Download Document

______________________________________________________________________________
PART B: THE PHYSICAL ENVIRONMENT
1. THE LITHOSPHERE
1.1 The Solar System
Space is made up of a vacuum which is interspersed with collections of planets, planetoids and
many other particles, collectively known as a galaxy. The galaxy to which Earth beolngs is the
Milky Way, whilst two others are the Alpha Centauri system and the 51 Pegasi system. Each
galaxy is made up of a number of solar systems, that is, a collection of planets surrounding a
central sun. The Sun and the group of celestial objects gravitationally bound to it (in the case of
the Earth’s solar system: nine planets and their 165 moons, as well as asteroids, meteoroids,
planetoids, comets, and interplanetary dust) together make a solar system.
The principal component of the solar system is the Sun that contains 99.86% of the system's
known mass and dominates it gravitationally. It creates the largest gravitational force (a pulling
force which causes gravity (the gravitational force is larger for larger objects and thus planetoids
like the moon have no gravity because they are relatively light). Because of its large mass, the
Sun has an interior density high enough to sustain nuclear fusion, releasing enormous amounts
of energy, most of which is radiated into space in the form of electromagnetic radiation, including
visible light. The Sun's two largest orbiting bodies, Jupiter and Saturn, together account for more
than 90% of the system's remaining mass.
The major planets are, in order of distance from the Sun, Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, Neptune, and Pluto (Most Volcanoes Erupt Mulberry Jam Strings Under Normal
Pressure). All planets but two are in turn orbited by natural satellites (usually termed "moons"
after Earth's Moon) and the largest (Jupiter and sturn) are encircled by planetary rings of dust
and other particles. The planets (with the exception of Earth) are named after gods and
goddesses from Greco-Roman mythology.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.2 Earth Structure
The earth is made up of several layers namely the crust, mantle and core. The crust is the thin
(30 to 100km at max) solid outer layer and beneath it there is the mantle, (which make sup 70%
of the structure), that
is the thick shell of
rock. Directly beneath
is the core. The
mantle is made up of
three layers that is
the
asthenosphere,
the outer mantle and
the inner mantle (30 2900km) , whilst the
core is made up of
the outer (2900- 4100
km) and inner core
(up to 6370km).
The asthenosphere
extend to 250km and
is different from other regions of the mantle since it is a liquid and is affected by seismic waves.
Note that the crust and the outer
mantle
together
form
the
lithosphere, which is a solid. Also
in general the crust is thicker
where
land
is
present
(continental crust; 100km) and
less dense, whereas where the
sea is present the crust is thinner
(oceanic crust; 30km). and more
dense (thus heavier).
This general structure is also similar in other planets.
The mantle differs substantially from the crust in its structure and composition (the chemicals
making it up). In fact, the distinction between the crust and mantle is based on chemistry, rock
types, and seismic characteristics. Typical mantle rocks have a higher portion of iron and
magnesium, and a smaller portion of silicon and aluminium than the crust. In addition, the
mantle differs from the core both in a physical and chemical manner.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
In the crust, temperatures are relatively low. This is the coolest part of the earth’s physical
structure. On the other hand mantle temperatures range between 100°C at the upper boundary
to over 4,000°C at the boundary with the core. Although these temperatures far exceed the
melting points of the mantle rocks, these rocks are solid (or semi solid), particularly in deeper
ranges. This is since although temperature increases from the outer to the inner mantle, the
pressure also increases. Therefore, while temperature should cause the rocks to melt and
become a liquid, the pressure will keep them compact and thus they remain a solid. The
enormous pressure caused by the overlying thick rocks is called the lithostatic pressure and
thus prevents the mantle from melting
The core is even hotter than the mantle. Hwoever, lithostatic pressure also causes it to be a
solid, as in the case of the mantle.
1.2 Plate Tectonic
The earth’s crust is rather thin and divided into several pieces called plates. The diagram below
shows the different plates which occur on earth. There are ten major and many minor plates.
The red arrows indicate the direction in which these plates move. (Note that a plate is normally
made up of both oceanic and continental crust)
It is well known that these plates move with time. In fact, the landmass on earth was once a
single continent called Pangaea rather than several continents. Pangaea eventually separated
over millions of years, as the plates floated on the asthenosphere, to give the present
continents. The study of this movement of plates is known as Plate tectonics (from Greek
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
tektōn "builder" or "mason"). This is a theory of geology developed to explain the phenomenon
of continental drift (movement).
Plate tectonics states that the cooler and more solid surface parts of the Earth's rock crust move
slowly over time across the hotter, liquid, underlying asthenosphere. In essence, the lithosphere
essentially floats on the asthenosphere. This movement occurs due to a temperature difference
between the Earth's crust and outer core, which causes a convective circulation in the mantle.
