THE
HISTORY OF OUR EARTH
Our planet was formed along with
the Sun and other members of the solar system about 4,500 million years ago
from a cloud of collapsing dust and gas in space. As our Earth started to form
an enormous amount of heat was produced - a heat that remains within the Earth
today.
More gas and dust meant more mass
and gravity, and soon the heavier elements of this vast ball, such as iron &
nickel sank to the centre to form the super-dense core. The lighter elements
rose to the surface and made the Earth's first thin granite-like crust. This
crust was constantly being broken open by large cracks known as fissures, and
volcanoes which let out the trapped gasses below. This gave the Earth its first
atmosphere of Ammonia, Carbon dioxide and Water vapour.
As the planet continued to cool,
so the atmosphere began to condense and then the rains fell for thousands of
years. Earthquakes, lightning and ultra-violet light from the Sun changed these
molecules into those that could begin a life process. Living organisms existed
on the earth at least 3100 million years ago. These early plants absorbed the
carbon-dioxide in the atmosphere using the carbon to build its tissue and threw
away the oxygen. And it was this process of giving out oxygen that thankfully
gave us an atmosphere suitable for life. The oxygen created the ozone layer
shielding the surface from the sun's ultra-violet light, and enabled plants
and animals to move onto the land. The first land plants appeared in the late
Silurian period, just over 400 million years ago.
Earlier than this, land surfaces
were either completely bare or possibly covered with a thin scum of algae, like
the green film seen on tree trunks today. The first land animals must have lived
in the moist habitats around ponds and streams, where they could feed on decaying
plant material. A cousin of the millipede would have been such a creature. By
300 million years ago forests covered the swamps and lagoons of Britain and
North America which formed our coal deposits of today.
Amphibians (today's examples include
frogs, toads, newts and salamanders) made the transition from water to land
and were the creatures that would evolve into reptiles. The ability to lay a
hard-egg shell which could survive on land was the reptiles initial key to success.
It was during the Permian period, 280 to 245 million years ago, that the reptiles
began their domination of the land. By the end of the following Triassic period
they had paved the way for the most terrifying group of creatures to dominate
the world - the Dinosaurs.
There were three main geological
periods in which Dinosaur life evolved, known collectively as the Mesozoic Era.
This started approximately 245 million years ago with the warm and dry Triassic
period. At this time all the land on the Earth was joined as one enormous continent
called Pangea. Then followed the Jurassic period, a time when the land began
to split, and finally came the warm and wetter Cretaceous period. The land was
continuing to move, the continents and mountains we know today were forming
as the Earth's plates drew themselves apart or crunched into each other.
The Cretaceous period ended 65
million years ago, so, all in all the dinosaurs had a reign of around 160 million
years! As far as our reptile-type friends are concerned it's simple - if it
lived in the sea or flew in the air then it was not a dinosaur, for they lived
on the land! So, for example the Pteranodon was not a dinosaur.' Examples include
the Tyrannosaurus Rex, Stegosaurus, Brachiosaurus, Kentrosaurus, Triceratops,
Diplodocus, Deinonychus, and his brother the nasty Velociraptor. As with most
reptiles, some dinosaurs laid their eggs in a circle, similar to the way that
birds do in a nest. New evidence also suggests that some dinosaurs may have
given live birth and were warm blooded, so we haven't reached the end of the
story yet.
One puzzle many people want to
know is exactly how did the Dinosaurs come to an end? Some palaeontologists
say that there may have been some small type of creature that roamed the Earth
which ate dinosaur eggs. Others think it may have been due to disease that spread
throughout the world. Others, and the majority of them, justify that about 65
million years ago a large meteorite came in a smashed into the Earth with such
force that it threw up millions upon million of tons of dust which polluted
our atmosphere and blanked out the light and heat from the Sun for a few thousand
years or so. This being the case, the Earth would have turned very cold, the
plants, unable to use photosynthesis, would have died, and the animals with
no heat light or food would also have perished. Evidence of this disaster comes
in the form of a crater over 200 miles across which has been found in the southern
Caribbean. To be this size the meteorite that crashed must have been huge and
certainly capable of causing such devastation. But we still don't know for sure.
