A2.2GZ2 Environmental Geology

1
A2.2GZ2 Environmental Geology

• GEOHAZARDS

2
SUMMARY

• Introduction
• Ground instability
• Seismicity
• Groundwater rebound
• Underground gas including Methane Hydrates
• Tsunamis

3
Introduction
4

• Geohazards are geologically-based situations or
  events that carry risk to life or property
• Common examples include
• Subsidence
• Landslides
• Earthquakes
• and many others.

5
Ground Instability

• Subsidence

6

• Subsidence arises from the underground extraction
  of minerals
• Coal
• limestone
• metals
• salt
• This creates a void that migrates upwards over
  time
• The nature of the surface disturbance is complex.
  It depends on the method of extraction, the depth
  of the workings and the nature of the overlying
  geology.

7
Subsidence at Ferniehill, Edinburgh - former
limestone workings
8
Crown hole above tin workings near Redruth
(Cornwall)
9
Salt subsidence bowl, County Antrim
10
Ground Instability

• Landsliding

11

• Landsliding is one of the most common geohazards
• It is relatively well understood and remedial
  measures can be designed to control it
• Problems arise in practice from
• unrecognised areas of old landslides
• ongoing erosion (especially coasts)
• exceptional events (usually rainfall in the UK)
• These lead to unexpected or unpredictable
  landslides that are difficult to design against
  before they occur.

12
Holbeck Hall landslide, Scarborough 1993
13
Holbeck Hall hotel, Scarborough
14
Warden Point landslides, Isle of Sheppy
15
Lochearnhead landslide, summer 2004
16
Sevenoaks by-pass after reconstruction
17
Seismicity
18

• Earthquakes are a well known geohazard
• The energy of an earthquake is usually measured
  using the Richter scale, which is a logarithmic
  scale based on the surface acceleration of the
  ground
• Although the larger earthquakes occur at plate
  boundaries, smaller quakes can occur within
  plates due to movement along old faults
• Britain regularly suffers quakes up to about
  magnitude 5, which are sufficient to damage
  buildings. They are particularly prevalent along
  the line of Caledonian faulting and in the
  Carboniferous sedimentary basins.

19
January 2005
20
Groundwater rebound

• Water table rise

21

• Many cities across Europe have lost industries
  that formerly abstracted groundwater
• London
• Birmingham
• Nottingham
• This has led to a long-term rise in groundwater
  levels as the water table has recovered
• In turn this has caused damage to infrastructure
  and building foundations
• The spatial details of the rise are geologically
  controlled and often follow subsurface features,
  expecially in Quaternary deposits.

22
Groundwater levels in the Berlin area
23
(No Transcript)
24
Groundwater rebound

• Acid mine drainage

25

• A particular problem is caused when rising
  groundwater (due to cessation of pumping) enters
  disused coal or metal mines and then exits into
  surface waters
• Coal beds and metal ores contain sulphides that
  oxidise to sulphates when exposed to the air
• Incursion of ground water then forms sulphuric
  acid (H2SO4) which in turn dissolves metals
  (mainly iron) from the remaining rocks.

26
Environmental Consequences

• The general name for this problem is Acid Mine
  Drainage (AMD)
• Acid wastes reduce water pH to below biologically
  viable limits
• Fine solids of Fe(OH)3 and Al(OH)3 (leached from
  soils) blanket sediment surfaces, coat plants,
  suffocate animals prevent spawning
• High metal uptake in aquatic plants may be toxic
  to the plant or consumer

27
Mining activities
Wheal Jane
28
UK Coalfields
29
Acid wastes
30
Acid wastes Carrick Roads
(some colour adjustment to emphasise plume)
31
Breich Water (Cuthill) following treatment
32
Cuthill treatment works - settling ponds
33
Cuthill treatment works - reed beds
34
Underground gas

• Radon

35

• Radon is a radioactive gas with a half-life of
  3.8 days. It is created by the decay of uranium,
  a minor constituent of acid igneous rocks such as
  granite
• High levels of natural radon are thus associated
  with areas of granitic rock, especially in
  Cornwall, northern England and NE Scotland.

