Rock movements
Rocks are not motionless. They not only move but
also break, and they can even change shape without breaking. When
buried deep within the earth under conditions of great pressure,
rocks can bend like soft metal or, like bread dough or modelling
clay, be squeezed into new shapes. Most deformed rocks that we now
see at the earth's surface underwent deformation when buried far
below the surface. A surface along which rocks of any kind have
broken and moved past one another is called a fault. Faults
have formed during the present century, and some of these have
intersected the surface of the earth, where earth movements along
them have left visible scars. Movements of the earth, which are
often associated with faulting, are partially responsible for the
origin of mountains, although buildup of volcanic igneous rocks has
also contributed to the growth of most mountain chains. Movement
along faults is sporadic. Sometimes after years of quiescence,
rocks will suddenly move several metres along a fault. Uplift or
subsidence (depression) of land, which may or may not be
accompanied by faulting, can also be sporadic or gradual. Sudden
uplift of local areas and subsidence of others accompanied the 1964
earthquake in Alaska.
Most of our knowledge about the structure of the
earth's interior derives from the study of oscillatory movements
called seismic waves, which travel through the earth as a
consequence of natural or artificial disturbances. An earthquake is
an example of a natural seismic disturbance that results from the
sudden movement of one portion of the earth against another along a
fault. Artificial explosions at the earth's surface also produce
seismic waves.
Primary and secondary waves are two types of
seismic waves that have provided particularly valuable insight into
the nature of the earth's interior. Primary waves, which are also
called P waves, propagate changes in volume as portions of the
earth are alternately compressed and expanded. Secondary waves, or
S waves, shake the material from side to side perpendicular to
their direction of movement. Primary waves are so designated
because they travel faster than secondary waves.
An earthquake always begins at a focus,
which is a place within the earth where rocks move against other
rocks along a fault, producing both P waves and S waves.
Earthquake foci lie within the earth's mantle and crust, far from
the centre of the earth—but the waves that foci emit often
pass great distances through the earth to emerge at the surface,
where they can be detected with machines called
seismographs. Geophysicists can evaluate the nature of the
earth's internal structure by recording at many locations the
arrival times of P waves and S waves of an earthquake.
The study of seismic waves reveals that the
materials that form the central part of the earth are much more
dense than those near the earth's surface. The density gradient
from the surface to the centre of the earth is not a gradual one,
however; instead, the planet is divided into several discrete
concentric layers. At the earth's centre is the core,whose
solid, spherical inner portion and liquid outer portion are thought
to consist primarily of iron. Forming a thick envelope around the
outer core is the mantle, a complex body of less dense rocky
material. Finally, capping the mantle is the crust, which
consists of still less dense rocky material. The density gradient
from the core to the crust developed early in the earth's history,
when molten materials of low density came to float on materials of
higher density.
There are several ways in which the study of
seismic waves has revealed the aspects of the earth's interior just
described and others as well. For example, when earth-quake waves
reach a boundary between two concentric layers within the earth,
they are usually both reflected from the boundary and transmitted
through it, just as light striking the surface of a body of water
is partially reflected and partially transmitted. S waves, however,
do not penetrate the earth's outer core, and since it is known that
liquids cannot transmit S waves, this strongly suggests that the
outer core is made of liquid. It is also known that both P
waves and S waves travel more rapidly through material of high
density than through material of low density. Changes in wave
velocity have thus revealed that the earth increases in density
with depth and, further, that this increase is not gradual. The
passage of seismic waves from the rocks of the crust to the denser
rocks of the mantle, for example, is signalled by an abrupt
increase in velocity known as the Mohorovicic discontinuity, or
Moho, for short . Because continental crust is much thicker
than the crust beneath the oceans, the Moho dips downward beneath
the continents.
The rocks that form oceanic crust are the type
known as mafic—a label whose first three letters indicate
that these dark rocks are rich in magnesium (Mg) and iron (Fe).