Once the dinosaurs had gone, little
mammals took advantage of their new opportunities. Also, because the world was
now split into continents much like today separated by vast seas, they could
evolve into many more different species. They grew larger and even more diverse
(look at today's kangaroo in Australia, and the South American Llama), and this
of course also applied to the plants..
Then, about 20 million years ago evolved a primate that was to be the ancestor
of both today's apes and man. The first real signs that man was on his way came
with the finding of jaw fragments dating back 14 million years. The reconstructed
features had a head like a chimpanzee, but with the arrangement of teeth more
like today's human. Human evolution was well under way. This must also have
been the time when our descendants began to walk on two legs. By one million
years ago they had evolved into beings that were undoubtedly true man.
THE VOLCANO
Picture of Mt Tiede in Tenerife from space.
A volcano forms where magma reaches the
Earth's surface. A volcano can be a lone peak or be one of a volcanic chain -
examples of these can be found in the Andes and the Indonesian islands; and chains
forming under the sea can create a string of islands like the Aleutians. The magma
under pressure inside the Earth contains varying amounts of gas and water. This
gas forces the magma to the surface through a vent where it becomes known as lava.
During an eruption the trapped gas is released, like opening a bottle of lemonade,
with differing results depending on the consistency of the lava.
When the lava is runny, the gas escapes easily, and the eruption is mild (these
types of volcano are generally found in the oceans where the Earth is splitting).
However, if it is thicker and more like a paste, the gasses explode out, and the
lava gets shattered into fragments (these types are usually found in mountain
chains where oceanic crust is sliding under a continental margin).
A volcano is fed by a single pipe and builds a cone with a summit crater. Eruptions
of runny basalt lava produce thin flows which spread over a wide area before hardening.
Thus a flattened volcano is formed, looking much like an upturned saucer, and
is known as a Shield volcano. Thicker lava produces a Strato-volcano which has
steeper sides because the lava cools faster once released. Initially the gas pressure
is high and activity is explosive. But, as the pressure is released, so activity
becomes milder and finally ends with lava flows. Such volcanoes are built of alternating
layers of lava and ash as each eruption adds its own material to the cone.
Volcanoes are normally active for short periods, a few months to a year or two
for each time. Between eruptions they remain dormant for longer periods of several
years to several hundred years. During this dormant time the lava in the crater
solidifies and plugs up the volcanic vent. Magma and gas pressure continually
builds until a new explosion opens the vent, and the volcano is active once more.
A Volcano is said to be extinct when it has not erupted in historic times. Sometimes,
however, volcanoes that were thought to have been extinct have erupted again,
like Vesuvius in AD 79. From the earliest recorded times Vesuvius had remained
dormant - the fertile soil of the cone was used to grow crops, and many people
lived on the mountain slopes. Earthquakes which took place between AD 63 and AD
79 were not recognised as a warning of the impending eruption, because people
were certain that Vesuvius was extinct. An enormous explosive eruption which shattered
much of the cone proved that it wasn't. A thick fall of ash buried the nearby
town of Pompeii and killed some people, although many had escaped. Nearby Herculaneum
was destroyed by an avalanche of volcanic ash turned to mud by torrential rain
from the volcanic cloud.
Other related areas of study include: parasitic cone, spatter cone, fissure eruption,
dyke, sill, batholith, pluton, igneous rock, lava tube, volcanic plug, caldera,
tuff, áá, pahoehoe, nuée ardente, pillow-lava, fumarole & geyser.
THE EARTH'S PLATES
We've seen what volcanoes are, but to understand where they
occur is important in understanding the workings of our planet Earth. Volcanoes
are found near the weakest points in the Earth's crust, namely along plate boundaries.
To explain: the surface of our planet is not like the peel of an orange - that's
all in one piece - it is made up of about fifteen pieces which fit very snugly
together. These pieces are known as plates, and they make up our world like an
enormous jigsaw puzzle. The major difference for our Earth puzzle is that all
of these plates are not 'locked', like the ones we put together from a box, they
are in a state of constant movement. This means that all the continents are changing
over time, some moving away from others, some moving closer. The term for this
is continental drift.