Due to the convective circulation, hot material ascends (goes up) from the border with the outer
core to the astenosphere since it is light, while cooler (and therefore heavier) material descends
(sinks) from the astenosphere downward to the outercore. Due top the convective currents, and
the consequent movement of the mantle, the tectonic plates (both continental and oceanic) are
caused to move millimetirically over years.
The convection currents of the Earth's mantle are a chaotic process, and thus movement of
plates is also rather chaotic.
1.2.1 Plate Boundaries
Plates move in relation to one another causing continents to change position (continental drift).
The movement of two plates at their boundary (called the plate boundary) in relation to each
other can occur in one of three ways that is: convergent, divergent, and transform. Earthquakes,
volcanic activity, mountain-building, sea bed spreading and oceanic trench formation occur
along plate boundaries.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
Divergent (constructive) boundaries
At divergent boundaries, two plates move apart from each other and the space that this creates
is filled with new crustal material which is formed from molten magma that rises from the
asthenosphere below. Divergent boundaries between oceanic plates form rifts such as of the
oceanic ridge system, including the Mid-Atlantic Ridge, which underlies Iceland and is widening
at a rate of a few centimeters per century. Therefore, these cause sea floor spreading (increase
in the sea floor). In the continental lithosphere rift valleys such as the famous East African Great
Rift Valley are formed. The Mid-Atlantic Ridge system
Divergent boundaries can create massive fault zones in the oceanic ridge system. They can
also cause seaquakes with resultant negative effects.
Convergent (destructive) boundaries
Convergent boundaries occur when two plates move towards each other and collide. The type
of convergent boundary depends on the type of lithospheres that are colliding. Where a dense
(heavier) oceanic plate collides with a less-dense (lighter) continental plate, the oceanic plate is
typically thrust underneath, forming a subduction zone. At the surface, the topography (form of
the land) is commonly a deep oceanic trench on the ocean side and a mountain range on the
continental side. Long chains of volcanoes form inland from the continental shelf and parallel to
it.
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. 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 compress into each other or one plate
burrows under or 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.
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 plate.
Good examples of this type of plate convergence would be Japan and the Aleutian Islands in
Alaska.
Transform (conservative) boundaries
The left- or right-lateral motion of one plate against another along a long transform faults can
cause highly visible surface effects. Because of friction, the plates cannot simply glide past each
other. Rather, stress builds up in both plates and when it reaches a certain level level the
accumulated potential energy is released as strain, or motion along the fault. The massive
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
amounts of energy that are released are the cause of earthquakes, a common phenomenon
along transform boundaries.
A good example of this type of plate boundary is the San Andreas Fault complex, 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.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.3 Earthquakes
An earthquake is a phenomenon that results from and is powered by the sudden release of
stored energy that radiates seismic waves. At the Earth's surface, earthquakes may manifest
themselves by a shaking of the ground and sometimes tsunamis, which may lead to loss of life
and destruction of property.
Earthquakes may occur naturally or as a result of human activities. In its most generic sense,
the word earthquake is used to describe any seismic event—whether a natural phenomenon or
an event caused by humans—that generates seismic waves
1.3.1 Naturally occurring earthquakes
Most naturally occurring earthquakes are related to the tectonic nature of the Earth. Such
earthquakes are called tectonic earthquakes. These are caused by the slow but constant
motion of the plates, resulting in plate boundaries gliding past each other thus creating frictional
stress. The stress builds up and when it exceeds a critical value, called local strength, a
sudden failure (yielding) of the crusto occurs. When the failure results in a violent displacement
of the Earth's crust, the energy is released suddenly from apoint and seismic waves are radiated
to the suface, thus causing an earthquake.
It is estimated that only 10 percent or less of an earthquake's total energy is ultimately radiated
as seismic energy, while most of the earthquake's energy is eventually converted into heat.
The majority of tectonic earthquakes originate at depths not exceeding a few tens of kilometers,
that is within the lithosphere. Earthquakes occurring at boundaries of tectonic plates are called
interplate earthquakes, while the less frequent events that occur in the interior of the
lithospheric plates are called intraplate earthquakes.
The energy of an earthquake originates from a particular point beneath the crust. This is the
point were apparently all seismic waves are originating and is called the focus. The location on
the surface directly above the focus is known as the epicentre, and is tha rea where most
damage is likely to occur. The the area below the focus is called the hypocentre.
Where the crust is thin the focus would be shallow and thus any earhtquakes would produce
more damage than when the focus is at greater depths of hundreds of kilometers (deep focus
earthquakes). However, the damage they produce is a direct result of the energy that is built up
and released. Earthquakes may also occur in volcanic regions and are caused by the movement
of magma in volcanoes. Such quakes can be an early warning of volcanic eruptions.