36
(No Transcript)
37
(No Transcript)
38

• Under some conditions radon can collect in
  enclosed areas such as buildings, cellars and so
  on
• It is a potential hazard since it raises the
  level of background radiation by a medically
  significant amount.

39
(No Transcript)
40
(No Transcript)
41
(No Transcript)
42
(No Transcript)
43

• The radon action level is 200 becquerels/m3. At
  this level lifetime risk of lung cancer is
  increased by about 3.
• Control measures include the installation of
  gas-proof membranes to prevent ingress and the
  use of actively pumped systems to remove radon
  from unventilated spaces.

44
(No Transcript)
45
(No Transcript)
46
(No Transcript)
47
Methane Hydrates
48
The Nature of Gas Hydrate

• Gas hydrate is a crystalline solid that consists
  of a gas trapped in a ring of water molecules
• There is no chemical bond between the gas
  molecule and the enclosing water molecules so the
  gas is effectively caged within an ice lattice.
• In fact, another name for this class of compound
  is clathrates, which is based on the Latin word
  meaning "to enclose with bars."

49

• Methane gas hydrates are a remarkable natural
  phenomenon that may represent a significant fuel
  source
• They may also constitute an important
  environmental hazard because their release could
  contribute to global warming
• Either way, researchers need to understand the
  distribution and quantities present in subsurface
  environments.

50

• Methane gas hydrates have been found in deep
  marine or sub-permafrost sediments along
  continental margins and along the Arctic Ocean
• But current methods of detection may not locate
  all deposits.

51

• In nature organic carbon from the detrital
  remains of dead organisms is converted into
  methane in two ways, biogenically (bacteria) and
  thermogenically (heat)
• Bacteria that live in marine sediments survive by
  consuming organic carbon from the dead biota
  deposited with the sediment
• This bacterial activity occurs in the top 10's of
  meters of the ocean floor.

52

• Thermogenic alteration occurs when sediments
  containing organic carbon are deeply buried
  within a sedimentary basin resulting in elevated
  temperatures
• When the temperature reaches 100C, the organic
  carbon breaks down to form methane (CH4) and
  carbon-dioxide (CO2)
• Methane formed by thermogenic alteration
  percolates up through the sedimentary pile and
  combines with water to form the methane hydrate.

53

• Methane can also exist as a gas phase (free
  methane) that is not trapped within gas hydrate
• This free methane can form thermogenically,
  biogenically or it can come from melted gas
  hydrate
• Free methane is generally found in the pore space
  within porous rocks such as sandstone.

54

• The positive buoyancy of free methane means that
  it tends to migrate up through the crust to the
  surface unless an impermeable layer of rock traps
  it
• As gas hydrate tends to cement sediments
  together it can in some circumstances act as
  traps to gas reservoirs.

55
Gas Traps
Two simple situations where layers of impermeable
sediment containing gas hydrate act as seals on
natural gas reservoirs
56
(No Transcript)
57
Hazards - Seeps, landslides, tsunamis and sinking
ships

• Underwater landslides can produce potentially
  fatal tsunamis
• Around 7000 years ago a tsunami generated by an
  underwater landslide off the Norwegian coast
  travelled across the North Sea and swamped
  Iceland and the Shetland Islands
• Its estimated that land as high as 20m above the
  sea level along the coast of Iceland was affected
  by the tsunami.

58

• Sea floor slopes on continental margins are
  stable if the slope is less than 5
• However, many continental margins with shallow
  slopes have scars from underwater landslides
• A potential trigger for shallow slope landslides
  is sudden gas release from the sediments.

59
(No Transcript)
60
(No Transcript)
61

• This can occur if the methane hydrate layer in
  the sediment becomes unstable
• The hydrate layer can melt if the temperature
  rises or there is a drop in the confining
  pressure
• Melting suddenly releases the methane trapped in
  the hydrate along with any natural gas trapped
  below the hydrate layer.

62
Sediment without Methane Hydrate
120m sea level drop
120m sea level drop
Methane released to atmosphere
Sediment without Methane Hydrate
Temperature
63
(No Transcript)
64

• Melting suddenly releases the methane trapped in
  the hydrate along with any natural gas trapped
  below the hydrate layer
• Twenty thousand years ago an ice age resulted in
  the formation of large ice cap that covered much
  of northern Europe and Canada, and resulted in a
  120m drop in sea level
• The drop in sea-level reduced the pressure at the
  sea floor (due to the fact that there was less
  overlying water).