Effects of the moving plates to our planet over long periods of time do not stop
at simply making the Earth look different: mountains are built, earthquakes and
of course volcanoes occur, and all where these plates join. This again is because
these boundaries are the weakest parts of the Earth's surface. Now these plates
are rigid structures floating on fluid layers within our planet, and it is currents
within this liquid that actually move the plates and hence the continents on the
surface. What happens is that the high temperatures from the Earth's core heat
the surrounding layer, known as the mantle. As we know, liquid or gas that is
heated generally tries to rise, and that's exactly what happens in the mantle.
Convection currents are set up which carry the heat towards to surface, where
the material cools and sinks back down to be reheated. This swirling motion in
the mantel actually moves the plates (consisting of the upper mantle and the crust),
and causes all the other effects mentioned before. The fifteen or so plates that
form the Earth's puzzle - seven large and about eight smaller ones - meet along
one of four zones: Subduction, Collision, Spreading Ridges or Transform Faults.
Here is a brief description of each type:
Subduction Zone: When one plate moves towards another, generally a lighter ocean
plate (containing water) moving towards a lighter continental plate (containing
land). In this situation the ocean plate is forced to sink under the continental
plate, hence the name subduction zone. This is an ideal place for volcanoes to
form. Collision Zone: When two plates push together and neither gives way. In
this case mountain chains can form - the Himalayas were made by this process.
Spreading Ridges: Where two plates are moving apart and new crust is formed. A
good example is the Mid-Atlantic Ridge. Transform Faults: This is where plates
are moving past one another in a sliding fashion and no crust is destroyed or
created. Plates here tend to slip suddenly, in which case an earthquake occurs.
All of the plates also have names: for example, Britain sits on one of the seven
larger sections called the Eurasian Plate.
OCEANS & SEAS
It is not surprising that the Earth is called the blue planet
by the astronauts when they are up in space, nearly 70% of the Earth's surface
is covered in water, which we call oceans and seas. We call the large areas of
water, oceans (vast distances between major continents) and the seas are known
as the smaller areas around coasts and islands.
Just over 200 million years ago there was only one land mass called PANGAEA and
one big ocean called THE TETHYS SEA. However, things are quite different today,
because the Earth's crust is made up of plates, which are sliding on a molten
mantle, which in turn, over millions of years has broken up the one big land mass
into several individual pieces, very much like a jig-saw puzzle, and these are
known as continents. This left large areas of water which we call the oceans,
there are five in all: the PACIFIC, ATLANTIC, INDIAN, ARCTIC and SOUTHERN OCEAN.
(Southern or Antarctic is now unofficially accepted as an ocean). The Pacific
being by far the largest, covers about 166 million sq. km.(nearly one-third of
the Earth's surface). Most of the seas are actually part of the oceans, or are
joined on to them.
Rivers are formed by rain falling on higher ground, then being filtered as it
drains through the land and flows as fresh water into the seas. If a river is
dammed or locked, part of it will be fresh water and the sea flowing side will
be salt water and tidal, and therefore different types of marine and fish life
will inhabit and survive. Rivers that are dammed have flow controls at the locks,
but after exceptionally heavy rainfall, can burst their banks and even flood towns
and cities, hence the Thames Barrier was built to stop London flooding. Lakes
are very much like large ponds which are normally completely surrounded by land.
They are normally found in low lying areas where rain water drains down from the
mountains or hills on higher ground, some are also fed by small surface streams
or underground streams which are also topped up by rain water. Ponds are no more
than low dips in the ground that become water-logged and therefore retain rain
and surface water. If the summer is long and dry the still water pond can completely
dry up due to evaporation.
Salt-Water amounts to nearly 96% of all the water on Earth, only 4 per cent is
fresh, 3 per cent of that is in our polar ice caps, and 1 per cent is in rivers
and underground streams. The water obtains its salt when rain falls over land
and is filtered through the minerals which dissolve from rocks, it then flows
into the sea. If all the oceans' salt was taken out and spread over the land,
the Earth would have a layer of salt nearly 152m (500ft) thick. Places like the
Dead Sea have such a concentration of salt (ten times more than the average ocean)
that no fish can live there; hence the name Dead Sea. However, when salt water
freezes most of the salt stays at the bottom and the ice contains little or no
salt, the ice can then be melted and used as fresh drinking water, a method frequently
used by the Eskimos.