Large earthquakes can cause serious destruction and massive loss of life through a variety of
agents of damage, including fault rupture, vibratory ground motion (shaking), inundation
(tsunami, seiche, or dam failure), various kinds of permanent ground failure (liquefaction,
landslides), and fire or a release of hazardous materials e.g gas leaks or petrol leaks. Most large
earthquakes are accompanied by other, smaller ones that can occur either before or after the
main shock; these are called foreshocks and aftershocks, respectively. Aftershocks can be
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
felt from half way round the world so in England you could feel an aftershock from New Zealand.
While almost all earthquakes have aftershocks, foreshocks occur in only about 10% of events.
The power of an earthquake is always distributed over a significant area, but in large
earthquakes, it can even spread over the entire planet.
Earthquakes that occur below sea level and have large vertical displacements can give rise to
tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake or as
a result of submarine landslides directly or indirectly triggered by the quake.
1.3.2 Severity
The severity of an earthquake is described by both magnitude and intensity. These two
frequently-confused terms both refer to different, but related, observations. Magnitude, usually
expressed as an Arabic numeral, characterizes the size of an earthquake by measuring
indirectly the energy released. It is measured using the Richter Magnitude scale which ranges
from 1 (low magnitude) to 10 (highmagnitude). By contrast, intensity indicates the local effects
and potential for damage produced by an earthquake on the Earth's surface as it affects
humans, animals, structures, and natural objects such as bodies of water. Intensities are usually
expressed in roman numerals, each representing the severity of the shaking resulting from an
earthquake. It is measured using the Mercalli scale.
Charles Richter, the creator of the Richter magnitude scale, distinguished intensity and
magnitude as follows: "I like to use the analogy with radio transmissions. It applies in seismology
because seismographs, or the receivers, record the waves of elastic disturbance, or radio
waves, that are radiated from the earthquake source, or the broadcasting station. Magnitude can
be compared to the power output in kilowatts of a broadcasting station. Local intensity on the
Mercalli scale is then comparable to the signal strength on a receiver at a given locality; in
effect, the quality of the signal. Intensity, like signal strength, will generally fall off with distance
from the source, although it also depends on the local conditions and the pathway from the
source to the point."
Therefore, an earthquake may have a large magnitude (such as 7 on the Richter scale) but have
a low intensity (since it is deep focus). However, shallow earthquakes which have a large
magnitude also have a large intensity. In addition, any given earthquake can be described by
only one magnitude, but many intensities since the earthquake effects vary with circumstances
such as distance from the epicenter and local soil conditions.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.4 Rocks and Minerals
A mineral is a natural compound formed through natural geological processes, made up of one
compound or element. For example, diamond and graphite are both minerals of carbon. Other
examples are alum, amber, calcite, cobaltite, copper, feldspar, jet, magnetite, opal and quartz.
A rock is a naturally occurring mixture of minerals and/ or mineraloids (a mineral-like substance
that does not form crystals such as iron and copper). In addition, rocks may encompass organic
remains. Rocks are therefore a mixture of different minerals and are classified by the mineral
and chemical content. For example, limestone is mainly made up of calcite mineral with iron
minerals and silicates. Their main chemical component is Calcium Carbonate (CaCO3).
Examples of rocks are coal, granite, basalt, dolomite, flint, marl and sandstone.
Rocks can be classified in one of three ways that is igneous, sedimentary and metamorphic.
1.4.1 Igneous Rocks
These are formed when molten rock (magma) cools and solidifies, either below the surface of
the crust (in this case forming intrusive/plutonic rocks) or on the surface of the crust (forming
extrusive/volcanic) rocks. (Note that magma that rises to the surface is called lava).
Magma can be derived from melting of pre-existing rocks in either the Earth's mantle or crust,
which melting is caused by an increase in temperature, a decrease in pressure, or a change in
composition. Over 700 types of igneous rocks have been described, most of them formed
beneath the surface of the Earth's crust.
Examples of igneous rocks are: granite, basalt, feldspars, quartz, olivines, pyroxenes,
amphiboles, and micas
1.4.2 Sedimentary Rocks
A type of rock which is and is formed from sediments, that is broken fragments of the three
types of rock. These are found to form in four main ways:
- by the deposition of the weathered remains of other rocks: weathering causes erosion of rocks,
which fragments accumulate in a mass forming sand or a similar formation. This can occur due
to numerous processes such as water erosion (through the sea or rivers), wind erosion etc.;
- by the accumulation and the consolidation of sediments: the accumulated fragments are
solidified by pressure to form sandstone, limestone etc;
- by the deposition of the results of biological activity activity (biogenic sedimentation):
sediments formed either through the activity of biota (for example erosion by biota or else
depostion of remains of biota); and
- by precipitation from solution: chemical compounds which precipitate from solution
Sedimentary rocks include common types such as chalk, limestone, sandstone, clay and shale.
