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CHAPTER 3
THE FORMATION OF ROCKS
SECTION 3.1. H O W IGNEOUS ROCKS ARE FORMED
Introduction
1. Igneous rocks may be defined as those rocks which have solidified from
liquid melts, or magmas as they are termed in geology. These rocks are classified
in two ways, firstly by their chemical composition and secondly by their grain
size. The grain size is largely dependent on the speed at which the rock has cooled;
if the cooling has been very slow then the individual crystals will be large and the
rock will be coarse-grained; if the cooling has been fast then the crystals will be
small, even microscopic, and the rock will be fine-grained. Thus molten lava
quickly extruded from a volcano will become a fine-grained igneous rock when it
has cooled and solidified.
2. The chemical composition of igneous rocks varies considerably and the
usual method of expressing this is by the percentage of silica ( S i 0 2 > contained.
A rock with a high silica content is termed 'acid' and one with a lower silica
content is known as 'basic'. These terms apply whether the grain-size is fine or
coarse. It is not necessary for a rock to be sent to a laboratory for chemical
analysis to determine its chemical composition; the various types may be deter­
mined, approximately enough, by field characteristics for almost all engineering
purposes. The most common types of igneous rock are as follows (their properties
and field identification are dealt with in Chapter 4):
Coarse-grained
Granite
Diorite
Gabbro
70% approx S i 0 (Acid)
60% approx Si02 (Intermediate)
50% approx S i 0 (Basic)
2
2
Fine-grained
Rhyolite
Andesite
Basalt
Igneous rocks are formed either from a complete melting of previouslyformed rock (of any type; igneous, metamorphic or sedimentary or any combina­
tion of these) or more often from magmas from deep in the Earth which had not
previously been near the surface, and which had been molten (or solid-state
above their melting point) for very long periods. If solid rock is forced down to a
sufficient depth by any means then it will melt. If subsequent forces push it to the
surface again then it will appear as an igneous rock.
3. Acid magma is much more viscous than basic magma, and this accounts for
the fact that the two most commonly found types of igneous rock are granite
(coarse-grained) and basalt (fine-grained), since basic basalts are extruded more
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Section 3.1
easily than acid rhyolites. Thus wide areas, such as the Deccan of India or much
of Northern Ireland, may be covered by sheets of basalt, which have spread out
quickly over the land or ocean floor before cooling. On the other hand acid
magmas tend to cool slowly at depth. When erosion subsequently occurs the
granites thus formed are exposed to view, such as the chain of granites at Dart­
moor, Bodmin Moor, St Austell, Land's End and the Scilly Isles, all in South­
west England (which may be connected at depth), or the immense Sierra Nevada
granite in the Western USA.
4. When igneous rocks appear on the surface as outcrops, the magma has
intruded the previously-existing rock in a number of different ways, giving rise to
differently shaped igneous bodies, the chief types of which are:
a. Dykes and sills (Figures 11 and 12). Both of these sheet-like structures
vary from a few centimetres to more than 100 metres in thickness, a most
usual thickness being about 1 to 5 metres for dykes and rather thicker for
sills. They are both injections of magma from underground into the overlying
rock, a dyke cutting more or less vertically through the bedding of the older
rock and a sill intruding roughly parallel to the bedding. Often the heat
from the liquid magma alters the surrounding rock for a short distance on
either side of the contact (rock is usually a rather poor conductor of heat)
and this phenomenon is known as 'contact metamorphism'. Dykes and sills
are always fine-grained at their edges, but in thicker intrusions the rock may
be coarse-grained towards the centre of the intrusion, where the magma has
cooled slowly.
b. Volcanic flows and ashes. These come from an active volcano and may
occur over land or over sea floor, or even flow from land to sea. As explained
above, flows of basic composition are much more common than acid flows.
