Download Are you ready for S279?

S279 Our Dynamic Planet: Earth and Life
Are you ready for S279?
Contents
1 Introduction
2
2 Suggested prior study
2
3 Mathematical skills
3
3.1 Self-assessed test for numeracy
3.1.1
3
Suggested further reading
4
4 Key concepts covered in prerequisite courses and developed in S279
5
4.1 Basic physical concepts
5
4.2 Chemical concepts
5
4.2.1
Atoms, molecules and compounds
5
4.2.2
Relative masses of molecules and compounds
6
4.2.3
Chemical bonding, valency and ions
6
4.2.4
Oxidation and reduction
7
4.2.5
Chemical equations
8
4.2.6
Suggested further reading
8
4.3 Biological concepts
8
4.3.1
Photosynthesis and respiration
8
4.3.2
Cells and cell division
8
4.3.3
Evolution
9
4.3.4
Suggested further reading
9
4.4 Geological concepts
9
4.4.1
Plate tectonics
4.4.2
Chemical weathering
11
4.4.3
Geological time
11
4.4.4
Suggested further reading
13
4.5 Other important concepts
9
13
4.5.1
Biogeochemical cycles
13
4.5.2
Feedback mechanisms
14
4.5.3
Suggested further reading
15
Answers to self-assessment questions
Copyright © 2010 The Open University
16
WEB 02254 5
4.1
1
Introduction
If you are intending to study S279, you will want to ensure that you have the
necessary background knowledge and skills to be able to enjoy the course fully
and to give yourself the best possible chance of completing it successfully.
Please read through these notes carefully and work through the accompanying
self-assessment questions. This will be a useful exercise for all prospective S279
students, including those who have already studied other Open University science
courses and/or have completed the recommended S279 preparatory courses (see
Section 2). Working through this booklet should serve as a reminder of some of
the facts, skills and conceptual knowledge you will be expected to have prior to
studying S279.
If you are coming to S279 without having studied a Level 1 science course at The
Open University, then it is essential that you establish whether or not your
background and experience will give you a sound platform from which to tackle
the work. If you find that you can answer most of the questions in these notes,
then it is likely that you are well prepared to take on S279. If, however, you find
that you have substantial difficulties with more than about five questions, then it
may be worthwhile taking a course that would prepare you for S279 instead (see
Section 2).
If you are still not quite sure whether S279 is the course for you, please seek
further help and advice from the Student Registration and Enquiries Service:
email: [email protected]
tel: +44 (0)870 333 4340
S279 Our Dynamic Planet: Earth and Life takes a holistic view of the Earth, and
requires a basic level of understanding of biology, chemistry and some physical
processes, in addition to geology (the Earth sciences), to appreciate how the
various components of the ‘Earth System’, i.e. the atmosphere, hydrosphere,
biosphere and geosphere, are linked. It also requires a thorough understanding of
basic maths skills and the ability to apply this understanding. The course as a
whole is attempting to address the issue of the changing conditions of the Earth
through time – and should encourage you to think about how human activity is
influencing these changes.
2
Suggested prior study
Recommend OU courses: S104 Exploring Science; S276 Geology; S151 Maths
for Science; S283 Planetary science and the search for life.
Before embarking on S279 you should have studied and passed the Level 1
science course, S104 Exploring Science (or one of its predecessors S103 or S102),
as S279 assumes that you have an understanding of basic scientific skills
equivalent at least to this level.
S279 also requires a basic understanding of mathematics, including algebra,
powers of ten, significant figures and the use and interpretation of graphs,
comparable to the Level 1 course S151 Maths for Science.
S279 forms one of two core Earth sciences courses at Level 2 and will build on
the general geological concepts developed in the other core course, S276 Geology
(and its predecessor S260). As such, we recommend that you study S276 before,
2
rather than at the same time as, S279. S283 is also very useful preparation and
should be studied before S279.
If you completed your study of science several years ago, we recommend that you
revise the key concepts listed in Section 3 before you begin the course. Your
Regional Centre can provide details of where to find reference copies of
S104 Exploring Science, or you can buy selected materials from: Open
University Worldwide, The Berrill Building, Walton Hall, Milton Keynes
MK7 6AA.
3
Mathematical skills
The amount of maths in S279 is significant, and essential. Throughout the course,
graphs are commonly used to show the relationship between two variables.
Angles are measured in degrees and the sine, cosine and tangent of angles are also
used at various points throughout the course. Several algebraic equations are
used, and you are asked to manipulate a few, relatively simple equations. You
should be able to put numerical values into algebraic equations to obtain the
required answers, calculate areas and volumes, and express quantities as ratios,
fractions and percentages. You also need to be familiar with positive and negative
powers of ten and quoting numerical answers to the correct number of decimal
places and significant figures. You do not have to memorise equations or the
values of physical constants, as, if needed, these will be given in the examination
paper.