Sedimentary rocks cover 75% of the Earth's surface. Four basic processes are involved in the
formation of a clastic sedimentary rock: weathering (erosion), transportation, deposition and
compaction.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.4.3 Metamorphic Rocks
These are formed by subjecting any rock type (including previously-formed metamorphic rocks)
to different temperature and pressure conditions than those in which the original rock was
formed. These temperatures and pressures are always higher than those at the Earth’s surface
and must be sufficiently high so as to change the original minerals into other types or else into
other forms if the same minerals.
Metamorphic rocks form a large part of the earth’s crust
Examples of metamorphic rocks are slate, phyllite, schist, gneiss; some marbles and quartzite
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.5 The Rock Cycle
The rock cycle describes the dynamic dtransitions that occur better the three main types of
rocks that is sedimentary, metamorphic and igneous rocks. This is best described through a
diagram (below).
Three main transitions are present:
i. Transition to igneous;
ii. Transition to metamorphic; and
iii. Transition to sedimentary.
1.5.1 Transition to Igneous
Any type of rock (igneous, metamorphic and sedimentary) may be pushed deeper under the
earth’s surface through subduction and burial by other rocks. When this occurs, the rising
temperatures liquefy the rock which forms magma, which is found in the mantle and the core. If
the magma rises up to the crust, it may either for intrusive or extrusive igneous rocks by cooling
below or above the crust respectively. Extrusive rock results thorugh volcanic activity in the
crust.
1.5.2 Transition to metamorphic
Any type of rock (igneous, metamorphic and sedimentary) may be subject to high temperatures
and pressure when buried. These cause a physical and chemical change to form a different rock
than the parent, called the metamorphic rock.
1.5.3 Transition to Sedimentary
Any rock exposed to the atmosphere is unstable and subject to weathering and erosion (such as
biological erosion). The result is that the original rock is broken down into smaller fragments or
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
else are taken up by organisms into their body to form things like shells of molluscs and the hard
bodies of corals. In some cases, rocks dissolve in water. From this point on, sedimentary rocks
may form in four ways, as mentioned further above:
- the sediment formed is deposited in a mass forming sand or a similar formation such as silt
from a river;
- the accumulated sediment solidifies by pressure to form sandstone, limestone etc;
- Biological activity will result in sediment formation, such as formation of the rocks of a coral
reef; and
- dissolved rocks will precipitation from solution, such as when rock salt is formed.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.6 Maltese Rocks
Maltese rocks are entirely sedimentary and are formed through erosion or biological activity. The
rock strata (stratigraphic sequence) in the Maltese Islands are as follows:
- Quaternary Sediments;
- Upper Coralline limestone;
- Greensand;
- Blue clay;
- Globigerina limestone; and
- Lower Coralline limestone.
All rock layers, except for the quaternary deposits were formed during the tertiary period through
biologival activity. Only quaternary deposits formed in the quaternary period (in which we live
nowadays. Quaternary deposits/sediments are sedimentary rocks layers found in mouths of
valleys and areas where water collects. They do not form solid rock but are fragmentary.
Upper Coralline Limestone (UCL): Il-Qawwi ta’ Fuq
This is the youngest solid rock formation in the Islands, which reaches a thickness of circa 160
metres in the Bingemma area of Malta. This rock type predominantly outcrops in western Malta
and Eatern Gozo where it forms the high ground of the islands. The rock is composed of
coralline algae Lithothamnion and Lithophyllum and thus is of biogenic sedimentary origin.
This rock was formed around 10million years ago when the Mediterranean Sea was tropical that
is warm and rather shallow. In such as environment, coralline algae grow forming large reefs.
Eventually the reefs grew very thick.
This rock is the most young therefore forming last. When formed, Malta rose from the sea bed
around 10million years ago.
Since it was the youngest to form, it is relatively soft. Yet it is still used in the building industry.
Greensand: Ir-Ramlija, Il-Gebla s-Safra
This sediment underlies the Upper Coralline Limestone deposit and consists of glauconite and
bioclastic limestones. It is soft and easily eroded and when exposed it gives a green colour. Its
thickness has been observed to be up to 11m thick on Gozo, but typically it is 1m or less thick.
This rock formed when the Mediterranean was shallow and characterised by numerous currents.
The bottom was andy and therefore this formed from compaction of sand.
The Greensand has no particular use. In fact, building on it is rather a big risk!