Ocean Waves are mainly caused by wind blowing across the surface of the water.
As much as the wind pushes the water upwards to form a wave crest, our gravity
pulls it back down again, into a wave trough.
The Sun and the Moons gravitational force also have an influence.Ocean Tides rise
and fall twice in every 24 hours & 50 minutes (two tides a day). Each tide comes
in about 25 minutes later each day and the reason for this is, the Moon (which
is mainly responsible for the tides) rises 50 minutes later each day. When we
have the Sun, the Moon and the Earth all lined up there is a tremendous gravitational
force on our Earth and it literally pulls our World out of shape, causing a bulge
like a large wave. This is called a high or spring tide (spring tide has nothing
to do with the season). As the earth spins, the wave travels around our planet,
causing a high tide.
DINOSAURS
ANCHISAURUS - Length: 2m (6½ft) and usually walked on all
fours during the Triassic period. It has been suggested that this creature ate
both meat and plants, and may have been an early dinosaur evolving from flesh-eating
ancestors into plant eating descendants. Fossils of the Anchisaurus were among
the very first dinosaur remains to be found in 1818, but nobody realised what
they were until much later.
APATOSAURUS - Length: 21m (70ft) or more and weighing about 30 tons, this was
one of the long-necked dinosaurs of the Jurassic period. Mistakenly, fossils from
what were thought to be two different dinosaurs were found and named Apatosaurus
and Brontosaurus. Later it was discovered these fossils came from the same animal
and so the famous Brontosaurus is now known as the Apatosaurus.
DEINONYCHUS - Length: 3m (10ft) and about 6 feet high, it lived during the early
Cretaceous period. This was a dinosaur that ran quickly on two strong and powerful
hind legs. They were also equipped with an effective frightening weapon - the
middle toe of both three-toed hind foot carried an enormous claw over 12 cm (5in)
long. It was this that gave the animal its name, which means 'terrible claw'.
DIPLODOCUS - Length: over 28m (90ft) and weighing over 11 tons, it was lighter
than many other dinosaurs half this length. The light weight was due to the very
long slender neck and the rather thin and slender body compared to other sauropods.
The Diplodocus lived during the Jurassic period.
IGUANODON - Length: 9m (30ft) and weighed about 5 tons. It probably walked on
all fours but was capable of running on just its hind legs. It was a herbivorous
dinosaur with many flattish teeth which were highly effective plant crushers.
STEGOSAURUS - Length: 9m (30ft) and weighing up to 4 tons, it was a plant-eater
lived during the Jurassic period. The most striking feature of this dinosaur was
the double row (or possibly only a single row as is now believed) of large diamond-shaped
plates that runs along its back, the largest of which was over 1m (3ft) in length.
TRICERATOPS - Length: 9m (30ft), up to 13 tons, and ate plants during the late
Cretaceous period. This is the most famous of the horned dinosaurs.
TYRANNOSAURUS REX - Length: 15m (50ft), standing 6m (20ft) high, and weighing
up to 10 tons, it was the largest and most powerful of the meat-eating dinosaurs.
The Tyrannosaurus lived during the Creataceous period and was one of the last
dinosaurs on the Earth. Its head was large and powerful, measuring over 1.3m (4ft)
in length with teeth that were sharp, serrated and up to 18cm (7in) long.
VELOCIRAPTOR - Length: 1.8m (6ft) and lived during the late Cretaceous period.
It ran on its hind legs, the front limbs being much shorter and bearing three
very sharp claws which it used to grip prey. Each hind foot contained a deadly
claw much like the Deinonychus, to which it is related. The Velociraptor probably
hunted in packs and so could tackle plant-eating dinosaurs much larger than itself.
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Copyright © 2001 Astrodome. All rights reserved.
Images courtesy of NASA.
THE
HISTORY OF OUR EARTH: Land & life