The following self-assessment questions outline some of the key mathematical
concepts needed for S279.
3.1 Self-assessed test for numeracy
Question 1
Express the following numbers in scientific notation:
(a) 100 000 000
(b) 400 000 000 000
(c) 3500
(d) 95 × 105
(e) 0.51 × 103
(f) 0.00025
(g)
1
1 000 000
(h) 0.0035
Question 2
Complete the following calculations and express your answers
(a) using the appropriate units and number of decimal places:
(i) 0.43 m + 1.217 m
(ii) 8.1 kg – 3.82 kg
(b) using the appropriate units and number of significant figures:
3
(i) 2.373 m × 3.6 m
(ii)
6342 kg
2.42 m 3
Question 3
Suppose x =
2z 2
and a = 4zr.
a
Combine these two equations to give an equation for x that does not involve a.
Question 4
Use your calculator to work out the following, expressing your answers to two
significant figures:
(a) sin 60°
(e) sin 45°
(b) cos 60°
(f) tan 10°
(c) cos 30°
(g) cos 10°
(d) tan 45°
Question 5
Study Figure 1 and then answer the following questions. lava flow/litres per second
200
150
100
50
0
00.00
04.00
08.00
12.00
16.00
20.00
24.00
time/hours
Figure 1 The volume of a lava flow flowing past a fixed point per second during
an eruption.
(a) What was the flow of the lava at 12:00 hours?
(b) What was the maximum flow of the lava and at what time did it occur?
(c) Describe in words how the lava flow has changed over the period plotted on
the graph.
3.1.1 Suggested further reading
S151 Maths for Science.
Northedge, A., Thomas, J., Lane, A. and Peasgood, A (1997) The Sciences Good
Study Guide, Open University Worldwide. (Especially Chapters 3–5)
4
4 Key concepts covered in prerequisite
courses and developed in S279
This section outlines the main scientific concepts that you should be familiar with
before beginning S279. These are all introduced in the Level 1 science course and
developed further in S279. (All references are to Books in S104 Exploring
Science and not to S279. Where appropriate, references are also made to specific
Level 2 Open University courses.)
4.1 Basic physical concepts
Force and energy are physics concepts that you should be familiar with and that
will be developed further in S279.
Force: S104 Exploring Science, Books 3 and 7 (also S207 The Physical World,
Book 2)
Newton’s second law states that the magnitude of an unbalanced force (F) on an
object is equal to the object’s mass (m) multiplied by its acceleration (a):
F = ma
where F is measured in newtons (N) and 1 N = 1 kg m s−2.
Newton’s law of gravitation states that two bodies of mass m1 and m2 separated
by a distance (d) between their centres attract each other with a gravitational force
(Fg) proportional to the product of their masses (m1m2) and inversely proportional
1
to the square of the separation distance ( 2 ).
r
Fg =
Gm1m2
r2
where G is the gravitational constant equal to 6.67 × 10–11 N m2 kg−2.
Energy: S104 Exploring Science, Book 3 (also S207 The Physical World,
Book 2)
The law of conservation of energy states that the total amount of energy in an
isolated system is constant, as energy can neither be created nor destroyed.
Question 6
The mass of the Earth is 5.98 × 1024 kg and the mass of the Moon is
7.35 × 1022 kg. On average, they are 384 500 km apart. Calculate the gravitational
force on the Earth due to the Moon. What is the force on the Moon due to the
Earth?
4.2 Chemical concepts
An understanding of chemical concepts underpins many of the processes
discussed in S279.
4.2.1 Atoms, molecules and compounds
You should be familiar with the idea that all ‘matter’ is composed of atoms.
Every atom contains a nucleus at its centre which contains protons and
neutrons. The nucleus is surrounded by particles known as electrons. Protons are
5
positively charged, neutrons have no charge and electrons are negatively charged.
Protons and neutrons each have approximately the same mass, whereas the mass
of an electron is negligible. The atom of any element is characterised by the
number of protons in its nucleus – called the atomic number, Z. Carbon for
example has six protons, whereas nitrogen has seven and oxygen has eight.
Different atoms of the same element, which differ only in the number of neutrons
they contain, are known as isotopes of that element, and have different atomic
masses. The mass of any atom is expressed by the relative atomic mass, which
compares the mass of that atom to the atomic mass of the isotope of carbon that
has six neutrons. On this scale, the relative atomic mass of 12C is defined as
12.000. A particular isotope of an element can be specified by the mass number,
A, which is the relative atomic mass rounded to the nearest whole number; it
gives the total number of protons and neutrons in an atom.
Question 7
The element carbon has an atomic number of 6. Carbon has three isotopes: 12C,
13C and 14C.
(a) How many neutrons do the isotopes 13C and 14C contain?
(b) How many electrons does a (neutral) atom of 14C contain?