When later preserved in a sequence of rocks, flows may be distinguished
from sills in two ways; (1) rocks above a flow show no contact meta­
morphism, and (2) flows do not transgress vertically from bed to bed as sills
Fig 11. Cross-section through rocks intruded by a dyke. (The stipple indicates
the area in which contact metamorphism is sometimes found)
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Section 3.1
Fig 12. Cross-section through dipping rocks intruded by a sill. (Stipple as in
Figure 11)
sometimes do (see Figure 71). Whilst some flows are of liquid magma, others
are density flows of hot gases, which can move large quantities of ash and
cinder at high speeds, and which on solidification have an aerated texture
when geologically young (sometimes termed ignimbrites). After some
geological time and burial, these small gas holes usually become filled with
secondary recrystallisation products. Ashes are also ejected from volcanoes,
sometimes to great heights, and come to rest some distance from the volcano,
both on land and out to sea. Beds of ash can eventually solidify into rock,
varying from many metres to less than a centimetre in thickness. Such rocks
may have similar engineering properties to mudstones, siltstones or sand­
stones of equivalent grain size, although in some sequences of solid
sedimentary rocks there are very fine-grained thin ashbands, known as 'bentonites' (for uses see Section 10.2, paragraph 6c and Section 12.7, para­
graph 6).
c. Volcanic necks. These are near-circular structures in plan, varying from
about 100 metres to over 1 kilometre in diameter, up which magma has
passed, usually into a once-active volcano. They may be filled with a variety
of igneous rocks, often with a conglomeratic or brecciated selection of
many different rocks. Sometimes the neck contains coarse or fine-grained
igneous rock crystallised from magma, which has come up at a late stage in
the volcano's history and solidified before reaching the surface.
d. Batholiths. These are much larger structures than necks, often elongate
in plan and ranging from one or two kilometres up to many hundreds of
kilometres in length. These usually represent the once-molten roots of
mountain chains which, after melting, have moved upwards, first to crystal­
lise and solidify and finally to become exposed on the surface when the
covering rock above has been eroded away. Batholiths are invariably made
up of coarse-grained igneous rocks. The largest ones are usually acid in
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Section 3.2
composition, made up of granite formed from the melting of continental
crust. Around the sides and at the roofs of batholiths (when these are
exposed) a zone of contact metamorphism is usually seen, with a thickness of
metres or even kilometres around a large intrusion. In addition large blocks
of pre-existing rock sometimes fall into a magma whilst the latter is still
molten, and may be much altered before the surrounding magma solidifies.
SECTION 3.2. H O W SEDIMENTARY ROCKS ARE FORMED
Introduction
1. The distinction between igneous and sedimentary rocks is that whilst the
former came from beneath the Earth's surfaces and crystallised from hot magmas,
sedimentary rocks were formed at the Earth's surface at normal atmospheric
temperature.
2. There are two main types of sedimentary rock, which will be considered
separately:
a. Those whose constituent particles have been transported to the place of
deposition, known as 'clastic' rocks.
b. Those which have been formed from nearby, either by aggregation of
organic matter or by chemical deposition.
Clastic rocks
3. These rocks are classified according to their average grain size as follows:
coarse-grained rocks are Conglomerate and Breccia (both consolidated) and
Gravel (unconsolidated); medium-grained rocks are termed Sandstone and finergrained rocks are known as Siltstone, Shale or Mudstone. The properties and
identification of these rocks are given in Chapter 4.
4. Most clastic rocks are formed under water, but some are formed on land, in
particular in desert conditions and also deposited by ice sheets in colder lati­
tudes. Desert sandstones are usually well-sorted, that is most of the constituent
particles are of approximately the same size, due to sifting by the wind. On the
other hand glacial deposits have been dumped haphazardly by melting glaciers
or ice sheets, and are usually un-sorted, with rock fragments of all sizes and shapes
jumbled to form tillites or breccias, which may be interspersed with lenses of
finer-grained rocks.
5. The clastic sedimentary rocks formed under water are usually of fairly
uniform grain size since they have been sorted to a greater or lesser extent by the
water currents which have transported them. The material is carried in the
first instance by rivers. Nearly all the transport of sediment occurs only at periods
immediately following heavy storms, since only quickly-moving water is capable
of carrying anything but the finest sediment. It is only at times of really violent
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Section 3.2
storms, such as occur perhaps two or three times in a century, that the largest
blocks are moved, e.g. Lynmouth disaster of 1953. Eventually the sediments will
come to rest in a lake, delta or sea and will compact into rock when the weight
of the overlying sediment has caused consolidation, thousands or even millions
of years later. The degree of consolidation of a rock bears no relation to its
geological age, for example Cambrian rocks laid down over 500 million years
ago near Leningrad, USSR, are much softer and weaker than rocks of the same
grain size laid down in the Alps less than 20 million years ago.
