Download Changes to the Earth`s rocks and atmosphere

Changes to the Earth’s
rocks and atmosphere
Course Notes
Chemistry C1
Section 6 2013-15
Inside the Earth
The Earth has a layered structure. Large-scale
movements of the Earth’s crust can cause changes in
the surface rocks.
The Earth consists of an inner and outer core, then a
mantle and finally a thin crust. The Earth’s crust and
the upper part of the mantle are cracked into a
number of large pieces called tectonic plates.
Convection currents within the Earth’s mantle,
driven by heat released by natural radioactive
processes there, cause these plates to move at speeds
of a few centimetres per year.
Earthquakes and volcanoes and
occur at the boundaries between
tectonic plates. These movements
can be sudden and disastrous.
Old ideas - bad science
Scientists once thought features
of the Earth’s surface happened
because the crust was shrinking
as the Earth cooled down.
Evolved atmosphere
Start
here
The Earth forms
from dust and gas
spiralling inwards.
Planet is rocky &
rather small
First single-celled plant-like
organisms live in deepish
water to avoid ultra-violet
rays - photosynthesis begins!
Huge quantities of CO2 are
turned into oxygen, O2 gas.
Plant-like organisms are the
first polluters!
First simple animals
evolve in water feeding on simple
plants - dead
organisms move
carbon from air to
rocks
Hydrogen and
helium gases escape
because Earth is not
massive enough
(lower gravity than
a gas giant planet)
Atmospheric oxygen builds
up. Ozone, O3, forms in the
stratosphere. Ozone shields
land from UV rays. With an
ozone shield, life can survive
on land!!
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Carbon dioxide and gases
blown out of volcanoes form
the first atmosphere
Steam, methane, ammonia
join CO2 - comets bring more
water to the Earth as the
molten surface solidifies.
Oceans form.
Steady state - atmosphere in balance
Atmosphere reaches a steady state
thanks to plants and animals.
Most of the high levels of the early
carbon dioxide atmosphere is now
locked up in rocks (limestone; chalk;
fossil fuels … ). Plants turn water and
carbon dioxide into sugar and oxygen.
Life is a powerful force controlling the
composition of the Earth’s atmosphere,
which in turn exerts a powerful control
on our planet’s climate!
Plate tectonics and sea-floor spreading
Alfred Wegener
He proposed an idea that the Earth’s continents were once
joined together. Over a few hundred million years, forces made
this supercontinent break up and drift apart. This was his
continental drift theory.
He offered this evidence:
 the shapes of Africa and South America look as though
they once joined together
 African and South American rocks which are 200 million
years old contain identical fossils and sequences of fossils
 rocks on the edges of these continents are similar where
they might once have joined.
New evidence supports Wegener’s theory
New evidence from exploring the ocean floor
rocks, including the magnetic striping patterns
of the mid-ocean rocks, gives us the idea that it is
the radioactivity from uranium and thorium that
provides the enormous heat source that creates
the convection currents that drive the tectonic
plates apart.
Under the Atlantic ocean, tectonic activity along
the mid-Atlantic ridge drives the plates apart.
This is called sea-floor spreading.
The new evidence proved that Alfred Wegener
was correct. The mechanism for Wegener’s
theory of crustal movement (continental drift) is
now generally accepted. Continents drift because
of sea-floor spreading.
Scientists cannot accurately predict when
earthquakes and volcanic eruptions will occur,
because Earthquakes are caused by sudden
movements of tectonic plates and volcanic
eruptions require sudden changes in pressure
deep underground.
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If recent Earthquakes and
volcanic eruptions are
plotted on a World map,
then a striking pattern
emerges.
The pattern exposes the
positions of the plate
boundaries.
These are parts of the
World where Earthquakes
and volcanic activity have
happened in the past , but
we can only wonder where
the next ‘quake will happen.
The Earth’s evolving atmosphere
For 200 million years, the proportions
of different gases in the atmosphere
have been much the same as they are
today:

about four-fifths (78%) nitrogen;

about one-fifth (21%) oxygen.

small proportions of various
other gases, including carbon
dioxide, water vapour and noble
gases.
During the first billion years of the Earth’s existence there
was intense volcanic activity. This activity released the
gases (called outgassing) that formed the early
atmosphere and water vapour that condensed to form the
oceans.
There are several theories about how the atmosphere was
formed. One theory suggests that during this period the
Earth’s atmosphere was mainly carbon dioxide and there
would have been little or no oxygen gas (like the
atmospheres of Mars and Venus today). There may also
have been water vapour and small proportions of
methane and ammonia.
There are many theories as to how life was formed
billions of years ago. One theory as to how life was
formed involves the interaction between hydrocarbons,
ammonia and lightning (the primordial soup theory).
This theory is supported by laboratory experiments done
by Miller and Urey.
Plants and algae produced the oxygen that is now in the
atmosphere (photosynthesis). This process takes out
carbon dioxide from the atmosphere. Respiration and
decay of organic material releases carbon dioxide into the
atmosphere.
Most of the carbon from the carbon dioxide in the air
gradually became locked up in sedimentary rocks as
carbonates and fossil fuels.
The oceans also act as a reservoir for carbon dioxide but
increased amounts of carbon dioxide absorbed by the
oceans has an impact on the marine environment.
Nowadays the release of carbon dioxide by burning fossil
fuels increases the level of carbon dioxide in the
atmosphere. Carbon dioxide is a greenhouse gas.
Air is a mixture of gases with different boiling points and
can be fractionally distilled to provide a source of raw
materials used in a variety of industrial processes.
Remember oxygen has a higher boiling point (–183℃)
than nitrogen (–196℃) because the more negative the
number, the lower is the boiling point.
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