Atoms may combine with other atoms. Some common elements exist as diatomic
molecules rather than simple atoms, e.g. the gases oxygen (O2), hydrogen (H2),
nitrogen (N2) and chlorine (C12). Atoms may also combine with atoms of
different elements to form compounds, e.g. calcium carbonate (CaCO3). In each
case, the chemical formula indicates the relative numbers of the different atoms
combining together.
Question 8
(a) How many atoms of oxygen are there in a molecule of oxygen?
(b) What are the relative numbers of the three different atoms calcium (Ca),
carbon (C), oxygen (O) in the compound calcium carbonate (CaCO3)?
4.2.2 Relative masses of molecules and compounds
The relative mass of molecules and compounds is the sum of the relative atomic
masses of all the atoms in the chemical formula.
Question 9
The relative atomic masses of some common elements are as follows: hydrogen
(H) = 1.01, carbon (C) = 12.0, oxygen (O) = 16.0, calcium (Ca) = 40.1. What are
the relative masses of:
(a) water (H2O)?
(b) carbon dioxide (CO2)?
(c) calcium carbonate (CaCO3)?
4.2.3 Chemical bonding, valency and ions
The electrons surrounding the nucleus of an atom occupy energy levels, or shells,
each of which is capable of holding a different number of electrons. The
relationship between the electrons and the electron shells is referred to as the
electronic configuration of an atom. The most stable electronic configurations are
6
those of the noble gases (e.g. helium, neon and argon), which take part in few
chemical reactions. Atoms chemically bond with other atoms in order to achieve
the stable electronic configuration of a noble gas. This can be achieved by either:
(i) transferring electrons (ionic bonding) to form positive or negative ions, or
(ii) sharing pairs of electrons (covalent bonding) to form molecules.
Calcium carbonate is an example of an ionic compound (it consists of positively
charged calcium ions and negatively charged carbonate ions) whereas the atoms
in the gases hydrogen (H2) and carbon dioxide (CO2) are covalently bonded.
Metals such as sodium (Na), calcium (Ca), magnesium (Mg) and iron (Fe) may
form ionic bonds with other atoms, and themselves become positively charged
ions.
In simple terms the valency of an atom is the number of bonds that an atom can
form within a molecule or compound. When bonding is ionic, valency can be
considered as the number of electrons gained or lost in the transfer. When
bonding is covalent, it is equal to the number of covalent bonds that an atom
forms with other atoms.
Question 10
In ionic compounds, sodium forms Na+ ions and calcium forms Ca2+ ions.
(a) What are the valencies of sodium and calcium?
(b) When calcium reacts with chlorine, calcium chloride (CaC12) is formed,
comprising calcium ions and chloride ions (i.e. charged chlorine (C1) atoms).
What is the charge on each chloride ion, and what is the valency of chlorine
in CaC12?
4.2.4 Oxidation and reduction
Oxidation is said to occur when the proportion of oxygen in a compound
increases, or the proportion of hydrogen decreases. Conversely, reduction occurs
when the proportion of hydrogen in a compound increases, or the proportion of
oxygen decreases. Any chemical reaction leading to oxidation of one substance
must be accompanied by the reduction of another substance, and vice versa. The
term oxidation is also used more generally to include any reaction in which an
atom loses electrons. For example, the change of an iron(II) ion, Fe2+, to an
iron(III) ion, Fe3+, is an oxidation (and the reverse process is a reduction). A
reaction which causes iron to share more of its electrons to form bonds can also
be considered as an oxidation reaction.
Question 11
(a) When oxygen forms ionic compounds with iron, negatively charged ions are
produced, O2−. What is the valency of oxygen?
(b) What is the charge on the iron ion in the following iron oxide compounds?
(i) FeO
(ii) Fe2O3? (Remember that in each case the compound must be electrically neutral.) (c) When FeO is converted into Fe2O3 is it reduced or oxidised?
7
4.2.5 Chemical equations
Reactions between various elements and compounds are written as chemical
equations showing the reactants on the left-hand side and the products on the
right-hand side. The reactants and products are linked by an arrow indicating the
direction of the reaction. As atoms are neither created nor destroyed during
chemical reactions, the number of atoms of each type present on one side of the
equation must be the same as the number of each type on the other side, i.e. the
equation must be balanced.
Question 12
During the process of photosynthesis green plants use atmospheric carbon dioxide
(CO2) and water (H2O) to produce glucose (C6H12O6). Balance the following
overall chemical equation for this reaction, so that you have the correct numbers
of molecules of the reactants.
CO2 + H2O → C6H12O6 + 6O2
4.2.6 Suggested further reading
S104 Exploring Science:
�
Book 4 ‘The right chemistry’ (basic concepts in chemistry, the Periodic
Table, chemical bonding and chemical reactions)
�
Book 8 ‘Quarks to quasars’ (atomic structure)
4.3 Biological concepts
A key biological concept that recurs in S279 is the relationship between
photosynthesis and respiration. In addition, you will need to understand the basic
structures of different types of cells, how cell division takes place and the basic
principles of evolutionary theory. Although all these concepts are re-taught in
S279, you will find the course less demanding if you already have some previous
knowledge of them.
