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Unit 11.1 The Structure of the Earth
Topic 1: Composition of the Earth –
the four spheres
ES
Unit 11.1 introduces students to the processes and forces that shape the landforms on the
surface of the Earth. Topic 1 gives a basic introduction to the four principal components,
each referred to as a ‘sphere’, that make up the biophysical environment of the Earth:
• The lithosphere – The outer surface and interior of the Earth.
• The atmosphere – The gases (air) that surround the Earth.
• The biosphere – All living things that inhabit the Earth.
• The hydrosphere – The waters of the Earth.
Unit 11.1 Topic 1 gives a brief overview of the four spheres. The remaining Topics in this
Unit cover the lithosphere in some detail.
G
The atmosphere is covered in more detail in Unit 11.2 ‘Natural Processes and Disasters’, in
particular Topic 1 ‘Air pressure and global wind systems’ (p. 97) and Topic 2 ‘Climates of
the world’ (p. 111). Unit 11.2 also looks at elements of the biosphere, in particular Topic 3
‘Natural vegetation’ (p. 119)
PA
Unit 11.3 looks at the hydrosphere and at oceanography in particular.
The four spheres – an introduction
E
The four principal components that make up the structure and biophysical environment of the
Earth are each referred to as a sphere (pronounced ‘sfeer’): the lithosphere, the atmosphere, the
hydrosphere and the biosphere.
PL
The lithosphere refers to the outer surface and the interior of the solid Earth. On the surface of
the Earth, the lithosphere is composed of three main types of rocks:
Igneous rock, created by volcanic activity from lava spewed out onto the surface of
planet Earth.
•
Sedimentary rock, created by the deposition of fine materials by water, in lakes and most
often under the ocean and later lifted up to become land.
•
Metamorphic rock, which was sedimentary and other rocks that have been ‘cooked’ and
changed chemically and structurally by intense pressure and heat, deep under the Earth’s
surface.
SA
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•
The atmosphere comprises the gases (air) that surround the Earth. The air varies in temperature
and pressure, depending mostly on where it is located on the Earth’s surface: coldest at the
poles and warmest on the equator. The qualities of the atmosphere – pressure, temperature and
humidity (the amount of water held in the atmosphere) – are responsible for the weather. There
is another term, the troposphere, which refers to the higher levels of the atmosphere. This also
has an influence on weather and climate.
The term ‘hydrosphere’ is used to refer to the waters of the Earth. Water exists on Earth in
the atmosphere, and in oceans, lakes, rivers, soils, glaciers and groundwater. Water moves
from these places to other places by way of: evaporation, condensation, run-off, precipitation,
infiltration and groundwater flow.
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34 Unit 11.1 The Structure of the Earth
The biosphere includes all living things, including plants and animals, from the very smallest
microscopic bacteria to the largest trees and animals. The biosphere is strongly influenced by the
other three spheres and most recently by human activity.
Unit 11.1 Activity 1A: Word analysis
1. Before reading much more of this Unit, do some research on your own and come up with
your own definitions of the terms:
• Lithosphere.
• Atmosphere.
ES
• Hydrosphere.
• Biosphere.
PA
G
Check as many dictionaries as you can to analyse how these terms were derived – for
example, what does the word ‘sphere’ mean? Look for definitions you think will be
relevant to the work you will be doing as outlined in the introduction.
Unit 11.1 Activity 1B: Your own research
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PL
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What lies beneath the lithosphere? List the layers that lie between the centre of the Earth and
the lithosphere and provide a brief description of each layer.
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Unit 11.1 The Structure of the Earth
Topic 2: Composition of the Earth –
its layered structure
Topic 1 gave a brief introduction to the four principal components that make up the
biophysical environment of the Earth: the lithosphere, the atmosphere, the hydrosphere and
the biosphere. Topic 2 focuses on the lithosphere and looks at the Earth’s layered structure:
• Core.
• Mantle.
• Crust.
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What is inside the Earth?
ES
By the end of this Topic you should understand what the inside of the Earth is made of, what
tectonic plates are and why they drift apart, what happens when plates collide and diverge,
and which parts of Papua New Guinea are parts of plates or parts of collision areas or parts
of divergence areas.