Blue Clay: It-Tafal
This deposit lies between the overlying Greensand and the underlying Globigerina. It is the most
fertile sediment on the islands and is essential to the agricultural well-being of the island. It also
acts as an aquifer since it accumulates water on it resulting in many of the springs on the island
Blue Clay erodes easily when wet and falls over the underlying hillslopes.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
Blue clay is of biogenic origin. It is a clay mixed with limestone and is thus a type of marl. In fact,
it formed from sediments which formed in open muddy water between 150 and 100 meters
deep. These sediments were derived from rivers which carried large amounts of silt towards the
sea, which silt deposited to form blue clay.
In its nature blue clay is impermeable, unlike all other Maltese rocks. It has a very low porosity
(few pores) and thus water does not pass through it, forming a perched aquifer.
Globigerina Limestone: Il-Franka
Globigerina limestone underlies the Blue Clay formation and overlies the Lower Coralline
limestone. Most of the low lying north-eastern part of Malta and Gozo is underlain by
Globigerina. Globigerina itself is made up of three layers called the upper, middle and lower
Globigerina layers.
The deposit is made up from planktonic animals called foraminifera. In particular, it is made up
of Globigerina, but vertebrate remains have been recovered including those of crocodiles,
turtles, sharks and seals. These microscopic foraminifera have shells and float around in open
water. When they die, their shells would fall to the bottom of the sea and over the years
accumulate into a thick layer, which compacts to become a rock.
Globigerina is rather a soft rock and thus easily quarried and cut into blocks and used as
building material. Try to get a look at the island from the air as the plane lands, you will see the
dramatic extent of quarrying, both legal and illegal on the island. Viewed from the air the extent
of this problem is obvious, and large areas appear to have been quarried away.
Lower Coralline Limestone (LCL): Il-Qawwi ta’ Taħt, Iz-Zonqor
This is the oldest rock on the island and underlies the Globigerina formation. On Malta it is rarely
exposed amd found in areas such as Mosta and Gharghur, but on Gozo uplift associated with
graben (valleys) and horst activity has thrown up the 140 m high Xlendi cliffs which are entirely
composed of Lower Coralline limestone.
The sediment is composed primarily of the coralline algae Lithothamnion and
Archaeolithothamnion and formed in a similar manner as UCL. This rock can be very hard since
it has been subject to a lot of pressure. In fact, the moat at Valletta is not as deep as planned as
the Knights were unable to quarry the LCL.
It is used in the building industry as material for cement and in road paving.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.7 Soil
Soil is a complex mixture found on the surface of the lithosphere composed on minerals and
organic matter, as well as living organisms, water and air. Living organisms include insects,
worm such as the earthworm, fungi, roots of plants and others. Minerals are derived from the
rocks making up the parent bedrock, from which the soil was formed. Therefore, in the Maltese
Islands, since the rocks are made up of calcium carbonate, the soil is basic and also made up of
calcium carbonate.
1.7.1 Soil Structure
Soil is made up of distinct horizontal layers called horizons, which is a layer of soil,
approximately parallel to the surface, having distinct characteristics. They range from rich,
organic upper layers (humus and topsoil) to underlying rocky layers (subsoil, regolith and
bedrock) and are split or differentiated by changes of color, texture, roots, structure and rock
fragments.
In general the top layers are rich in organic matter. The organic matter content decreases on
going down since organic matter is produced from plants. However, the opposite is true of
minerals and salts. Since these originate from the parent material from which soil forms (called
the bedrock or parent rock) from top to bottom mineral content increases. Leaching also
causes this process.
O Horizon - The top, organic layer of soil, made up mostly of leaf litter and humus (decomposed
organic matter). It is made up of 12-18 % organic carbon.
A Horizon - The layer called topsoil; it is found below the O horizon and above the E horizon.
Seeds germinate and plant roots grow in this dark-colored layer. It is made up of humus
(decomposed
organic
matter)
mixed
with
mineral
particles.
E Horizon - This eluviation (leaching) layer is light in color; this layer is beneath the A Horizon
and above the B Horizon. It is made up mostly of sand and silt, having lost most of its minerals
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
and clay as water drips through the soil (in the process of eluviation i.e. water percolation
through soil).
B Horizon - Also called the subsoil - this layer is beneath the E Horizon and above the C
Horizon. It contains clay and mineral deposits (like iron, aluminum oxides, and calcium
carbonate) that it receives from layers above it when mineralized percolates from the soil above.
C Horizon - Also called regolith: the layer beneath the B Horizon and above the R Horizon. It
consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer; very little
organic material is found in this layer.
R Horizon - The unweathered rock (bedrock) layer that is beneath all the other layers.
Note: Often on four soil horizons are described that is O, A, B and C.
1.7.2 What makes up soil?
What exactly makes up soil? The following is a brief account…
50% is composed of organic substances (including organism) and inorganic (dead) material.