4.3.1 Photosynthesis and respiration
Photosynthesis is the basis of almost all of the primary production on the Earth.
Using the energy derived from sunlight, green plants manufacture carbohydrates
(glucose) from the raw materials of water and carbon dioxide by the reaction
shown in the answer to Question 6. There is a related reaction called respiration
in which oxygen is used to break down carbohydrate, producing carbon dioxide
and water again.
Question 13
(a) Write down the overall chemical equation for respiration.
(b) Explain which of the reactants in your equation is oxidised during the
reaction, and which is reduced.
4.3.2 Cells and cell division
At the broadest level organisms are classified into three domains: Archaea,
Bacteria and Eukarya. All Archaea and Bacteria have prokaryotic cells and most
are unicellular. Eukarya include plants, animals and fungi; they have eukaryotic
cells and are mostly multicellular. There are fundamental structural differences
between prokaryotic and eukaryotic cells, and between the features of plant and
animal cells.
8
Question 14
The items in the following list describe features of cells. For each, state in which
of the following three types of cell it occurs: a prokaryotic cell, a eukaryotic
animal cell and/or a eukaryotic plant cell.
(a) contains a nucleus;
(b) possesses cell walls;
(c) contains organelles;
(d) DNA is contained within the nucleus;
(e) contains chloroplasts;
(f) DNA is free within the cytoplasm. Cell division in eukaryotes takes place either by mitosis or meiosis. Question 15
Which type of cell division takes place during growth?
4.3.3 Evolution
Over the course of geological time, plants and animals have evolved; in other
words they have changed both in their morphology and their genetic make-up,
resulting in new species. The basis for evolutionary change is the operation of
natural selection upon heritable variation in organisms. Such variation is
underwritten by the genes of the organism. These are composed of long strands of
the complex organic molecule, deoxyribonucleic acid (DNA). Variation may be
brought about by recombination, the genetic ‘shuffling’ that takes place as the
result of sexual reproduction, and by mutation, involving spontaneous changes in
the composition of the DNA. Eventually, the changes may be so significant that
the DNA of related organisms becomes incompatible (i.e. they can no longer
interbreed) and thus new species arise.
Question 16
Is recombination a feature of mitosis or meiosis?
4.3.4 Suggested further reading
S104 Exploring Science:
�
Book 5 ‘Life’ (photosynthesis, mitosis, meiosis, genetics and evolution)
�
Book 6 ‘Exploring Earth’s history’ (evolution, the fossil record)
4.4 Geological concepts
The three most important concepts with which you should be familiar are plate
tectonics, chemical weathering and geological time. These concepts are re-taught
in S279, but again you will find the course less demanding if you already have
some previous knowledge of them.
4.4.1 Plate tectonics
The outer layer of the Earth, consisting of the crust and the top-most part of the
mantle, is known as the lithosphere. The lithosphere is divided into a number of
rigid plates which are in constant motion relative to each other across the
underlying asthenosphere. Plates may consist of oceanic lithosphere alone or
9
oceanic and continental lithosphere. Plate tectonics is the name given to the
processes involving the movement and deformation of lithospheric plates.
Question 17
During plate movement the following situations may arise:
(i) divergence of two oceanic plates;
(ii) convergence of two oceanic plates;
(iii) convergence of an oceanic plate and a continental plate;
(iv) convergence of two continental plates.
(a) In which situation(s) is one plate subducted into the mantle beneath the other?
(b) In which situation(s) does an ocean ridge form and sea-floor spreading take
place?
(c) Which situation(s) are associated with volcanic activity?
(d) In which situation(s) does a continental mountain range develop?
(e) In which situation(s) is an island arc produced?
Question 18
(a) Use the following definitions to describe the mode of formation and texture
of igneous, metamorphic and sedimentary rocks. (Select two definitions for
each rock type.)
(i) The rocks have a granular or fragmental texture.
(ii) The rocks form from a molten state, either within the Earth or at its
surface.
(iii) The rocks form by the deposition of sediment and fossil remains.
(iv) The texture of these rocks is often aligned or foliated.
(v) The rocks form in the solid state, when another rock is subjected to
changing pressures and/or temperatures.
(vi) The rocks have a crystalline, interlocking texture.
(b) Place the following rock types in the correct columns in Table 1:
basalt
quartzite
chalk
schist
gneiss
sandstone
phyllite
gabbro
diorite
breccia
shale
limestone
siltstone
granite
migmatite
slate
rhyolite
greywacke
marble
peridotite
10
Table 1
Examples of common igneous, metamorphic and sedimentary rocks.
