快猫短视频

FOR 600 million years, continents have broken up and drifted across the
globe, weird and wonderful creatures have come and gone, but the Earth has
maintained a calm equilibrium. This does not mean that the processes maintaining
the status quo are static鈥攆ar from it. Just take a look at the changes
happening all around us. A biting winter wind carries particles from nearby rock
or soil. A muddy river carries sediment towards the sea and waves crashing on a
beach grind pebbles to sand, or slowly eat away at cliffs each year. And on the
larger scale, volcanoes spew out lava adding to the volume of the continents and
creating new ocean floor each year. Ocean floor vanishes, too. And all the time
the plates that make up the surface of the Earth are moving a few centimetres
each year.

To reconcile the long-term stability of the planet that we can see in the
geological record with the short-term changes we see all around us is no easy
task. Just think about the different timescales, for a start. We can track
flotsam floating across the ocean and create a picture of ocean currents, we can
even track these currents across the globe from satellites, but how do we
combine such snapshots of ocean circulation with the waxing and waning of the
sea level over millions of years?

What gives our planet its distinctive character is the continual recycling of
our rocks. A process we can only understand if we study the underlying physical
processes, such as erosion and combine these studies with observations of the
Earth today, plus a look at the historical and geological records.

Think of the surface of the Earth now. Almost everywhere you look, the rocks
are continually eroding away. This is not a trivial process. At today鈥檚 rate,
half the exposed sediments in the world will be destroyed in just 100 million
years. But that鈥檚 a mere blink of an eye in geological terms. Wind and water
play their part in the physical wear and tear, but chemical effects speed the
disintegration of solid rock. Slightly acid rainwater dissolves limestone by
breaking the bonds between its calcium and carbonate ions. And other natural
fluids can rapidly break down minerals within rocks.

Erosion dominates in high mountain areas, helped by gravity and often
assisted by frost. Erosion enhances cracks and joints in supposedly solid rock
at the surface of the Earth. Far underground, the rocks are under great pressure
from the weight of rocks above them and the confining pressure of the rocks
around them. But as they edge closer to the surface as a result of uplift and
wear and tear on the rocks above, the stress in the rock decreases and changes,
often becoming weaker in one direction than in others. This creates the
fractures where weathering often begins.

The joints and other lines of weakness in the rock trap water that freezes,
expands and enlarges the cracks. Rock falls shatter large pieces of rock into
smaller fragments, mountain streams smooth and smash boulders into pebbles and
the mountains slowly wear away
(Figure 2).

Physical erosion, though, is only part of the story. The dramatic erosion of
limestone to form deep caverns, gullies and caves, a landscape known as karst,
is almost entirely chemical. The process picks up speed as channels in the rock
concentrate the water flow. In other types of rock, too, chemical erosion speeds
up the growth of cracks and hastens the disintegration of individual mineral
grains. Feldspars, for example, can degenerate into clays. Although quartz is
the main physical component of rocks to survive into the sea, much of the
complex chemical make-up of rocks also gets there.

Once in the sea, waves and currents carry the sediment out onto the shallow
sandy shelves beyond the shore zone, the so-called continental shelves. Like
avalanches down mountain sides sediment tumbles off the shelves and slides down
valleys or canyons, onto the deep ocean floor. One important form of 鈥渁valanche鈥�
is a turbidity current, in which a mixture of sediment and water moves as a
turbulent flow: it is denser than the surrounding seawater and can travel long
distances close to the ocean floor. Such currents carry sandy sediment
considerable distances, one of the few supplies of relatively coarse material
into the quiet world of the deep ocean.

Ancient landscapes

Ripples in the sands

SEDIMENT is accumulating on the deep ocean floor all the time, in a modest
and slow way. The many tiny plants and animals that float in the seas constantly
rain down fine particles, mainly carbonates and silicates, from their excrement
or skeletons. Layers of sediment buildup on top of the basalt of the ocean
floor. Carbonates are precipitated in the shallower parts of the deep seafloor,
where they dominate. But where the seas are especially deep, the pressure of the
water above means that carbonate minerals dissolve in water and no longer
accumulate as sediment. Below this level, the so called carbonate compensation
depth, even existing carbonate mud becomes soluble. What is left is the
siliceous debris, which forms deposits of chert, a flinty, often layered
material.

Although these deep-sea sediments accumulate over a large area, they don鈥檛
often appear in the rock record because they are relatively thin. As we shall
see, when tectonic plates run into each other these sediments are mostly carried
down beneath the plates. This tends to happen within a few hundred million
years.

