of these zones are hundreds to thousands of kilometers long and
as much as 8 km deep. Examples of these large scars include the Clarion,
Molokai, and Pioneer fracture zones in the Northeast Pacific off the coast of
California and Mexico. These zones are presently inactive, but the offsets of the
patterns of magnetic striping provide evidence of their previous transform-fault
activity.
Plate-boundary zones
Not all plate boundaries are as simple as the main types discussed above. In
some regions, the boundaries are not well defined because the plate-movement
deformation occurring there extends over a broad belt (called a plate-boundary
zone). One of these zones marks the Mediterranean-Alpine region between the
Eurasian and African Plates, within which several smaller fragments of plates
(microplates) have been recognized. Because plate-boundary zones involve at
least two large plates and one or more microplates caught up between them,
they tend to have complicated geological structures and earthquake patterns.
Rates of motion
We can measure how fast tectonic plates are moving today, but how do
scientists know what the rates of plate movement have been over geologic time?
The oceans hold one of the key pieces to the puzzle. Because the ocean-floor
magnetic striping records the flip-flops in the Earth's magnetic field, scientists,
knowing the approximate duration of the reversal, can calculate the average rate
of plate movement during a given time span. These average rates of plate
separations can range widely. The Arctic Ridge has the slowest rate (less than
2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific
about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).
Easter Island monolith [80 k]
Evidence of past rates of plate movement also can be obtained from geologic
mapping studies. If a rock formation of known age -- with distinctive composition,
structure, or fossils -- mapped on one side of a plate boundary can be matched
with the same formation on the other side of the boundary, then measuring the
distance that the formation has been offset can give an estimate of the average
rate of plate motion. This simple but effective technique has been used to
determine the rates of plate motion at divergent boundaries, for example the MidAtlantic Ridge, and transform boundaries, such as the San Andreas Fault.
GPS Satellite and Ground Receiver [63 k]
Current plate movement can be tracked directly by means of ground-based or
space-based geodetic measurements; geodesy is the science of the size and
shape of the Earth. Ground-based measurements are taken with conventional
but very precise ground-surveying techniques, using laser-electronic instruments.
However, because plate motions are global in scale, they are best measured by
satellite-based methods. The late 1970s witnessed the rapid growth of space
geodesy, a term applied to space-based techniques for taking precise, repeated
measurements of carefully chosen points on the Earth's surface separated by
hundreds to thousands of kilometers. The three most commonly used spacegeodetic techniques -- very long baseline interferometry (VLBI), satellite laser
ranging (SLR), and the Global Positioning System (GPS) -- are based on
technologies developed for military and aerospace research, notably radio
astronomy and satellite tracking.
Among the three techniques, to date the GPS has been the most useful for
studying the Earth's crustal movements. Twenty-one satellites are currently in
orbit 20,000 km above the Earth as part of the NavStar system of the U.S.
Department of Defense. These satellites continuously transmit radio signals back
to Earth. To determine its precise position on Earth (longitude, latitude,
elevation), each GPS ground site must simultaneously receive signals from at
least four satellites, recording the exact time and location of each satellite when
its signal was received. By repeatedly measuring distances between specific
points, geologists can determine if there has been active movement along faults
or between plates. The separations between GPS sites are already being
measured regularly around the Pacific basin. By monitoring the interaction
between the Pacific Plate and the surrounding, largely continental plates,
scientists hope to learn more about the events building up to earthquakes and
volcanic eruptions in the circum-Pacific Ring of Fire. Space-geodetic data have
already confirmed that the rates and direction of plate movement, averaged over
several years, compare well with rates and direction of plate movement averaged
over millions of years.
Tasty lab time!
MATERIALS:
For each student:
• One large graham cracker broken in half (i.e., two square graham crackers)
• Two 3-inch squares (approx.) of fruit roll up
• Cup of water
• Frosting
• Sheet of wax paper
• Plastic knife or spoon
DIRECTIONS:
1 Make the model
a Give each student about a square foot of wax paper and a large dollop of
frosting. Instruct students to spread frosting into a layer about half a cm thick.
b Tell students that the frosting in this model represents the asthenosphere, the
viscous layer on which Earth’s plates ride. The plates in this model are
represented by fruit roll up (oceanic crust which is thin and dense) and
graham crackers (continental crust which is thick but less dense).
2 Divergent plate boundary
a Instruct students to place the two squares of fruit roll up (oceanic plates) onto
the frosting right next to each other.
b Press down slowly on the fruit roll ups (because they are dense and will sink a
bit into the asthenosphere) as you slowly push them apart about half a cm.
c Notice how the frosting is exposed and pushed up where the plates are
separated? This is analogous to how magma comes to the surface where real
plates are moving apart at divergent plate boundaries. Most divergent plates
boundaries are located within oceanic crust. When plates begin to pull apart at
continents, rift valleys are made, like the great rift valley in Africa, which can
become the bottom of the sea floor if the plates continue to pull apart.
3 Continental-oceanic collision
a Instruct students to remove one of the fruit roll ups from the frosting. (They
can eat it if they wish!)
b Tell students to place one of the graham cracker halves lightly onto the
frosting asthenosphere next to the remaining fruit roll up piece. The graham
cracker represents continental crust, which is thicker and less dense than
oceanic crust (fruit roll up). It floats high on the asthenosphere so don't push it
down.
c Gently push the continent (graham cracker) towards the ocean plate (fruit roll
up) until the two overlap and the graham cracker is on top. The oceanic plate
is subducted below the continental one.
4 Continent-continent collision
a Tell students that they will next model what happens when tow continents
collide. Have them remove both the cracker and fruit roll up from the frosting
asthenosphere. (Students can eat or discard the fruit roll up.)
b Place one edge of both crackers into the glass of water for just a few seconds.
c Place the crackers onto the frosting with wet edges next to each other.
d Slowly push the graham crackers towards each other.
e Notice how the wet edges crumple? This is how mountains are made at
convergent plate boundaries! When continents move towards each other
there is nowhere for the rock to go but up!
5 Transform plate boundaries
a Pick the two crackers up off the frosting and turn them around so that two dry
edges are next to each other.
b Push one cracker past the other to simulate a transform plate boundary like
the San Andreas fault!
6 Final step: eat all remaining model materials (except, of course, wax paper
and plastic utensils!)
ASSESSMENT:
Have students draw what each situation looks like in cross section (by looking at
the edge of their model).
Sources:
Lab:
http://www.windows.ucar.edu/tour/link=/teacher_resources/teach_snacktectonics.
html
Other information:
http://en.wikipedia.org/wiki/Plate_tectonics
This Dynamic Earth: The Story of Plate Tectonics by W. Jacquelyne Kious and
Robert I. Tilling (online book through usgs.org)
http://pubs.usgs.gov/gip/dynamic/understanding.html#anchor6715825
http://volcano.und.edu/vwdocs/vwlessons/lessons/Plates/Plates11.html
http://www.windows.ucar.edu/tour/link=/teacher_resources/teach_snacktectonics.
html
http://www.astronomynotes.com/solarsys/plates-collide.png