Mafic rocks are much less common in continental crust than are
lighter coloured, less dense rocks that are described as
felsic—an adjective derived from the first three letters of
feldspar, which is the most common mineral of continental crust. In
comparison to mafic rocks, felsic rocks are rich in silicon and
aluminum and poor in the heavier elements magnesium and iron. Rocks
of the mantle are even richer in magnesium and iron than is the
oceanic crust—hence their great density—and they are
known as ultra-mafic rocks.
Continental surfaces not only stand above the
surface of the oceanic crust but also extend farther down into the
mantle than oceanic crust. It also shows that continental crust
beneath a mountain range extends even farther down into the mantle
than that located elsewhere. Isostatic adjustment, or the
upward or downward movement that keeps crust in gravitational
equilibrium as it floats on the mantle, is responsible for this
phenomenon. In effect, the root beneath a mountain acts to balance
the mountain.
Although the crust and the upper mantle are
separated by a difference in density, they are firmly attached to
one another, forming a rigid layer known as the lithosphere.
Below the lithosphere is the asthenosphere, which is also known as
the "low-velocity zone" of the mantle because it has been found
that seismic waves slow down as they pass through it. This property
tells us that the asthenosphere is composed of partially molten
rock—slushlike material consisting of solid particles with
liquid occupying the spaces in between. Although the asthenosphere
represents no more than 6 percent of the thickness of the mantle,
the mobility of this layer allows the overlying lithosphere to
move. The lithosphere does not move as a unit, however; instead it
is divided into plates that move in relation to one another. Some
plates carry continents with them as they move, while others carry
only oceanic crust.
Some plates, such as the one that includes the
continent of Asia, are enormous, while others, such as the one that
forms the floor of the Caribbean Sea, constitute only a minute
fraction of the skin of the earth. Plates move over the surface of
the earth about as rapidly as your fingernails grow. Slow as this
rate may seem, the progress of plates over millions of years has
been considerable. Many have moved about 500 or 1000 kilometres
(300 to 600 miles) in 10 million years. Since the early 1960s, it
has been recognized that many earth movements can be attributed to
the motions of plates. Movement of the edge of one plate over the
edge of another is in fact a major source of mountain
building.
Ocean barriers
In the seventy million years between the Lower Cretaceous and the
Eocene (one of the Epochs of the Tertiary Period) the world changed
from a single land-mass dominated by gymnosperms and reptiles to a
pattern of separating continents dominated by flowering plants and
mammals. Because the rise of the flowering plants took place in the
early Cretaceous, before that of the mammals in the Paleocene, the
effects of continental drift upon the distributions of these two
groups were rather different.
Continental drift facilitated the development of
separate, distinctive faunas and floras, not merely because of the
physical separation of the new continents by ocean barriers, but in
other ways also. The climates of land areas newly bordered by seas
became milder and less variable. Where new mountain ranges lay
across the path of the prevailing rain- bringing winds, new deserts
grew in their lee. Finally, as the continents continued northward,
their northern fringes reached such a high latitude that they
became covered by permanent ice-sheets. This may have been the
reason for the exaggeration and narrowing of the climatic zones; it
may also have led in turn to the great Ice Ages of the Pleistocene,
which wrought havoc upon the plant and animal life of the Northern
Hemisphere. It is, perhaps, no mere coincidence that both the
Permo-Carboniferous glaciation and the Pleistocene glaciations
occurred at times when a considerable area of land lay near to one
of the poles.
It is worth considering for a moment what
patterns of distribution we might expect to find had the continents
always had their present positions, so that the only changes would
then have been the relatively minor climatic variations of the
Northern Hemisphere Ice Ages, and changes in sea level making or
breaking the intercontinental Bering and Panama land bridges. At
times when the climate was warmer than it is today, the spread of
animals and plants across the Bering region between Siberia and
Alaska would have been possible. Similarly, in the absence of the
deserts of the Middle East, there would also have been a single
tropical fauna and flora stretching from West Africa to South- East
Asia. It would not be surprising, however, if the later development
of these deserts, dividing the tropical region into African and
Asian sections, had allowed distinctive features to appear in the
faunas and floras of each. Finally, it might have been expected
that the complete isolation of Australia and the almost complete
isolation of South America would have led to the development of
unique faunas and floras on these continents.