PA
The Earth is made up of three parts, each one wrapped
round the other like the skins of an onion. They are, from the
outside in:
Crust
Upper mantle
Lower mantle
1. The crust, a thin layer which forms the surface of
the Earth where we live. The crust is thicker under
continents than under oceans.
Outer core
E
Inner
core
The layers of the Earth
Most plates move less than 1 cm in a year. However, the
plates around Papua New Guinea are quite fast moving;
Plate, 70
Scientists believe that the upper part of the Earth is
covered by a number of thin plates of very rigid rock.
The zone containing these plates is called the lithosphere.
The lithosphere is made up of the crust and the upper
part of the mantle and it ‘floats’ on the more liquid, lower
part of the mantle. Because these lithospheric plates are
floating, they move very slowly about the Earth’s surface.
This movement is called plate tectonics.
kms thick
Plates on the move
Oceanic crust
Continental crust
SA
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If you could drill a hole from one side of the Earth to the
other, it would be 12 757 km deep! However, the deepest
hole ever drilled goes down only a tiny fraction of that
depth. We can only really guess what happens deep
beneath the surface of the earth.
Ocean
3. The core, which gives off enormous heat, a very small
part of which reaches the Earth’s surface.
Upper part of the mantle
PL
2. The mantle, which is made up of semi-solid rocks.
The deeper you go, the more liquid the rocks become.
A cross-section through one of
the Earth’s plates, showing the
oceanic and continental crusts
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36 Unit 11.1 The Structure of the Earth
some even travel 11 cm or more each year! All plates do not move in the same direction.
In some parts of the world they are moving away from one another; in other parts they are
colliding. Landforms such as fold mountain ranges, deep valleys and ocean ridges and trenches
form the boundaries between two plates. (We will learn what these terms mean later.)
Let us look more closely at what happens when plates either move apart or collide.
Scientists believe that currents flow through the half-liquid rock in the Earth’s mantle. (Parts of
the molten rock are moving or flowing in a certain direction in relation to surrounding rock
which remains stiller.) It is probably these currents which cause the plates to move. In some
places, the currents come together (converge); in other places they move apart (diverge).
Ocean ridge
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Ocean floor
SOUTH AMERICA
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AFRICA
PA
Trench
PL
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Lower mantle
Asthenosphere
Lithosphere
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Plate tectonics: plates moving apart under the Atlantic Ocean, and colliding along
the west coast of South America. The ‘asthenosphere’ is the liquid part of the mantle
on which the lithosphere floats. The arrows indicate the direction of currents.
SA
Plates moving apart
Where they diverge, they are slowly pulling apart the crust which lies on top of them. As a result
the crust gets thinner and thinner until it splits. The half-liquid rock below will then come up
to the surface, where it cools and becomes part of the crust. These new parts of the crust will in
turn be pulled apart and more of the mantle will seep to the surface.
This might sound rather dangerous. But remember a) that it is happening very, very slowly
and b) that plates usually move apart only under the largest oceans. The main landforms that
result from liquid rock seeping to the surface in areas of plate divergence are ocean ridges. Plate
divergence affects very few land areas. However, some scientists think that Murua (Woodlark)
Island in Milne Bay is sitting right on top of such a plate divergence and is slowly being pulled
apart.
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Topic 2: Composition of the Earth – its layered structure 37
Plates moving together
Mountain
ranges
PA
G
ES
Many of you will have seen a car accident when two cars
run into each other. Cars travel quite quickly (say 50
kph) but are quite small; when plates collide they do it
very slowly (at a speed of around 1 cm a year), but they
are billions of times bigger than cars. When they collide
As plates move together, the
one plate gets pushed upwards and the other slides
Earth’s crust is ‘crumpled up’
underneath and is pushed back down towards the Earth’s producing fold mountain ranges.
mantle. When cars crash, they are squashed up and
The arrows show the direction of
twisted out of shape; when plates collide the same things
the pressure causing the folding.
happen. Rocks are twisted (folded) and
sometimes big cracks (faults) appear in the
Scarp Mountain range
Plateau
earth’s surface. If the cracks are big enough,
half-liquid rock from the mantle may come
Rift valley
up to the earth’s surface, forming volcanoes.
The plate that is pushed up in the collision
will form mountains, and the plate that
is pushed down will form deep valleys or
trenches.
FAULTS
When plates move together, faults are
produced in the Earth’s crust. The blocks
of land between fault lines may move,
producing new landforms.