More common are the sediments of the shallow seas and continental shelves,
with deltas, estuaries and large rivers. They leave behind patterns called
sedimentary structures which are distinctive enough to build up a picture of the
land surface at different times in the past. The basis for understanding
sedimentary structures is fluid dynamics: water and wind carry particles of
sediment in different ways depending on their speed and turbulence. Ripples on
the beach, sand dunes and the shapes of deltas are all a result of different
sorts of flow.

Imagine water flowing over a sandy bed, perhaps a stream on a sandy beach.
The sand grains jump and bounce (saltation) along with the current. A slightly
faster current picks up bigger grains and all the particles are carried forward,
until some obstacle intervenes. An irregularity in the bed of the stream creates
an eddy in the flow and a small whirlpool develops. The water flow reverses and
slows immediately downstream of the bump. Because the water is flowing more
slowly some of the particles are now too heavy to be carried along, so they fall
to the floor. Sediment continues to build up on the upstream face of the bump,
which grows in the direction of the current. Such small irregularities are the
beginnings of ripples in the sand.

The size and shape of the ripples depends on the currents that create them.
Ripples formed in a current flowing in one direction, such as a stream, have a
gently sloping upstream face and a steep downstream face. Ripples on a beach
form when the water washes back and forth. They are symmetrical and have a less
ordered internal structure. Finding symmetrical ripples in rocks tells a
geologist that they formed on a beach or in shallow coastal waters washed by
tides. Symmetrical ripples also give clues to the size of the sea in which they
formed: the bigger the ripples, the greater the sea swell.

When rivers flow faster, the range of sedimentary structures also increases:
sand dunes and sand waves form in similar ways to ripples, for example. Flow
speed is not the only variable: the sizes and types of sediment in the river
also play their parts. In many lowland rivers in Britain, there is little sand
and the sediment is predominantly mud. In many circumstances, mud is cohesive
enough to retain the marks made in it by, for example, pebbles bouncing along
the riverbed. These are called tool marks, and if you find several of them
together in a rock you can tell the direction the river flowed. Rocks on the
riverbed can also disrupt the water flow and produce eddies, just as in a sandy
stream. But if there is no sand to deposit, the eddies scour depressions in the
river-bed; these flute marks, like tool marks, show the direction the river
flowed.

Together, these observations of sedimentary structures make it possible
for geologists to build up a map of the Earth鈥檚 surface. And because the rocks
preserve a sequence of structures through time, they can trace the waxing and
waning of rivers and oceans over millions of years.

The same pattern of sediment accumulation, burial, then uplift and erosion,
happens across the world. This is why it is common to find fossils of sea
creatures in rocks from the Pennine landscape of England to the top of Mount
Everest. They formed under the sea, were buried and became rocks, then rose to
the surface, where they are now eroding. The pattern ties in with the
large-scale landscape of ocean basins and continents. So it makes sense to look
to the framework of the crustal plates to understand the shifting structures of
the Earth鈥檚 surface. These plates are vast sheets of the outer rigid layer of
the Earth. They float on the hotter, denser layers below, and constantly shift
relative to each other.

Plate movements are a useful framework for understanding these cycles,
because they can be measured now and traced back through geological history. The
key to this historical jigsaw is the continual formation of the ocean floor.
Ranges of volcanic mountains where the Earth鈥檚 crust splits, or rifts apart,
form the spines of the ocean floors. Here volcanoes steadily erupt lava, perhaps
a third of a cubic kilometre a year along the Mid-Atlantic Ridge alone. It is
like a conveyor belt, always moving from the ridges to the shores of the oceans,
but a conveyor belt in which the belt itself is formed at the central point. As
the rocks solidify, iron-rich minerals in them acquire magnetism that matches
the Earth鈥檚 magnetic field at the place and time where they form. The movement
of the conveyor belt means that the rocks act like a tape recorder as time goes
by: when the magnetic field reverses, so that north becomes south, as it has
done many times in the past, the magnetism recorded in the rocks changes too
(see Inside Science No. 26).

Rocks on the move

Plate tectonics

SO the oceans of the world have 鈥渟tripes鈥� in their rocks鈥攊n some the
polarity of the magnetic field is normal and in others it is reversed. The
longer the period of normal or reversed polarity, the wider the stripe on the
ocean floor. And because the reversals do not take place at regular intervals, a
distinctive pattern is left which lets us run our imaginary conveyor belt back
to discover the shapes and sizes of the oceans in the past. If you know where
the oceans are, you can also reconstruct the positions of the continents.