6. Sedimentary rocks formed under fresh water are often indistinguishable
from those formed under sea water, apart from the fossil animals and plants
which they may contain. Some types of animal and plant thrive in fresh water
or on land, others in sea water; only a small proportion of living things can
tolerate both fresh and salt water. Fossils are also useful in determining the age
of the rock, since different animals and plants existed at different evolutionary
stages in the Earth's history. However the age of the rock is not usually of direct
importance to the engineer, since its physical properties do not correlate with its
age.
7. Although much rock-forming sediment is deposited directly by rivers,
ocean currents and waves are also important in shifting loose sediment on the sea
floor, and even eroding submarine and coastal rock outcrops. Banks of sediment
near the edges of ocean basins or continental shelves are often unstable, and the
addition of more sediment, storm conditions, or perhaps an earthquake shock,
may cause such banks to slide into the depressions. Such subaqueous slides often
form turbidity currents of sedimentary particles in water suspension which can
reach speeds up to 50 km/h and thus spread the sediment evenly over a large
area, perhaps some tens of kilometres wide. The rocks eventually formed from
these deposits, 'turbidites', usually consist of a mixture of grain sizes, each
turbidite bed having coarser particles at its base and finer particles at its top,
termed a 'graded bed'. The tops and bottoms of beds in a turbidite sequence are
usually more parallel than other types of sedimentary rock, with each bed
extending as far as the original turbidite flow.
Rocks formed in situ
8. These fall under four headings, each with a different mode of origin:
a. Limestones, dolostones and cherts. The first two rocks are mainly com­
posed of the carbonate minerals calcite (CaCCfe) and dolomite CaMg(COa)2
respectively. Most are organic in origin, being largely made up of the remains
of fossil animals and plants; some are inorganic, made up of chemicals
precipitated from sea water in shallow high temperature conditions, some
are a mixture of the two. However they are often recrystallised during
subsequent geological time, so that both organic and inorganic limestones
may be considered to be alike by the engineer. Many limestones contain
small spherical objects, varying much in size but often about 1 mm in
diameter, called ooliths. These are carbonate aggregations formed by
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Section 33
crystallisation around a minute central particle, and kept rounded by con­
tinual wave or current action during growth. Some limestones, such as the
Jurassic limestones forming much of the Cotswold Hills in England, are
largely made up of ooliths. Chalk is another type of limestone, formed by
immense numbers of algal skeletons so small as to be invisible to the human
eye (about 10 microns diameter).
Chert, including its variety flint, consists of amorphous silica (SiC>2) often
occurring as bands or nodules within limestone sequences. The silica is
deposited originally under sea water, and concentrated into the bands or
nodules during the rock-forming period subsequent to deposition.
b. Evaporites. When sea water evaporates, usually by the sun's heat acting
in an enclosed, or semi-enclosed basin, then the various salts remain behind,
which can eventually accumulate as rock. This process occurs on the shores
of the Persian Gulf today, when sea water occasionally covers wide coastal
areas and subsequently dries out. Thick beds of salt (NaCl) and other
evaporite minerals exist under many parts of the Earth. They are not seen
at the surface except in arid areas, since the salts will have been dissolved
and carried away by circulating groundwater.
c. Coal. Some large rivers, such as the Nile, form deltas at their mouths
where sediment spreads out over a considerable area. At some periods in the
Earth's history very large deltas have been covered by vegetation, which on
death has become buried. Thick deposits of dead vegetation become coal
seams when the subsiding deltaic deposits subsequently become preserved as
rock. These seams are usually interbedded with the sand, silt and mud
brought down by the river. Coal-forming forests flourished at particular
times, for example nearly all the coal in Britain and the eastern USA is of
Carboniferous age (about 300 million years ago) and is formed from large
extinct trees and fern-like plants. However coal of other geological periods
from the Devonian onwards is to be found in many parts of the world, for
example in the Tertiary of Spain.
d. Oil and natural gas. Since they occur in rock, oil and gas are mentioned
here for completeness as geological phenomena. Both are formed from the
decay of microscopic marine animals and plants without hard parts, leaving
organic hydrocarbons which are deposited in most types of sedimentary
rocks. To become of economic importance they have to be concentrated by
subsequent geological processes into traps capable of being tapped by drill­
ing. Gas often originates by emigration from coal deposits.