SA
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PL
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Once cars have crashed, that is it.
Everything is over in a few seconds. But a
plate collision keeps on going over millions
of years. The collision will affect nearby
parts of the Earth’s surface causing them
to shake. These ‘shakings’ are known as
earthquakes or earth tremors.
Passive plate
boundary
Constructive plate
boundary
Destructive plate
boundary
Plates on the surface of the Earth
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38 Unit 11.1 The Structure of the Earth
How Papua New Guinea was created
Nearly all of Papua New Guinea has been formed by the collision of the Australian and Pacific
plates. These two plates are still colliding today along the north coast of the main island and
through New Britain. This is why earthquakes are so common there. Millions of years ago the
collision was happening where the highlands are now, but the Australian Plate has gradually
pushed northwards. There are extinct volcanoes all over the highlands, showing where the
collision used to occur. It is the active ones which show us where the collision is today. As you
can see from the map below, most of Western Province and parts of Gulf Province are really part
of the Australian Plate. Almost everywhere else in Papua New Guinea has been formed by the
collision.
G
ES
Because of this, folds and faults are found nearly everywhere in Papua New Guinea. The
biggest fault runs between Lae and Madang; the Markham River runs along it. The land north
of the Markham is slowly being pushed up as it collides with the Pacific Plate. That is why the
Main collision areas
Old or minor collisions
Direction of plate movement
PA
Philippines
So
lom
PNG
on
Is
Murua
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Indonesia
PL
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PACIFIC PLATE
Fiji
SA
Vanuatu
Australia
New
Caledonia
AUSTRALIAN PLATE
N
0
1000
New
Zealand
2000 km
Plate tectonics near Papua New Guinea
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Topic 2: Composition of the Earth – its layered structure 39
Finisterre and other mountains there are so high. Perhaps in millions of years time the highest
mountain in Papua New Guinea will be Mount Sarawaget, not Mount Wilhelm.
SA
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PL
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PA
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Papua New Guinea has been created by plate movements that are still going on. This is only part
of the picture. The next Topic will explain what happens to landforms after they have formed
and the part the water cycle plays in wearing down the land.
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Unit 11.1 The Structure of the Earth
Topic 3: Composition of the Earth – rocks
and soil-forming processes
The previous Topic gave a brief introduction in Topic 1 to the four principal components
that make up the biophysical environment of the Earth: the lithosphere, the atmosphere,
the hydrosphere and the biosphere. Topic 2 now focuses on the lithosphere and looks at the
Earth’s layered structure:
•core
•mantle
•crust
ES
By the end of this Topic you should understand what the inside of the Earth is made of, what
tectonic plates are and why they drift apart, what happens when plates collide and diverge,
and which parts of Papua New Guinea are parts of plates or parts of collision areas or parts
of divergence areas.
G
Introduction
PA
The rocks that are found on the surface of the Earth are always changing. New rocks are formed
when very hot, liquid lava cools down. Igneous rock can form underground, where the magma
cools slowly. Or, igneous rock can form above ground, where the magma cools quickly. When
lava cools it forms crystals. The type of crystals that form depends on the chemical composition
of the lava, which in turn determines the type of igneous rock formed.
M
PL
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When igneous rock is exposed to air and water it begins to break down and erode away. Over
very long periods of time, very hard igneous rocks can be turned into fine sand, which can be
carried away by water or blown away by wind. Water deposits the fine material on the bottom
of lakes or in the ocean, where it settles and, over very long times, becomes sedimentary rock.
Sedimentary rock is characterised by horizontal layers formed as different material is deposited.
Sedimentary rock may also be formed from the bodies of tiny ocean-living creatures, such as
corals and shellfish. Limestone, a very common rock in PNG, is formed in this way.
SA
If sedimentary rocks are pushed deep down into the earth’s crust, where it is hot and there
are great pressures from the weight of all the rocks above, sedimentary rocks can ‘cook’, or be
changed in their chemical and crystalline form, to become metamorphic rocks. ‘Metamorphic’ is
Latin for changed form. When metamorphic rocks become exposed on the Earth’s surface, they
also erode and decompose, to form sedimentary rocks.