The rest is composed of air and water which fill up the spaces between the particles. The
inorganic part is made up of many components such as leterite (clay), vermiculite, smectite,
illinite, chlorite, gibbsite and kaolinite (both aluminium compounds), goethite and kaolinite (both
iron compounds).
The living compounds are either microscopic or else macroscopic (large). Microscopic
organisms include algae, protozoa, bacteria and fungi. Large organisms include animals such as
insects, millipedes, mites and worms, such as the earthworm and nematodes). Plants also occur
or parts thereof such as the roots.
The inorganic component is often made up of particles of different sizes. These are classified as
given in the diagram below:
Very small particles (from 0 to 0.002mm) are
called clay. These are responsible for trapping
nutrients since they are charged molecules
(positive clay particles trap anions such as
Nitrate, NO3-, phosphate, PO43- and Sulphate,
SO42- whilst negative clay particles trap cations
such as potassium, K+, calcium, Ca2+ and Iron,
Fe2+). These also trap water and thus clay is an
very important component.
Silt is larger than clay. It retains less water and
nutrients and thus gives good drainage in the
soil.
Sand is also important. Since it is relatively
large in size, it holds less water than clay.
Therefore water drains easily from sand and
with it nutrients are lost.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
The best soils are thus a mixture of sand, silt and clay since a balance between water logging
and drought is given. Also, a balanced nutrient holding capacity is given for mixed soils.
According to the composition, soil is given a name. The diagram below helps determine the
name of a soil according to the composition of clay, sand and silt.
Loam soils contain a mixture of sand, silt and clay and are the best types of soil. Adding a bit
more clay gives a clay loam whilst adding a bit more sand or silt gives a sandy or silty loam
respectively.
1.7.3 Soil Formation
Soil formation (pedogenesis) is the process by which soil is created. Soil formation is a very
complex procedure and depends on numerous factors, outlined below:
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
All soils initially come from rocks, that are called the ‘parent material’. The parent material may
be directly below the soil, or great distances away if wind, water or glaciers (ice) have
transported the soil. In addition to the soil parent material, soil formation is also dependent upon
other prevailing processes namely climate and the organisms present. Climatic conditions are
especially important factors affecting both the form and rate at which soil forms. This is since
weather causes erosion of the parent material by physical and chemical weathering of the
parent material to form soil. Biological erosion also occurs.
There are two main categories of weathering, having different effects:
Physical Weathering
• Freezing and Thawing – as water freezes it expands and this pushes the soil structures apart.
Water expands considerably when frozen and this expansion literally pushes the soil apart,
breaking it down. When the ice thaws the soil can slump back again. The overall process is
rather like a very slow but can literally grind mountains down over time!
• Heating and Cooling - here soils subjected to extremes of temperature are affected as they
expand and contract. The effect is less pronounced that that of freezing and thawing but over
time this can become significant as is seen in the desert.
• Wetting and Drying - soils that are wetted up may be prone to swelling. Clay minerals in
particular exhibit this property. The soils that have thus expanded then shrink when the soils dry
out. These seasonal effects are termed shrinkage and swelling.
• Grinding or Rubbing - most obvious on the beach, grinding of particles against each other
leads to particle disintegration. This is why beach pebbles become smooth. Abrasion similarly
breaks down the soil particles.
•Organisms - the effect of organisms, plants and animals, living in the soil cannot be
overstated. Soil is home for a wide range of organisms. If plants can push through concrete soil presents little obstacle! Worms churn their way through soil, mixing and aerating it all
through their lives and there can be thousands of worms in a field.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
Chemical Weathering
•Solution - certain solid components in the soil can be dissolved in water. In this way
underground caverns can form in limestone karst landscapes.
•Acid – acids break down certain rocks to give smaller particles. In fact, rocks break down due
to carbonic acid, naturally found in rain.
Weathering and erosion results in breakdown of the parent material to form small particles. This
accumulates in holes and plants (colonizers) may start growing on it. Due to the advent of
plants, organic matter is added to the soil. After plants come the animals, which start mixing the
soil and introduce air into it. They also carry organic matter deeper into the soil. In fact, animals
like the earth worm carry leaves into deep burrows. The more plants grow, the more organic
matter is added to the soil, thus rendering it rich.
1.7.4 Other notes on soil
Further processes are known to occur in soil. The following is a list:
Leaching - leaching is the removal of soluble components of the soil column. As water washes
down (percolates) through the soil it can carry away minerals and ions such as calcium and
magnesium. Leaching causes the top soil to be poor.
Eluviation - soil particles held in suspension, such as clay, are washed away with floods. This
results in loss of soil from habitats and fields, a process called soil erosion. However, soil
erosion is also a result of wind erosion and not only eluviation.
Nutrient content of soil: It is the amounts of nutrients found in soil.