Though these features are clearly visible in the
accepted patterns of animal and plant distribution, other aspects
of these patterns are less simple to explain. Today's floral realms
are based upon the distribution of the flowering plants, or
angiosperms. Perhaps the most fundamental feature of angiosperm
distribution is the fact that, almost everywhere in the world, four
families are among the six most numerous —the Compositae,
Graminae, Leguminosae and Cyperaceae. Similarly, dicotyledonous
angiosperms are almost everywhere more abundant and diverse than
the monocotyledonous types. Even the floras of the isolated
continents, Australia and South America, though they certainly
display unusual features, are not basically unique, composed of
major groups found nowhere else.
Furthermore, if the spread of angiosperms through
the world had been through the pattern of continents we see today,
one would expect each of the three southern temperate regions
(Australia and the southern parts of South America and Africa) to
have a flora derived from that of its own adjacent tropical region.
This is not the case, as is shown most clearly by the contrast
between the floras of Australia and of New Guinea. Twelve of the 28
most dominant families of angiosperm in each region are completely
absent from the other region, and the Australian flora appears as
an intrusive element in an otherwise uniform southern Pacific
flora. In fact, rather than being related to the floras
neighbouring them to the north, the floras of the three south
temperate regions show considerable similarity to one another,
despite their separation by wide expanses of ocean. For example,
over 700 species of angiosperm are more or less entirely restricted
to two or three of these regions, and six families (the
Cunoniaceae, Escalloniaceae, Gunner-aceae, Philesiaceae, Proteaceae
and Restionaceae) are found in all three. Even more well defined is
the flora of the extreme cold temperate region south of 45°s,
which is composed largely of groups of angiosperm which are
scarcely, if at all, represented further north. This "Antarctic"
flora is characteristic of the extreme southern end of South
America, southeast Australia, Tasmania and New Zealand—it is
absent from South Africa, which extends only to 35°S. This
type of distribution has long puzzled biogeographers—see, for
example, Darlington's discussion of the distribution of the
southern beech, Nothofagus.
All these facts clearly suggest that the
angiosperms spread through the world at a time when the continents
had not yet split apart, so that a flora of fairly uniform
composition (at family level) spread everywhere. Part of this
uniform flora became adapted to the temperate climate of the
southern part of Gondwanaland. When this supercontinent split up,
the flora found itself on three separate continents and, though
some genera have since become extinct, many are still to be found
in more than one of these areas. This theory is compatible with
what is known of the sequence of events.
The angiosperms originated in the Jurassic, were
still a relatively insignificant part of the floras of the Lower
Cretaceous, and became dominant in the Mid-Cretaceous. Gondwanaland
is thought to have broken up during the Cretaceous and, though the
precise timing is still uncertain, it is clearly possible that the
angiosperms spread throughout that supercontinent before the
break-up—some types still characteristic of the south
temperate region today, such as the Proteaceae and the southern
beech, Nothofagus, had already appeared in Australia in the
Mid-Cretaceous. In Laurasia, two separate angiosperm floral realms
had already appeared by the Upper Cretaceous. One covered eastern
North America and the whole of Europe, the other extended
throughout Asia and western North America. The two were separated
by the shallow epicontinental Turgai Straits, which lay east of the
Urals and ran across Asia from north to south, and this region
still marks the separation between the European and the Asiatic
sub-regions of the Boreal floral region today.
Though it is not yet possible to trace in detail
the transition between the two, the basic characteristics of
today's floral realms are already discernible in the Cretaceous
floral distributions.