Igneous rocks
Metamorphic rocks
Sedimentary rocks
4.4.2 Chemical weathering
This is the process whereby rocks at the Earth’s surface are chemically
decomposed by reactions between the minerals they contain and slightly acidic
surface waters.
Question 19
Explain, with the aid of a simple chemical equation, the role of atmospheric
carbon dioxide in the formation of slightly acidic surface waters.
4.4.3 Geological time
Geologists routinely refer to enormous lengths of time with apparent
nonchalance. The age of the Earth is calculated at around 4.6 billion years (4600
million years, or 4600 Ma) and Earth processes take place over time periods up to
hundreds of Ma in length. Discussion of geological time generally takes place in
two ways. First, there is the relative measurement of geological time, in which
the age of one geological event or rock layer is considered as simply older or
younger than another. Second, there is absolute measurement in which the
actual age of an event or rock layer can be calculated to within a few Ma.
Relative measurements (or dating): the relative timescale is complied as the
geological column, which is divided into Eons, Eras, Periods, as shown in
Table 2. The Tertiary and Quaternary Periods are further divided into Epochs.
The age of an event is referred to by the Eon, Era, Period or Epoch within which
it falls.
11
Table 2
The geological timescale*.
Eon
Era
Period
Age of lower boundary
of Period or Era/Ma
Cenozoic
Quaternary
1.75
Tertiary
65
Cretaceous
142
Jurassic
205
Triassic
245–248
Permian
290
Carboniferous
354
Devonian
417
Silurian
443
Ordovician
495
Cambrian
540–545
PHANEROZOIC
Mesozoic
Paleozoic
CRYPTOZOIC
Proterozoic
2500
Archean
2800
Hadean
4600
*You may find slight differences in the geological timescale represented in other texts.
The timescale shown in Table 2 is the one developed in S279.
Question 20
(a) A lava flow within a succession of limestones in northern England is dated as
approximately 340 Ma old. During which geological Period and Era was it
erupted?
(b) The appearance of anatomically modern humans has been dated as
150 000 years ago. During which geological Period and Era was this?
Absolute measurements (or dating) allows the boundaries between various Eons,
Eras and Periods to be assigned an age in Ma (see Table 2). The technique used is
radiometric dating.
The isotopes of some elements are not stable, and they breakdown (or decay)
through the loss of particles at the subatomic level until they are in a stable
configuration. The length of time it takes to halve the number of original atoms is
defined as the half-life of the isotope. The proportion of the number of original
atoms of the isotope to the number of atoms of the decay product in a naturally
occurring material can be used to determine how many half-lives have elapsed
and, from this, the age of the sample can be calculated.
Question 21
The proportion of the carbon isotope 14C in the remains of an ancient wooden
shelter is only one-quarter of that found in modern-day trees. The half-life of 14C
is 5700 years, so what is the approximate age of the shelter?
12
4.4.4 Suggested further reading
It would be helpful if you were familiar with the following concepts, which are
developed extensively in S279.
S104 Exploring Science:
�
Book 1 ‘Global warming’ (an introduction to the interactions between the
atmosphere, biosphere, hydrosphere and lithosphere; climate change)
�
Book 2 ‘Earth and space’ (igneous, metamorphic and sedimentary rocks;
plate tectonics)
�
Book 6 ‘Exploring Earth’s history’ (distribution of silicate minerals in crustal
rocks; crystallisation processes; partial melting; weathering; geological time)
S260 Geology:
�
igneous, metamorphic and sedimentary rocks: Books 1–4
�
distribution of silicate minerals in crustal rocks: Books 2–3
�
crystallisation processes: Book 3
�
partial melting: Book 3
�
palaeoenvironments: Book 4.
Or S276 Geology:
�
igneous, metamorphic and sedimentary rocks: Books 1–3
�
distribution of silicate minerals in crustal rocks: Books 2–3
�
crystallisation processes: Book 2
�
partial melting: Book 2
�
palaeoenvironments: Book 3.
Geological concepts are also covered in any good elementary geological
textbook, such as:
Rothery, D.A. (2003) Teach Yourself Geology, Hodder & Stoughton.
Skinner, B.J. and Porter, S.C. (2004) The Dynamic Earth: an introduction to
physical geology, 5th edition, Wiley, Chichester.
4.5 Other important concepts
Two other important concepts that recur in S279 are the concepts of
biogeochemical cycles and feedback mechanisms.
4.5.1 Biogeochemical cycles
The Earth’s system (atmosphere, hydrosphere, biosphere and lithosphere) operate
to recycle the biologically important elements through chemical transformations
of these elements as they move between organisms (bio-) and the rest of the Earth
(geo-). The pathways that these elements take through the Earth’s systems are
known as biogeochemical cycles.
The cycle for an atom of a particular element comprises two main parts:
reservoirs where the atom is stored for various lengths of time and transfers
between the reservoirs.