SECTION 3.3.
H O W ROCKS MAY BE CHANGED
UNDERGROUND
Faulting, folding, jointing and earthquakes
1. When rocks lie at an angle to the horizontal they are known as 'dipping
rocks' (see Figure 13). The dip angle is expressed in degrees from the horizontal,
followed by the compass orientation of the maximum dip, for example 'the
limestone dips 45 degrees at 120 degrees' or sometimes simply 'Dip 45/120'.
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Section 3.3
'Strike' is by definition the direction at right angles to the maximum dip when
seen in plan view, so that on a geological map the line of outcrop at a level
surface of a thin rock bed follows the strike.
Fig 13. Block diagram to demonstrate dip and strike
2. When rocks fracture and move this is termed 'faulting' (see Figure 14). The
plane of fracture is termed a 'fault'. The amount by which the rocks appear to
have been displaced is termed 'throw', and is expressed in linear measurements,
usually metres. There are many different types of fault (see also Chapter 7), but
the most common are:
a. Normal (or gravity) fault (as in Figure 14), where relative movement
between two blocks of rock is vertical along the fault plane.
b. Tear fault, where relative movement between two blocks of rock is
horizontal along the fault plane.
c. Thrust fault, where one block of rock has been driven over another.
Fig 14. Cross-section through faulted rocks
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Section 33
3. In the first two types, the fault will appear as an approximately straight line
on a small-scale geological map; in thrust faults the line on the map will be most
irregular, since the fault plane will be approximately horizontal, and its outcrop
pattern will be very dependent on the local topography. Some tear faults may be
extremely large, for example the San Andreas fault in California USA is over
1000 km long, and has a lateral throw of more than 200 km separating two con­
tinental plates (see Chapter 2). Movement along this fault caused an earthquake,
followed by a disastrous fire in San Francisco in 1906, when the relative lateral
movement was 6 metres at one time along a large part of the fault. However
many faults may be seen in quarry faces which are completely stable today, and
whose throw may be as little as 10 mm.
4. Often when rock strata have been subjected to pressure, usually horizontal;
instead of breaking they will have buckled. This is termed 'folding' (Figure 15).
The scale of folding varies widely, on the one hand it is possible to find a piece of
gneiss perhaps 100 mm across with twenty or thirty small folds across its face;
on the other hand some folds have amplitudes of several hundred kilometres.
A syncline and anticline are illustrated in Figure 15. Sometimes folding is
associated with faulting, at other times there is no such relationship (see also
Chapter 2). When pressure has occurred from more than one direction, then
domes and basins are formed, the first with the rocks dipping outwards on all
sides, the second with the rocks dipping inwards towards the centre. When the
core of a dome is formed of a material such as salt which is plastic at the depth
and pressure concerned, this may be squeezed upwards into the overlying rock
to form a diapiric structure.
Anticline
Syncline
Fig 15. Cross-section through folded rocks
5. When folding takes place, very often cracks develop in the strata, even
though no movement may take place along them. Such cracks are termed 'joints',
and joint patterns often make up a meshwork of cracks, usually at right angles
to each other. The cracks are often enlarged, particularly in limestones, by the
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Section 3.3
subsequent passage of groundwater. Joints are usually at right angles to the
bedding planes, and can also form in thick beds without external pressure,
simply due to the internal stresses which exist in the very first stages of rock
formation, at the same time as the pore water is driven out. Jointing can also be
formed when igneous rocks cool, a common form being the hexagonal-sided
columnar jointing to be seen in many basalts. This is the origin of the impressive
structures at the Giant's Causeway, Ireland, and Fingal's Cave, Scotland.
6. After rocks have been folded, perhaps faulted, uplifted and partially
eroded, then very often they are subsequently submerged and again have fresh
sediment deposited upon them. When this second sedimentary sequence has been
changed into rock, both sequences together may be brought up to the surface
again and exposed as a rock outcrop. The junction between the first sequence and
the second sequence is known as an 'unconformity' (Figure 16). An unconformity
is recognisable in even a small outcrop by the difference in the angle of the dips
above and below the plane of unconformity. The rocks beneath an unconformity
do not necessarily have to be sedimentary rocks; where sedimentary rocks rest
upon igneous or metamorphic rocks, that is also known as an unconformity.