The rock cycle refers to the way in which the rocks exposed on the Earth’s surface are changed
physical and chemically into other rocks, which are then transported to a different place, or
taken down into the Earth’s crust and re-formed again.
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42 Unit 11.1 The Structure of the Earth
Cooling
Magma
(deep in the Earth’s crust)
Melting
Metamorphic rock
Igneous rock
Erosion, weathering
Heat, pressure deep
in the earth’s crust
Erosion, weathering
Sand, mud, silt
(sediments)
Erosion, weathering
Heat, pressure deep
in the earth’s crust
The rock cycle
G
Sedimentary rock
ES
Transport by water and wind;
compaction
PL
Soils
E
PA
The different kinds of rock that form the surface of the Earth can change over millions
of years. Igneous rock and metamorphic rock can be eroded and weathered to form
sediments (sand, mud, silt). Sediments can be carried by wind and water and compacted
under pressure to form sedimentary rock. Heat and pressure deep within the Earth’s
surface can change sedimentary rock into metamorphic rock. Metamorphic rock can be
melted under pressure deep within the Earth’s crust and can form igneous rock. This takes
millions of years but the rock cycle is always changing.
M
Soil is material on the Earth’s surface produced from rocks (parent material) which has been
altered by physical, chemical and biological weathering.
Factors affecting soil characteristics
•
SA
Most soils contain varying quantities of minerals, humus (decaying organic plant and animal
material), water, air and living organisms (such as worms). These elements give soil its
characteristics – the quantities of each present in soil depend on the following factors:
Climate – Particularly temperature and precipitation. Cold climates, such as tundra areas
(eg Siberia in Russia) or high, mountainous areas (eg Andes in South America, Southern
Alps in New Zealand) have soils of little use for agriculture. Often these soils are frozen, and
the decomposition of material to form humus is slow. High or low rainfall areas also have
different soil types.
•
Relief – In particular, slope angle, which affects the rate of both water run-off and erosion.
Hilly or mountainous areas with steep slopes have high rates of erosion – surface material is
carried downslope quickly and soils are shallow as a result. On flatter land, erosion rates are
slower, and the deposition of alluvium by rivers in flood occurs. Consequently, soils on flat
land are often deeper and more fertile than other soils, but may not always be well-drained.
•
Parent material – Includes the structure and mineral composition of the original rock
weathered to form the soil. Different rock types give rise to different soils because of their
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Topic 3: Composition of the Earth – rocks and soil-forming processes 43
differing mineral compositions, eg the tropical, lateritic soils of the Amazon Basin reflect the
iron or alluvium in their parent material.
•
Time – Young soils, known as azonal soils, reflect the characteristics of the parent material,
because little weathering has taken place to change the character of the soil. Older or
zonal soils have developed their own characteristics, as organic material is added or
further processes occur. Alluvium and recent volcanic tephra (eg the soils surrounding
Mount Hagen in the Western Highlands) are examples of azonal soils. A podsol soil formed
under cool climates with high rainfall such as any high-altitude areas in PNG is an example
of a zonal soil.
Variations in these factors within a natural landscape will produce different soils.
Unit 11.1 Activity 3A: Soils
ES
In a brief paragraph, describe how climate and relief can influence soil type.
Soil profile
PA
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The influence of soil-formation factors can be shown in a soil profile, which is a vertical section,
from the surface of the soil to the underlying rock. It clearly shows the characteristics of a soil
(such as colour and texture), and that soils are organised in layers, or horizons.
PL
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The depth of each layer varies from several centimetres to several metres, depending on all the
aspects of soil formation (ie climate, time, parent material, biological conditions and relief).
O – Organic material at the surface.
SA
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A horizon – the topsoil; contains some weathered
material and organic elements, is rich in humus.
B horizon – contains weathered parent and animal
material and some organic elements; material leached
from the A horizon may be deposited.
C horizon – the subsoil (weathered parent material).
D horizon – the parent material.
Soils in Papua New Guinea
The type of soil found in any particular place is the outcome of five influences: the climate,
the vegetation growing on the site, the slope, the parent material (the underlying material from
which the soil has developed) and the length of time the soil has been developing. Changes in
the combination of these factors can result in soils varying over relatively short distances.