65

• A drop in sea-level reduces the pressure at the
  sea floor and causes the melting of methane
  hydrate
• The sudden release of gas results in landslides
  and slumps
• It can also result in a plume of gas rapidly
  rising to the ocean surface
• Gas in the water reduces the density of water
  leading to the loss of buoyancy of ocean going
  craft.

66

• Is this what causes the mysterious sinking of
  ships in the Bermuda Triangle

67

• Tsunamis

68
Tsunamis

• Tsunami is a Japanese word with the English
  translation, "harbour wave."
• Represented by two characters, the top character,
  "tsu," means harbour, while the bottom character,
  "nami," means "wave."
• In the past, tsunamis were sometimes referred to
  as "tidal waves" by the general public, and as
  "seismic sea waves" by the scientific community
• The term "tidal wave" is a misnomer although a
  tsunami's impact upon a coastline is dependent
  upon the tidal level at the time a tsunami
  strikes, tsunamis are unrelated to the tides.

69

• Tides result from the imbalanced,
  extraterrestrial, gravitational influences of the
  moon, sun, and planets
• The term "seismic sea wave" is also misleading.
  "Seismic" implies an earthquake-related
  generation mechanism, but a tsunami can also be
  caused by a nonseismic event, such as a landslide
  or meteorite impact.

70

• Tsunamis are unlike wind-generated waves, which
  many of us may have observed on a local lake or
  at a coastal beach, in that they are
  characterized as shallow-water waves, with long
  periods and wave lengths
• The wind-generated swell one sees at a California
  beach, for example, spawned by a storm out in the
  Pacific and rhythmically rolling in, one wave
  after another, might have a period of about 10
  seconds and a wave length of 150 m
• A tsunami, on the other hand, can have a
  wavelength in excess of 100 km and period on the
  order of one hour.

71

• As a result of their long wave lengths, tsunamis
  behave as shallow-water waves
• A wave becomes a shallow-water wave when the
  ratio between the water depth and its wave length
  gets very small
• Shallow-water waves move at a speed that is equal
  to the square root of the product of the
  acceleration of gravity (9.8 m/s/s) and the water
  depth - let's see what this implies

72

• In the Pacific Ocean, where the typical water
  depth is about 4000 m, a tsunami travels at about
  200 m/s, or over 700 km/hr
• Because the rate at which a wave loses its energy
  is inversely related to its wave length, tsunamis
  not only propagate at high speeds, they can also
  travel great, transoceanic distances with limited
  energy losses.

73
(No Transcript)
74
Causes

• Sea Floor Deformation
• Tectonic Earthquakes
• Landslides
• Rock Falls

75

• Tsunamis can be generated when the sea floor
  abruptly deforms and vertically displaces the
  overlying water
• Tectonic earthquakes are a particular kind of
  earthquake that are associated with the earth's
  crustal deformation when these earthquakes occur
  beneath the sea, the water above the deformed
  area is displaced from its equilibrium position
• Waves are formed as the displaced water mass,
  which acts under the influence of gravity,
  attempts to regain its equilibrium. When large
  areas of the sea floor elevate or subside, a
  tsunami can be created.

76

• Large vertical movements of the earth's crust can
  occur at plate boundaries
• Plates interact along these boundaries called
  faults. Around the margins of the Pacific Ocean,
  for example, denser oceanic plates slip under
  continental plates in a process known as
  subduction
• Subduction earthquakes are particularly effective
  in generating tsunamis.

77
(No Transcript)
78
Chilean Tsunami
79
Subduction Zone
80
(No Transcript)
81
Side scan Image (RNHO)
82
(No Transcript)
83
(No Transcript)
84
(No Transcript)
85
(No Transcript)
86
HMS SCOTT
87
BEFORE
88
(No Transcript)
89
WITHDRAWAL
90
(No Transcript)
91
(No Transcript)
92
(No Transcript)
93

• THE END