Nutrients holding capacity: The ability of soil to store nutrients. It is often low in sandy soils but
high in clay soils. Therefore, loam soils are preferred.
Water holding capacity: The ability of soil to store water. Soils which are rich in clay normally
have a high water holding capacity and often become water-logged. On the other hand soils rich
in sand have a very low water holding capacity since water percolates through. The best soils
are thuis loam soils (mixture of clay and sand.
Irrigation: The process of adding water to soil. This may take many forms such as fdrip
irrigation, sprinklers etc.
Fertilization: The process of improving the soil’s ability to hold plants i.e. its fertility. This is
done by using artificial fertilizers such as NPK (Nitrogen, Phosphorous, Potassium) and
Ammonium nitrate or natural fertilizers (manure)
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.8 Biogeochemical cycles
These depict the movement of nutrients on earth through living organisms and the non living
environment. They may be very complex since many processes are involved in many parts of
the earth.
The following terms may be used:
Source: A structure or organism containing a particular nutrient.
Sink: A structure or organism which consumes a nutrient. A sink is also, very often, a source.
Reservoir: A sink in which the nutrient is stored.
1.8.1 The Carbon Cycle
Carbon (C) is the 4th most abundant element in the universe after hydrogen (H), Helium
(He) and oxygen (O). It is the building block of life since it forms all substances in organisms
such as DNA and also other substances, such as fossil fuels.
The movement of carbon, in its many forms, between the biosphere, atmosphere,
hydrosphere and lithosphere are described by the carbon cycle, which can be depicted by a
simple diagram, as below
CO2
1.8.1.1 Forms of Carbon
Carbon is present in many forms in the environment. The main forms are below:
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
-
-
-
Carbon Dioxide (CO2): A gas present in the atmosphere at a concentration of 0.04%. It
also occurs dissolved in water. The atmosphere is thus a main reservoir of carbon, whilst
the oceans are a carbon sink.
Organic carbon: Occurs in the form of sugars, fats, proteins, DNA and other molecules in
the biosphere. The largest percentage of carbon is found in cellulose, which forms plant
cell walls. These are present in all living organisms and decomposing animal and plant
matter in the soil.
Carbonates: An inorganic form of carbon present in rocks such as limestone and soils,
and dissolved in water.
Coil, oil and gas: fossil fuels contain complex molecules of carbon derived from living
organisms. These are stored in the lithosphere.
1.8.1.2 Processes in the Carbon Cycle
The cycle has four main reservoirs that is the atmosphere, biosphere, lithosphere and fossil
fuels (within the lithospohere). Several processes carry carbon from one reservoir to another.
CO2 (Atmosphere, hydrosphere)  Biosphere
Several processes change CO2 into another form of carbon:
- Photosynthesis: Carbon dioxide in the atmosphere or the hydrosphere (in the form of CO 32-)
is converted into the sugar glucose by autotrophs, with the help of light energy. The glucose
formed is either stored in the body, used to release energy for work or else used to build the
plant body, such as by being converted into cellulose or proteins.
Once heteretrophs eat plants, they convert the organic carbon molecules into other molecules.
Therefore, photosynthesis introduces C into the biosphere.
- The ocean absorbs a large amount if carbon dioxide by dissolution. This is used by
phytoplankton and marine algae to perform photosynthesis.
Biosphere CO2 (Atmosphere, hydrosphere)
Heteretrophs and autotrophs respire to produce energy, in the process liberating carbon dioxide.
This gas is released into the atmosphere.
Biosphere  Fossil fuels
Organisms produce waste and die. In the process organic nutrients are released into the
environment, which are decomposed by saprotrophs to form CO2. However, some organic
material does not decay. In fact, it fossilizes and forms fossil fuels. The process of fossilization is
complex and takes a long time to occur. In the process, organic compounds do not decompose
but are preserved. However, over time they change in structure, in the end giving a very
complex molecule, which is the basis of fossil fuel. Remember that coal is formed from plant
remains, whilst oil and natural gas are formed from the remains of phytoplankton and
zooplankton.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
Fossil fuels  Atmosphere
Combustion is the process by which biomass and fossil fuels are burnt in the process releasing
heat and CO2. There are two forms of combustion: aerobic and anaerobic, as shown below
Aerobic: C (organic matter) + O2  CO2
CH4 + 2O2  CO2 + 2H2O
Anaerobic: 2C (organic matter) + O2  2CO
4CH4 + 5O2  2C + 2CO + 8H2O
Combustion releases several thousands of tonnes carbon dioxide in the atmosphere annually.