Soils are also significantly influenced by human activities, for example, by tillage, composting,
mounding or erosion caused by cultivation on steep slopes. As a result, it is difficult to
generalise about soils and their properties in PNG. Because much food production in PNG is
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44 Unit 11.1 The Structure of the Earth
from small, discontinuous areas cleared from fallow land as part of a shifting cultivation cycle, it
is also difficult to be specific about the relationships between soil type and food production.
Soil type is one of the factors taken into account in the assessment of resource potential
described above, however, and here we attempt to make some general statements about PNG
soils and their long-term association with food production.
Soil science is frequently confusing and difficult to understand because of the way in which
soils are classified and named. In the United States Department of Agriculture (USDA) Soil
Taxonomy, which is used in PNG, the highest level of classification is the ‘soil order’. The
following soil orders occur in PNG:
Entisols – Very young soils, with little or no profile. These soils occur mainly on recent
alluvium or on steep slopes where soil erosion takes place.
•
Histosols – Soils that contain very high levels of organic matter (peat soils). These soils are
mostly dark-brown to black in colour, and occur in swampy areas. They are saturated with
water for much of the year.
•
Inceptisols – Moderately weathered soils, with no strongly contrasting horizons. They
include soils derived from volcanic ash.
•
Andisols – Volcanic ash soils. These soils are very important in PNG.
•
Vertisols – Soils with high clay content that are sticky when wet and very hard when dry.
These soils swell when wet and crack when dry and are generally of high fertility. They are
not common in PNG.
•
Mollisols – Soils in which there is accumulation and decomposition of organic matter.
These soils generally have a high base content (eg calcium, magnesium).
•
Alfisols – Moderately weathered soils that have an argillic horizon (a soil layer with higher
clay content due to movement of clay from the top to lower layers). These soils are usually
highly fertile.
•
Ultisols – Strongly weathered and acid soils with an argillic horizon. These soils have a low
base saturation and thus are relatively infertile.
•
Oxisols – Very strongly weathered soils, with low fertility. These soils occur on old land
surfaces and are not common in PNG.
SA
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PL
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PA
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The most common soils in PNG are inceptisols (including andisols), which are found over
almost half of the total land area. Inceptisols cover more than 80% of the land area of Western
Highlands, Simbu, Eastern Highlands and West New Britain provinces. Inceptisols are least
common in Western, East Sepik and Gulf provinces.
The next most common soils are entisols. These young soils cover more than a quarter of the
total land area, reflecting the high geological activity that occurs in PNG. Entisols are common
soils in East Sepik, Gulf, Morobe, Central, Western, Oro, Sandaun and Madang provinces.
Ultisols (strongly weathered soils) cover approximately 14% of the land area of PNG, and occur
mainly in Western Province, where they occupy more than half of the land area. Alfisols are
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Topic 3: Composition of the Earth – rocks and soil-forming processes 45
common in New Ireland, where they cover over a quarter of the land area. Mollisols are most
prevalent in East New Britain, where they occur on one-third of the land area.
Almost 2.5 million people (59% of PNG’s rural population) live on inceptisols. Inceptisols
(with andisols) support more than 80% of the populations of Simbu, West New Britain, Western
Highlands and Enga provinces. Entisols support 20% of the population and are most important
in Central Province, where 78% of the population use these soils. Alfisols and mollisols together
support a further 16% of the rural population. Alfisols are important in New Ireland Province
and mollisols are important in Manus Province. Around 32% of the population in Western
Province cultivate ultisols.
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ES
The availability of nutrients for plants depends on several factors. Low levels of nutrients in the
soil may be caused by low amounts of nutrients in the parent material from which the soil is
derived. The nutrients may also become chemically fixed in the soil and therefore not available
to plants. Nutrient imbalances in the soil (for example, high calcium, low potassium) may have
a similar effect. High rainfall can leach nutrients from the soil. Low nutrient levels may also
result from cultivation, when agricultural crops remove nutrients which are then not replaced by
humans or by natural processes.
PA
Soil nutrients that plants require in relatively large amounts are nitrogen (N), phosphorus (P),
potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S). A number of other elements
that plants require in very small amounts are called micronutrients; these include boron, zinc,
manganese, iron and copper. Nutrient deficiencies affect crop production. This problem is likely
to worsen in PNG in the future as our population increases and land is used more intensively.