The pattern of plate movement back through the years reveals cycles in which
the continents reassemble to form one huge landmass, Gondwana or Pangaea, for
example, then rift and separate again
(Figure 4). This separation in itself has
major consequences for the volume of the oceans. More mid-ocean ridges in the
oceans take up more space, displacing the water onto the continental shelves and
the land, creating what we see in the geological record as a high sea level. And
when there are many small continents, the shallow seas around them take up a
proportionately greater area than when all the continents combine into one
supercontinent.

But, such major changes in the configuration of the continents don鈥檛 just
affect the sea. The distribution and position of land masses play a huge part in
the climate, affecting air circulation and rainfall.

Plate tectonics is one of the main mechanisms for recycling rocks through
time. Although the floor of today鈥檚 oceans is 200 million years old, we have
fossils of sea creatures from long before this. Indeed the Earth and the oceans
themselves are hundreds of millions of years older. How can this be? Although
the ocean floor has been forming for 2500 million years it is also being
destroyed. We know the ocean floor is being recycled because there is no sign
that the Earth has had to expand its radius to accommodate the enlarging ocean
floor at the mid-ocean rifts.

So where has all the ocean crust gone? Back to where it came from. Some
continents ride across the face of the Earth with the oceans, slowly getting
further from the central ridge where the crust forms. This is what is happening
to Europe and the Americas, on each side of the Atlantic Ocean. But other
continental edges seem to swallow the ocean crust as it grows, riding over the
moving edge of the ocean floor.

Continental crust is made of rocks that are much less dense than ocean crust
and so they float higher on the hotter, denser layer below. As a consequence,
when the spreading ocean floor and the continental crust collide, the denser
ocean crust slides beneath the continental one into the mantle below
(Figure 1).
The region where the ocean crust nudges under the edge of the continent is
called a subduction zone. This is what is happening on the Pacific coast of
South America (see Inside Science No. 96). You can trace the path of the
subduction zone by mapping earthquakes that begin under the Andes. The ocean
crust is cool when it starts to descend into the hotter mantle. As it moves and
bends, it deforms in a series of short sharp shocks, as if it is cracking; this
is what we see as earthquakes, and they mark a dipping zone called a Benioff
Zone. In some cases these zones reach down 500 to 700 kilometres into the crust.
The sense of movement on these faults shows that they generally start from the
slab sliding downwards. The earthquakes continue until the slab warms up enough
to flow rather than crack. So a fast-moving slab will descend deeper than a
slow-moving one. In some cases, the sense of movement shown by deep earthquakes
indicates vertical stretching鈥攁s if the bottom of the slab were dropping
off.

Metamorphic changes

Signs of deep stress

GEOLOGISTS use seismic tomography to make images of the descending slabs. It
depends on the seismic properties of the slabs differing from those of the
surrounding mantle. It works in much the same way as medical tomography but
depends on seismic waves from earthquakes rather than X-rays. After the waves
have passed through the area studied, they are picked up by an array of
receivers. A computer processes the signals, building up a picture of the slabs
descending into the mantle.

Parts of the slabs seem to survive as far down as the boundary between the
core and the mantle. The layer is known as the D鈥� boundary, and has been called
a slab graveyard. As to the slabs, are they assimilated into the deep mantle or
circulate as discrete chunks? We don鈥檛 yet know.

What is certain is that not all the material that dives down into a
subduction zone goes all the way down. Instead some of it is scraped back into
overlapping sheets on the seafloor
(inset Figure 1). Some of these wedges of
sediment build up to form islands: parts of Japan began in this way.FIG-mg21178701.JPG

The sediments that do descend are among the first parts of the slab to
respond to the increasing temperature. Volatile materials such as water and
carbon dioxide respond to the increasing heat by moving upwards into the mantle
rocks. Here they change the composition of these rocks enough to make the mantle
melt. And magma or molten rock moves upwards and eventually erupts from
volcanoes. Japan鈥檚 volcanoes are the result of this mechanism, the simplest and
most obvious recycling of rocks from sediment to volcano.

There is also a more subtle recycling going on around subduction zones. At
these boundaries rocks are being transformed from one type to another by heat or
pressure or both. So metamorphism is a sure sign that rocks have been buried to
great depths and returned to the surface. In many cases, the combinations of
minerals that are stable in different conditions of temperature and pressure can
be deduced, either from experiments or from theoretical calculations.
Metamorphic rocks provide evidence of otherwise hidden movements deep in the
Earth. Certain rocks in the Alps, for example, contain coesite, a type of quartz
that forms only under very high pressure.

Metamorphic rocks around the world began life either as igneous rocks, which
crystallised from molten rocks, or as sedimentary rocks. Rocks that were once
deeply buried are uplifted to form the raw material for sediments once again.
There is a continual recycling of rocks from the surface of the Earth into its
depths.