Atmosphere  Hydrosphere  Lithosphere
Carbon dioxide in the air dissolves in water to form carbonic acid. Carbonic acid may be used by
animals to form carbonates, that make up shells and other hard structures
CO2 + H2O  H2CO3
H2CO3  2H+ + CO32CO32- + Ca2+ CaCO3
When these organisms dies,their shells and hard body parts sink to the ocean floor where they
accumulate in carbonate rich deposits. After a long period of time and the process of lithification
they form carbonate rocks, such as limestone, which are rich in calcite, CaCO3.
The carbon cycle is rather complex and each otf the mentioned processes is important. Of
particular importance is combustion, which is used by humans to generate energy. This releases
enormous amounts of CO2 into ht atmosphere and is thus risking imbalancing the Carbon cycle.
In fact the CO2 content of the atmosphere has already increased by 0.005%. However, the
ocean and autotrophs are good sinks (especially the ocean)taking up a large amount of CO 2
annually.
1.8.2 The Nitrogen Cycle
Life requires nitrogen (N) since it makes up proteins and DNA. Proteins are very important
molecules since they act as catalysts of body reactions and therefore, N is also important. The
cycling of the different forms of N can be represented by a biogeochemical cycle, which is more
complex than that of C. It is shown in the diagram further below.
1.8.2.1 Forms of Nitrogen
-
N is present in many forms in the environment. The main forms are below:
Nitrogen gas (N2): The atmosphere contains 79% nitrogen gas and thus is the biggest
reservoir;
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
-
Inorganic nitrogen: Many compounds exist such as nitrates (NO3-, nitrite (NO2-)
-
ammonia (NH3), ammonium (NH4+) and urea (CO(NH2)2).
Organic nitrogen: in amino acids (components of acids) and nucleic acids (components
of DNA).
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
1.8.2.2 Processes in the Nitrogen Cycle
The cycle has three main reservoirs that is the atmosphere, biosphere and lithosphere. Several
processes carry carbon from one reservoir to another.
Atmosphere  Biosphere
Nitrogen from the atmosphere must be taken up by organisms due to its importance. However,
,ost organisms cannot make use of nitrogen gas (N2). Therefore, N2 must be converted into a
form that can be taken up by organisms (plants) and distributed into the food web. The process
by which N2 is taken up by the biosphere is called Nitrogen fixation. It may occur in three way:
-Biological fixation: Specific bacteria absorb N2 and convert it into organic compounds such as
proteins. These bacteria may either be free living or else in symbiosis with legumes.
- Industrial fixation: N2 in the atmosphere is converted into ammonia, which is often used as a
fertilizer.
N2 + 6H+ + 6e- 2NH3
- Atmospheric fixation: The enormous energy of lightning breaks N2 and enables the atoms to
combine with O2 in air to form nitrates. These dissolve in rain and are carried down to earth.
N2 + 3O2 + 2e- 2NO3Biosphere Biosphere Lithosphere
The proteins made by plants enter and pass through the food web. At each trophic level organic
nitrogen compounds are produced such as proteins. These are released into the environment
either through excretion (urea is formed due to excess proteins in the diet), through egesta, or
else when the organism dies. In each case, decay in the soil will break down animal and plant
molecules containing nitrogen into ammonia.
Lithosphere  Lithosphere Biosphere: Nitrification
Ammonia cannot be taken up directly by plants through their roots. However, the ammonia
produced by decay is converted into nitrates. This is accomplished in two steps as shown below:
2NH3 + 5O2 + 2e- 2NO2- + 3H2O
2NO2- + O2 2NO3Nitrifying bacteria present in the soil carry out the process when oxygen is available (the soil is
not waterlogged). Due to this activity, the plants can absorb nitrogen from the soil.
Lithosphere  Atmosphere: Denitrification
Nitrogen fixation removes N2 from the atmosphere. If only this were present, the process would
not be a cycle. However, a process called Denitrification changes nitrate into N2 as shown
below:
NO3- + 6H+ + 5e- N2 + 3H2O
Bacteria called denitrifying bacteria act as agents. These live deep in the soil and in aquatic
sediments where conditions are anaerobic.
J. Henwood
The Physical Environment- Lithosphere
______________________________________________________________________________
Other processes
In the diagram, other processes are mentioned such as:
Emissions from fossil fuels: The have nitrogen in them, which on burning are released as NO
and NO2, collectively called NOX. These precipitate to earth during rain and are taken up by
plants to form proteins.
Eutrophication: Excess artificial fertilizers (e.g. NH3NO3, ammonium nitrate) are soluble in water.
Therefore these are carried by surface runoff or by leaching into water bodies. They are used by
water plants and phytoplankton to grow, thus entering the nitrogen cycle in water, but in excess
may cause eutrophication.
Final remark: The role of saprotrophs
Saprotrophs have a very important role in all cycles, These essentially recycle living
organisms and their components, therefore also recycling the nutrients in them through decay.
J. Henwood
The Physical Environment- Lithosphere