PL
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Soil nitrogen availability is determined in part by the length and type of fallow, the introduction
of organic matter (plant materials) into the soil, rainfall and temperature. Most nitrogen in PNG
soils is derived from organic (plant) matter. Soil nitrogen tends to be higher in highlands soils,
where temperatures are cooler and organic matter decomposes more slowly.
M
Phosphorus fixation (where phosphorus becomes chemically fixed in the soil and is unavailable
to plants) is widespread in the volcanic ash soils (andisols) that support large numbers of people
in the highlands, and in Oro and Bougainville provinces. Phosphorus fixation can also be severe
in ultisols and oxisols.
SA
Potassium-deficient soils are usually highly weathered and leached, with limited amounts of
mineral reserves. Volcanic ash soils usually have high levels of potassium. Soils that develop on
limestone and that have high levels of calcium and magnesium may have a potassium deficiency
because the calcium holds the potassium in the soil and makes it unavailable to plants. This is
common, for example, in New Ireland Province.
In many parts of PNG, the people manage land use (through shifting cultivation systems)
and the techniques they use to maintain soil conditions (eg green manuring) are at least as
important to the long-term production of food as the basic characteristics of the soil. However,
intensification of land use will affect soil fertility, and nutrient deficiencies are therefore likely to
increase, particularly in food crop species where inorganic fertilisers are not used.
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PA
Ultisols cover 14% of the land area of PNG, but occur mainly in Western, where they cover more than half of the area of that province.
Alfisols are common in New Ireland and Mollisols in East New Britain.
The next most important soils in PNG are Entisols. They cover 25% of the total land area. They are most common in East Sepik, Gulf,
Morobe, Central, Western, Oro, Sandaun and Madang.
The most common soils in PNG are inceptisols and andisols, which cover half of the country. Inceptisols cover more than 80% of the area of
Western Highlands, Simbu, Eastern Highlands and West New Britain. Inceptisols are least common in East Sepik, Western and Gulf.
Papua New Guinea soils.
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PL
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SA
46 Unit 11.1 The Structure of the Earth
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Topic 3: Composition of the Earth – rocks and soil-forming processes 47
Soil processes
Water is important in soil formation. An important soil process operating in PNG is alluviation
(material being deposited by the action of rivers) – this includes material laid down in river
channels, on flood plains, in lakes and in fans at the foot of mountain slopes. This type of
material can range from coarse (eg gravel and sand) to fine (eg clays and silts), and is often high
in nutrients and very fertile.
Two other important soil processes occur when rain is abundant or lacking.
Leaching occurs in conditions of heavy, consistent rainfall, such as in the tropical rainforest
environment, when large volumes of water filter down through the soil. The water dissolves
and carries away minerals and nutrients in the upper layers of the soil, depositing them
deep in the soil (perhaps to a depth of 30–40 metres). The result is a tropical laterite soil,
often red or yellow in colour, with high concentrations of iron or alluvium.
Laterite soils (latosols) support lush tropical rainforests, so people often imagine these
soils are fertile and rich; however, the upper horizons of the soil are thin and mineraldeficient. Although there is plentiful vegetation debris, the rapid organic decomposition
caused by the high temperatures combined with constant leaching means humus does not
accumulate.
•
Calcification occurs in soils in arid or very dry areas, such as deserts. Water in the soil moves
upwards towards the surface by capillary action (a process similar to sucking liquid through
a straw), driven by constant evaporation at the surface. The water, coming from a deep
water table, brings up dissolved minerals such as calcium carbonate, nitrates and other
salts, where they are deposited just below the surface in strata or layers. The humus
content of these soils is low because vegetation is sparse. In places such as the Atacama
Desert in South America, mineral deposits below the surface are sufficiently rich to allow
mining to occur.
PL
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•
Unit 11.1 Activity 3B: Papua New Guinea soils
M
1. List the United States Department of Agriculture (USDA) Soil Taxonomy name for the
following PNG soil orders:
SA
a. Soils derived from volcanic ash falls, with no strongly contrasting horizons.
b. What other soil order in PNG includes soils which come from volcanic ash falls?
c. Very young soils with little or no profiles.
2. What are the most important soil orders in Papua New Guinea?
3.a.List the most important chemical elements that plants get from soils and which are
critical to their growth. Include their chemical symbol.
b. Give two reasons why nutrients found in soils important to plant growth may not be
available to plants.
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