13
residence time =
mass of element in reservoir
rate at which element enters (and/or leaves) reservoir
In other words, residence time is the average time that an atom of an element
spends in a reservoir.
The most important biogeochemical cycle is the carbon cycle.
Question 22
Figure 2 illustrates the main reservoirs (boxes) and transfer processes (arrows) of
carbon in and on the Earth, and the timescales involved for the return of carbon to
the atmosphere (as CO2).
(a) What are the main reservoirs for carbon in decreasing order of size?
(b) The processes by which atmospheric carbon dioxide is transferred to plants
and animals on the land are not shown on Figure 2. What are they?
(c) Which are by far the most long-term storage reservoirs for carbon?
(d) What is the residence time for carbon in the atmosphere? (Assume an average
rate of transfer of carbon from the atmosphere to the surface ocean to be
95 × 1012 kgC y−1.)
atmospheric CO2
760
120
60
60
0.4
soil and detritus
1500
burial and lithification
uplift and weathering, volcanism
90–100
90–100
plant biomass
560
preserved organic carbon
(incl. fossil fuels) ~10000000
surface
ocean
1000
36
biomass
2
mixing
upwelling
37
deep ocean
4−5
38000
sinking
detritus
33
0.4 0.6
0.05
burial and lithification
carbonate rocks
~40000000
40
marine sediments
~3000
0.2
Figure 2 Summary diagram for the carbon cycle. The approximate mass of
carbon in each reservoir is given in units of 1012 kgC (kilograms of carbon), and
the transfers (or fluxes) in units of 1012 kgC y−1. Biomass is the total mass of
organisms in a given area, expressed as the mass of carbohydrate in the dry
material.
4.5.2 Feedback mechanisms
The Earth’s systems are interlinked. Processes should not be treated in isolation
as they affect other processes. When a change in one quantity causes changes to
others, which lead to a further change in the original quantity, this is called
feedback. If an increase in a quantity leads, eventually, to a further increase in
this quantity (or a decrease leads to a further decrease), it is called positive
14
feedback. However, if an increase in a quantity leads, eventually, to decrease in
this quantity (or a decrease leads to an increase), it is called negative feedback.
Question 23
Consider the following situation: an increase in snow cover increases the amount
of solar radiation reflected from the Earth, which in turn lowers the amount of
energy from the Sun which is retained on the planet, which causes cooling. This
may allow more snow to fall and to remain for longer, which in turn increases the
reflectance, causing more cooling, which may allow more snow to fall, and so on.
Is this an example of positive or negative feedback?
4.5.3 Suggested further reading
S104 Exploring Science:
�
Book 1 ‘Global warming’ (water cycle, carbon cycle, global warming and
feedbacks)
15
Answers to self-assessment questions
Question 1
(a) 1.0 × 108
(b) 4.0 × 1011
(c) 3.5 × 103
(d) 9.5 × 106
(e) 5.1 × 102
(f) 2.5 × 10–4
(g) 1.0 × 10–6
(h) 3.5 × 10–3
Question 2
(a) You were asked to quote to your answer to the appropriate number of decimal
places:
(i) 0.43 m + 1.217 m = 1.647 m, which should be quoted as 1.65 m to two
decimal places. Note: the number of significant figures in the result
should be the same as the measurement with the fewest significant
figures.
(ii) 8.1 kg – 3.82 kg = 4.28 kg, which should be quoted as 4.3 kg to one
decimal place. Note: the number of decimal places in the result should be
the same as the measurement with the fewest decimal places.
(b) You were asked to quote to your answer to the appropriate number of
significant figures:
(i) 2.373 m × 3.6 m = 8.5428 m2, which is 8.5 m2 to two significant figures.
6342 kg
(ii)
= 2620.6612 kg m−3, which is 2.62 × 103 kg m−3 to three
2.42 m 3
significant figures.
Question 3
Since a = 4zr (second equation), we can eliminate a from the first equation by
substituting 4zr in its place:
x=
2z 2 2z 2
z
=
=
a
4zr 2r
As z2 = z × z, one of these z terms will cancel the z on the bottom of the fraction:
x=
2z × z
4zr
x=
z
2r
So:
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Question 4
(a) sin 60° = 0.87
(b) cos 60° = 0.50
(c) cos 30° = 0.87
(d) tan 45° = 1.0
(e) sin 45° = 0.71
(f) tan 10° = 0.18
(g) cos 10° = 0.98
Question 5
(a) At 12:00 hours, the lava flow was about 95 litres per second. (To find this
flow, you first locate 12:00 hours on the horizontal axis, and then draw a
vertical line from there up to the point where it intersects the curve. From this
point you then draw a horizontal line to meet the vertical axis, and you read
the appropriate flow from the scale. In this case, the line intersects the scale
one division below 100. Since ten divisions correspond to 50 litres per
second, one division corresponds to 5 litres per second, and so the flow is
(100 – 5) litres per second, or 95 litres per second.)