So the movement of the plates seems to be driving all the changes we see at
the surface: they control where new rocks form, and where the ocean floor is
swallowed up. And, on an even longer timescale, plate collisions drive
metamorphism and the uplift of mountains. But what drives the plates in the
first place?

The Earth is a thermally and dynamically active planet,
internally as well as externally. The layered structure of the Earth and the
steady movements of the plates are a result of the continual cooling. The plates
move as a result of huge convection currents, stirring the mantle and gradually
bringing heat from the interior of the planet to the surface. The layering of
the Earth insulates the core from the surface, but even so, the planet is
cooling. The outer core of the Earth is liquid and the inner core is solid: iron
is solidifying on the inner core at the rate of a million kilograms of iron per
second. This sounds like a lot, but in fact the core is solidifying by less than
a tenth of one per cent of its size every million years. Even at this slow rate,
it will be completely solid in 10 000 million years.

When this happens, the whole engine that drives the Earth will grind to a
halt. There will be no more heating from below to drive convection in the
mantle. Plates will slow and stop. Volcanoes will all become extinct.
Earthquakes will continue for longer as the crust cools and becomes thicker. But
the Earth will no longer be a hospitable planet when the core solidifies
totally. For its magnetic field will decay and the magnetosphere, the area
around the Earth where our magnetic field dominates the Sun鈥檚, will have gone.
We will have no protection from radiation and the charged particles of the solar
wind.

But this is the least of our worries. The Sun will probably have changed
beyond all recognition in 10 000 million years鈥� it is due to start burning
helium instead of hydrogen in about 5000 million years. When that happens our
star will expand into a red giant, engulfing the Earth鈥攔ocks, plates,
core, us and all.

* * *

Water transport and graded deposition

FRAGMENTS of material are carried away from the mountains by the streams and
gradually worn down into sediment as they move down the watercourse: only the
finest particles reach the sea. Rivers carry this sediment by bouncing particles
along their beds, saltation, or in suspension. The faster the river flows, the
more material can be carried.

Different minerals from the original rocks respond differently to the
hurly-burly of the riverbed. Quartz and feldspar are the two most common
minerals in continental rocks. While quartz is made of silicon and oxygen,
feldspar also has aluminium and calcium, potassium and/or sodium, and both have
crystal structures classified as frameworks.

But this is where the similarity ends. In rivers, quartz wears into rounded
pebbles, then to rounded grains that survive in the sediment to reach the sea.
But because they have three strong planes of cleavage, feldspar crystals break
easily into fragments shaped roughly like bricks. The chunks soon break down
into smaller pieces and then into very fine particles and clay which does not
travel far in rivers.

In fact, most of the sediment in the sea around the edges of the continents
is quartz. And if you go to a beach you will find that much of it is made of
those same rounded quartz grains. There will be shell fragments and particles of
the nearby cliffs too, but much of the material that reaches the sea has
survived from erosion of rocks far inland.

* * *

Changing shapes of deltaic structures

Another key to reconstructing the past world is the arrangement of sediment
on a larger scale than individual layers. This often involves building up a
picture of the landscape at different times as the rocks formed. Rivers carry
and deposit different types of sediment at different stages of their journey
from mountains to the sea. Conditions in the sea are a big change from the
river. For a start, there is no longer a channel in which to flow: the water and
its sediment spreads out and the flow slows down. As it slows, the sediment it
carries drops to the seafloor and a delta begins to form where the river meets
the sea. Once a wedge-shaped mass of sediment has built up on the seafloor, the
delta develops its own geography, depending on the balance between the amount of
sediment, how fast the current is flowing in the river and the effects of waves
and tides carrying material away.

Most deltas exist in a delicate balance between the sediment deposited by the
river and erosion by waves and tides. Often these combine to produce lines of
barrier islands offshore, as at Venice. Big deltas are heavy enough to sink
under the weight of the sediment: regular seasonal floods from the rivers
deposit enough sediment on the top of the delta to maintain the balance. But
this balance is easily disrupted. A regular influx of sediment on the lands
around a delta make them fertile and valuable for farming. They remain valuable
as long as the flooding continues. Attempts to reduce the flooding鈥攐ften
to save lives, when many people live on the delta lands鈥攎akes the land
less fertile. And without regular deposits of sediment the land gradually
sinks.

The changing shapes of deltas can be seen maps of the surface of the Earth
and in the record of the rocks below ground which show the history of a delta,
followed by its abandonment and subsidence.