The difference in age between rocks below and above an unconformity varies
considerably, anything between perhaps 50 000 years and thousands of millions
of years.
Fig 16. Cross-section through an unconformity
7. Earthquakes are sudden tremors in the Earth, and usually result from
the movement of faults. The chief occurrences of earthquakes may be mapped
out as a series of belts (see Figure 17). These earthquake belts mark the margins
of continental plates (see Chapter 2). Thus in stable areas, not near the edges of
plates, such as Britain, the danger from earthquakes is small; but in areas of high
seismic activity, such as Japan or Greece, then extra care is needed in the design
and construction of any permanent, or even semi-permanent structure.
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Section 33
8. Earthquakes are measured either by local intensity or by magnitude.
The scale used for intensity is the modified Mercali scale, which is a largely
subjective scale based on the actual effect of the earthquake at the place of
measurement. The scale runs from I (detected only by seismographs), through VI
(slight damage), to XII (catastrophic). Thus the intensity of a single earthquake
will differ in value from place to place, depending on its varied local effects.
The magnitude scale, on the other hand, reflects the total energy released by an
earthquake, and will only have a single value for each shock. The magnitude is
calculated by complex equations relating the ground motions recorded by a seis­
mograph to the distance of the instrument from the epicentre. Several magnitude
scales have been proposed, but that most commonly used is by Gutenberg and
Richter, which is logarithmic. Small detectable disturbances, with energy releases
of about 6-3 x lO^ergs, have a magnitude of 0, whilst the largest, with an energy
release of about 2 x lO^ergs, have a magnitude of 8-5. There are more than a
million earthquakes each year, ranging from an annual average of one earthquake
with magnitude 8 or more, to a majority with magnitude less than 3.
Metamorphism and hydrothermal activity
9. When rocks of all types are subjected to very high temperature and pressure,
they will melt and remobilise to form igneous magmas. However since the
melting points of each of the many constituents within a rock varies widely (and
also the melting point increases with increase of pressure), some constituents will
recrystallise and reform before others. Rocks which have been noticeably altered,
but which still reflect some traces of their original bedding and structure are
termed 'metamorphic' rocks. A special case of metamorphism, contact meta­
morphism, has been mentioned above, but this is only a local phenomenon,
occurring close to intruded igneous rocks. Most metamorphic rocks are found in
much larger areas, which have been pushed down to substantial depths in the
Earth's crust, perhaps 20 km deep or more, and then raised again and finally
eroded in some subsequent period of Earth history. The whole of the Highlands
of Scotland is one such area, where regional metamorphism has occurred, inter­
spersed by a few granites where the rock has been completely melted and remobilised upwards into the metamorphic horizons. Much larger areas of metamorphic
rocks occur for example in Canada and Central Africa.
10. The four principal types of metamorphic rock are as follows:
a. Slate. Shales may have the orientation of their constituent particles
altered by a relatively small increase in pressure and temperature to form
slate. When this has occurred the rocks will become hard and brittle, and
split along directions often at angles unrelated to their original bedding
planes. This property is termed rock cleavage. Slate from North Wales was
used extensively as roofing material during the nineteenth century, and is
still common.
b. Schist. Finer-grained rocks which have become completely recrystallised
are termed 'schist'. The minerals recrystallise under high temperatures and
pressures parallel to one another; especially conspicuous in most schists
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Section 3.4
are shiny flakes of mica minerals, but many other minerals also occur. The
schistosity of a rock is similar to the cleavage of slates, but on a coarser and
less uniform scale. Sometimes the schistosity parallels the bedding planes,
at other times it lies oblique to the bedding planes, depending on the direc­
tion of the pressure at the time of recrystallisation.
c. Gneiss. When rock is coarsely recrystallised it is termed 'gneiss' (pro­
nounced 'nice'). The crystal size may be as coarse as igneous rocks such as
granite, but gneiss is distinguished from granite by banding, which may
reflect the relic structure of bedding planes and by the fact that the crystals
are preferentially orientated in one direction, as opposed to the crystals in
granite, which grow slowly from small nuclei in the liquid magmas and have
random orientation.
d. Marble. When limestones are metamorphosed, they recrystallise to form
marble. The streaks often seen in true marble are caused either by the local
separation of original impurities in the limestone, excluded on recrystallisa­
tion of the calcite, or else by the cracking or brecciation of the limestone
under stress. This geological usage of 'marble' is more restricted than the
commercial use, which incorporates any kind of limestone used for orna­
mental work under the term, whether metamorphosed or not. It is even
sometimes erroneously used to describe any sort of polished stone, perhaps
granite or even slate.