(b) The maximum flow is about 155 litres per second, and this occurs at
approximately 08:00 hours. (The maximum flow corresponds to the peak of
the curve; by drawing horizontal and vertical lines from the peak to the axes
you can read off the flow and the time, respectively.)
(c) The flow was slow and steady at 20 litres per second until 04:00 hours. It then
increased very rapidly for about 2 hours. After this it increased more slowly
for approximately 1 hour before increasing more rapidly once again for a
further hour. At 08:00 hours, a maximum flow was reached. After this time,
the flow then started to decrease; there was a small peak at about 11:00 hours,
and the flow gradually decreased until it became fairly steady after 20:00
hours.
Question 6
Fg =
Gm1 m 2
r2
Substituting values:
Fg =
(6.67 ×10−11 N m 2 kg −2 ) × (5.98 ×1024 kg) × (7.35 ×1022 kg)
(3.845 ×108 m) 2
Fg = 1.98 × 1020 N (to 3 significant figures).
This is also equal to the magnitude of the force on the Moon due to the Earth.
Question 7
The atomic number of 6 for carbon refers to the number of protons in the atom,
whereas the mass number (written as a superscript preceding the chemical
symbol) is the number of protons plus neutrons.
(a)
13C
therefore contains 13 – 6 = 7 neutrons. 14C contains 14 – 6 = 8 neutrons.
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(b) In a (neutral) atom, the number of electrons equals the number of protons.
There are 6 electrons in an atom of 14C (and all other isotopes of carbon).
Question 8
(a) The subscript ‘2’ in O2 indicates that there are two oxygen atoms.
(b) The symbols for carbon and calcium have no subscripts, so there is only one
atom of each. However, the symbol for oxygen has the subscript ‘3’ (O3) so
there are three atoms of oxygen indicated in the chemical formula. The
relative numbers of atoms indicated by the chemical formula are therefore
1 calcium, 1 carbon, 3 oxygen.
Question 9
Relative masses of molecules and compounds are calculated by summing the
relative atomic masses of the elements according to the weighting indicated by
the chemical formula.
(a) A molecule of water contains two hydrogen atoms and one oxygen atom, so
its relative mass is (2 × 1.01) + 16.0 = 18.02 (or 18.0 to 3 significant figures).
(b) A molecule of carbon dioxide contains one atom of carbon and two atoms of
oxygen so its relative mass is 12.0 + (2 × 16.0) = 44.0.
(c) The compound calcium carbonate contains calcium, carbon, oxygen in the
ratio 1:1:3 so its relative mass is 40.1 + 12.0 + (3 × 16.0) = 100.1 (or 100 to 3
significant figures).
Question 10
(a) The sodium ion carries only a single positive charge (+), implying that only
one electron has been transferred, therefore sodium has a valency of 1. The
calcium ion carries two positive charges (2+), implying that two electrons
have been transferred, therefore calcium has a valency of 2.
(b) The charge on the calcium ion is 2+, so two electrons have been transferred to
chlorine atoms to form chloride ions. Calcium chloride contains twice as
many chloride ions as calcium ions so each chloride ion must have received
one of the electrons. Therefore the charge on each chloride ion that is formed
is a single negative charge (C1–), and so the valency of chlorine must be 1.
Question 11
(a) The superscript ‘2−’ on the symbol for oxygen indicates that two electrons
have been received, so the valency of oxygen is 2.
(b) (i) In FeO, the ratio of Fe to O is 1:1, so the iron must be in its ferrous form,
Fe2+, in order to balance the 2− charge on the oxygen.
(ii) In Fe2 O3 the ratio of iron atoms to oxygen atoms is 2:3. The total charge
contributed by the oxygen ions is 3 × (2−) = 6−. In order to maintain
electrical neutrality the iron must be in its ferric form, Fe3+, i.e. 2 × (3+) =
6+.
(c) It is oxidised. The ratio of Fe to O in FeO is 1:1 and in Fe2O3 is 1:1.5. Also
Fe2+ has converted into Fe3+, with the loss of an electron.
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Question 12
The balanced equation is:
6CO2
(6 × C, 12 × O)
+
6H2O
→
(12 × H, 6 × O)
C6H12O6
(6 × C, 12 × H, 6 × O)
+
6O2
(12 × O)
Question 13
(a) The equation or respiration is the reverse of that shown in the answer to
Question 12:
C6H12O6 + 6O2 → 6CO2 + 6H2O
(b) During respiration the carbohydrate is oxidised to carbon dioxide (by removal
of hydrogen) and the oxygen is reduced to water (by addition of hydrogen).
Note that this is effectively the reverse of photosynthesis, during which
carbon dioxide is reduced to carbohydrate by the addition of hydrogen and
water is oxidised to oxygen by the removal of hydrogen.
