Unit 11.1 The Structure of the Earth Topic 1:...

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Unit 11.1 The Structure of the Earth Topic 1: Composition of the Earth –

the four spheres 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:• Thelithosphere–TheoutersurfaceandinterioroftheEarth.• Theatmosphere–Thegases(air)thatsurroundtheEarth.• Thebiosphere–AlllivingthingsthatinhabittheEarth.• Thehydrosphere–ThewatersoftheEarth.Unit11.1Topic1givesabriefoverviewofthefourspheres.TheremainingTopicsinthisUnit cover the lithosphere in some detail.

The atmosphere is covered in more detail in Unit 11.2 ‘Natural Processes and Disasters’, in particularTopic1‘Airpressureandglobalwindsystems’(p. 97)andTopic 2‘Climatesoftheworld’(p. 111).Unit11.2alsolooksatelementsofthebiosphere,inparticularTopic 3‘Naturalvegetation’(p. 119)

Unit11.3looksatthehydrosphereandatoceanographyinparticular.

The four spheres – an introductionThe 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.

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.

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

© Oxford University Press www.oup.com.au

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 analysis1. 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.

• Hydrosphere.

• Biosphere.

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 researchWhat 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 EarthTopic 2: Composition of the Earth –

its layered structureTopic 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.

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.

What is inside the Earth?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:

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.

2. The mantle, which is made up of semi-solid rocks. The deeper you go, the more liquid the rocks become.

3. The core, which gives off enormous heat, a very small part of which reaches the Earth’s surface.

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.

Plates on the moveScientists 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.

Most plates move less than 1 cm in a year. However, the plates around Papua New Guinea are quite fast moving;

Crust

Lower mantle

Outer core

Upper mantle

Innercore

The layers of the Earth

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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).

Plates moving apartWhere 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.

Ocean ridgeOcean floor

AFRICA

SOUTH AMERICA

Trench

Lower mantle

Asthenosphere LithospherePlate 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.


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Topic 2: Composition of the Earth – its layered structure 37


Plates moving togetherMany 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 one plate gets pushed upwards and the other slides underneath and is pushed back down towards the Earth’s mantle. When cars crash, they are squashed up and twisted out of shape; when plates collide the same things happen. Rocks are twisted (folded) and sometimes big cracks (faults) appear in the earth’s surface. If the cracks are big enough, half-liquid rock from the mantle may come 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.

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.

Mountainranges

As plates move together, the Earth’s crust is ‘crumpled up’ producing fold mountain ranges. The arrows show the direction of the pressure causing the folding.

When plates move together, faults are produced in the Earth’s crust. The blocks of land between fault lines may move, producing new landforms.

Plateau Scarp

Rift valley

FAULTS

Mountain range

Plates on the surface of the Earth

Passive plateboundary

Destructive plateboundary

Constructive plateboundary


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Plate tectonics near Papua New Guinea

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Australia

AUSTRALIAN PLATE

PNG

Murua

VanuatuFiji

PACIFIC PLATE

Main collision areasOld or minor collisionsDirection of plate movement

N

How Papua New Guinea was createdNearly 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.

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


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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.

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

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.

IntroductionThe 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.

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.

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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SoilsSoil 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 characteristicsMost 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

Metamorphic rock

Igneous rock

Sand, mud, silt(sediments)

Magma

Sedimentary rock

The rock cycle

Heat, pressure deepin the earth’s crust

Cooling

Melting

Heat, pressure deepin the earth’s crust

(deep in the Earth’s crust)

Erosion, weathering

Erosion, weathering

Erosion, weathering

Transport by water and wind;compaction

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.


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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.

Soil profileThe 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.

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).

Unit 11.1 Activity 3A: SoilsIn a brief paragraph, describe how climate and relief can influence soil type.

O – Organic material at the surface.

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 GuineaThe 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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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.

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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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.

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.

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.

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.

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.

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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Soil processesWater 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 mineral-deficient. 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.

Unit 11.1 Activity 3B: Papua New Guinea soils1. List the United States Department of Agriculture (USDA) Soil Taxonomy name for the

following PNG soil orders:

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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