1 1 . Rocks may be slightly affected by hydrothermal activity which is the action
of water circulating deep enough to become first hot and then often superheated
under pressure, penetrating the pore spaces of rocks and acting as a lubricant in
joints. Occasionally water reaches the surface in the form of hot springs, which
are usually of local occurrence and not necessarily associated with active vol­
canic areas. Geysers, which are hot fountains of waters ejecting at sporadic
intervals, are usually confined to volcanically active areas.
SECTION 3 . 4 . H O W R O C K S M A Y B E C H A N G E D
THE SURFACE
NEAR
1 . Rock decay with little or no transport of the products is termed 'weathering';
when the rock is simultaneously removed this is termed 'erosion'. The effects of
weathering on a rock may be considered on the large scale and on the local scale.
The large scale is discussed in Chapter 5 in the description of landforms, which
also covers erosion. The local scale is also discussed in Chapter 5 under the forma­
tion of soils.
2. All rocks near the surface are affected to a greater or lesser degree by
weathering. The principal types of weathering are:
a. Surface weathering. The zone of surface weathering varies greatly, from
less than 1 mm in some rocks to perhaps as much as 2 0 0 metres in another.
Climatic conditions affect the depth of weathering; in tropical climates the
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Chapter 3
zone of weathering tends to be deeper than in temperate climates, but there
is also a difference in weathering between extremes of climate. Weathering
is achieved firstly by physical means, in which rocks may be shattered by
temperature changes, by gravity effects, and by the pressure of growing
organisms in cracks; and secondly by chemical means, in which the individual
minerals that make up a rock are dissolved or decomposed by the water,
oxygen and carbon dioxide of the atmosphere, and also by the chemicals
produced by live or decaying organisms. The usual effect is to make a
weathered zone structurally much weaker from the engineering point of view,
and thus in all site work it is most important firstly to differentiate between
weathered and unweathered rock, and secondly to try to find out the depth
of weathering, which may vary between different parts of the site.
b. Weathering below the surface. Joint systems occur in most types of rock,
both igneous and sedimentary, and joint planes make natural zones of
attack along which and down which water and chemicals in solution can
penetrate. Limestones are particularly prone to subsurface weathering of this
sort, since calcium carbonate is water soluble, and large caves and channels
can form underground. Well-known examples include the Cheddar Caves,
England, and the Carlsbad Caverns, USA. Such caves in limestones are an
engineering hazard, since the building of any heavy surface structure can
cause the sudden collapse of cave roofs.
3. Diagenesis is the name given to the processes which alter the character of a
sedimentary rock after it has been deposited, either by the reactions between the
various constituent minerals and particles with each other, or by the reactions
of the various constituents with pore or circulating fluids. Such reactions are
termed diagenetic at the lower temperature range, and metamorphic at the higher
temperature range, but there is no rigid division between the two. Diagenesis
occurs at two principal times, firstly when the original deposit is still in contact
with sea or lake water soon after the time of formation, and secondly, after this
period, when such direct contact with the original water has been removed.
Diagenesis usually goes on until the constituents and pore fluids are all in chemical
equilibrium.
4. Water also percolates through all types of rock, firstly through pores in the
rock itself and secondly along fault zones and other surfaces, such as bedding
planes and unconformities, which can lead to planes of weakness in apparently
massive rock. Professional geological advice should always be sought in regard to
large civil engineering works.
REFERENCE LIST—CHAPTER 3
BARAZANGI M and
DORM AN J, 1969
— World Seismicity Map of ESSA Coast
and Geodetic Survey epicenter data for
1961-1967. Bulletin of the Seismological
Society of America, Volume 59, No. 1.
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