Question 14
The items are features of the cells as follows:
(a) A eukaryotic cell contains a nucleus
(b) Prokaryotic cells and eukaryotic plant cells possess cell walls
(c) Eukaryotic cells contain organelles
(d) Eukaryotic cells have DNA contained within the nucleus
(e) Eukaryotic plant cells contains chloroplasts
(f) Prokaryotic cells have DNA free within the cytoplasm.
Question 15
Mitosis is the type of cell division that takes place during growth. Mitosis
produces diploid cells each with a set of chromosomes identical to that produced
by the parent cell.
Question 16
It must be a feature of meiosis. The progeny cells are genetically different to the
parent cell, which indicates that genetic shuffling has taken place. (The progeny
cells from mitosis are genetically identical to the parent cell (see Question 9).)
Question 17
(a) (ii) and (iii). During oceanic/oceanic plate convergence either plate may be
subducted as both have the same density. During oceanic/continental plate
convergence, the oceanic plate is always the one to be subducted as it is
denser than the continental plate.
(b) (i). Where two oceanic plates diverge the hot, underlying asthenosphere rises
up in the gap and an ocean ridge is formed. Igneous activity at the ridge helps
to push the two plates apart by the process of sea-floor spreading.
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(c) (i), (ii) and (iii). Melting the upper mantle beneath an ocean ridge, results in
igneous activity, including submarine volcanic eruptions. Partial melting of a
subducted plate occurs as it descends into the hotter mantle. The heat of the
rising magma may partially melt the overlying plate as well. Magma reaching
the surface gives rise to volcanic activity.
(d) (iii) and (iv). In situation (iii) a volcanic mountain range forms along the
continental margin, e.g. the South American Andes. In situation (iv) both
plates are too buoyant (low density) for either to be subducted into the denser
mantle and so convergence results in crumpling of the continental margins
and the folding of the continental margin sediments to produce a mountain
range, e.g. the Himalayas.
(e) (ii). Volcanic activity above the plate that is being subducted leads to the
formation of volcanic islands on the surface of the plate that is not subducted,
e.g. the Philippine Islands or the Caribbean Islands.
Question 18
(a) Igneous rocks: (ii) and (vi); metamorphic rocks: (v) and (iv); sedimentary
rocks: (i) and (iii).
(b) Check your answers with Table 1 (completed) overleaf.
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Table 1 (Completed) Examples of common igneous, metamorphic and
sedimentary rocks.
Igneous rocks
Metamorphic rocks
Sedimentary rocks
basalt
quartzite
chalk
gabbro
schist
sandstone
diorite
gneiss
breccia
granite
phyllite
shale
migmatite*
migmatite*
limestone
rhyolite
slate
siltstone
peridotite
marble
greywacke
*Migmatite (meaning ‘mixed rock’) can be placed in either the igneous or metamorphic
column, as it represents a transitional state, where a rock under high grades of
metamorphism is just beginning to melt.
Question 19
Carbon dioxide from the atmosphere dissolves in rain and river water. Hydrogen
ions (H+) and bicarbonate ions (HCO3−) are produced. It is the H+ ions that make
the water acidic.
H2O + CO2 → H+ + HCO3−
(Organic acids in soils increase the acidity of soil water.)
Question 20
(a) The Carboniferous Period of the Paleozoic Era.
(b) The Quaternary Period of the Cenozoic Era.
Question 21
For the proportion of 14C to be reduced to one-quarter, the number of 14C atoms
must have halved and then halved again. This means that two half-lives have
elapsed, so the approximate age of the shelter is 2 × 5700 = 11 400 years.
Question 22
(a) In order of decreasing size the reservoirs are: carbonate rocks, preserved
organic carbon (including fossil fuels), the deep ocean, marine sediments, soil
and detritus, surface ocean, atmospheric CO2, plant biomass, biomass (in
ocean).
(b) From Section 4.3, you know that initially CO2 is taken up by plants in the
process of photosynthesis. Animals then consume the plants to take in carbon.
(c) The most long-term storage reservoirs for carbon are the carbonate rocks and
the preserved organic carbon. It takes hundreds of millions of years for the
carbon in these reservoirs to be returned to the atmosphere.
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(d) residence time =
mass of element in reservoir
rate at which element enters (and/or leaves) reservoir
= 760 × 1012 kgC
Mass of carbon in atmosphere
Rate at which carbon is transferred from atmosphere to plant biomass
= 120 × 1012 kgC y−1.
Rate at which carbon is transferred from atmosphere to surface ocean
= 95 × 1012 kgC y−1.
Therefore, total rate at which carbon leaves the atmosphere
= 215 × 1012 kgC y−1.
Residence time of carbon in the atmosphere
=
760 ×1012 kgC
= 3.53 years
215 ×1012 kgC y −1
Question 23
It is an example of positive feedback because an increase in snowfall leads
eventually to a further increase in snowfall.
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