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EARTH, ENVIRONMENT AND SOCIETY 1901
THEME 1 – GEOLOGY
Shi-Ling Kou 2011
LECTURE 1-2 – INTRODUCTION
• Geologic theory of relativity
• Tools for objective and quantitative analysis of climate and environmental challenges
LECTURE 3 – SCIENTIFIC METHODS AND PRINCIPLES IN GEOLOGY
• Experimental procedures – observation-hypothesis experimentation
• Scientific method: fact (verifiable observation), hypothesis (proposed explanation), theory
(hypothesis that has gained broader acceptance through repeated verification), law (theory
which is so consistent that its validity is no longer questioned)
Principles in geology – Steno (1660), Hutton (1788)
• Principle of superposition: in a sequence of undeformed sedimentary rock, the oldest
layers are at the bottom and the youngest at the surface
o Assumptions: layers were deposited horizontally, rocks haven’t been overturned
• Principle of original horizontality: under gravity, layers are originally deposited
horizontally
• Principle of lateral continuity: sediment deposition will form a continuous layer which
can end by thinning out, or by obstruction by another geologic body (eg shoreline, older
outcrop)
• Principle of Uniformitarianism: the laws that govern natural processes today are the
same as those in the past and will not change with time
• Principle of cross-cutting relationships: a geological body must be younger than the
other body which it cuts across
o Can use to work out relative ages of rocks
• Principle of inclusion: rock inclusions are older than the surrounding rock which
envelopes it
Related Principles and other ideas
• Angular unconformities are interruptions in rock-forming processes which reveal
significant developments in geologic time eg Siccar Point, SE Scotland
• Disconformities occur when erosion strips off the top of an older sequence and/or cuts
channels into the older beds but there is no structural discordance between the older,
eroded rock and the younger, overlying rock
• Catastrophism (opposite of Uniformitarianism) – Earth was created in a series of
catastrophic events and have remained relatively unchanged since
• Correlation of rock formations – by looking at surrounding rock types can correlate a
particular rock formation between 2 different locations
o Same sequence of rock strata in different places can suggest rocks are similar
ages
• Principle of faunal succession: groups of fossil animals and plants occur in the geologic
record in a definite chronologic order, thus in relation to superposition the occurrence of
particular fossils can be used to date and relative-date rocks
• Succession in landscape development: a feature’s relative age can be determined from
degree of erosion
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Incomplete geologic records
Geologic change and deep time
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Precambrian eon (80% earth time), Phanerozoic eon (540Ma) ! Paleozoic,
Mesozoic, Cenozoic
Radiometric dating (half life): C-14 (5730y), K-40 (1.25bn), Rb-87 (48.8bn), Sm-147
(106bn), Th-232 (14bn), U-238 (4.5bn)
Tree rings
Varves (rhythmic changes in sedimentary environment), ice layers, layered volcanic
rocks
LECTURE 4 – THE ROCK CYCLE
Igneous rocks
• Igneous rock: a rock formed from the cooling and solidifying of magma and/or lava
• Magma: molten rock originating from partial melting of the lower crust and upper mantle
(10-200km below Earth’s surface)
o Less dense than solid rock so often migrates upward through the mantle
o Magma composition dependent on: rock composition, partial melting, fractional
crystallisation, mixing and assimilation of solid rock into molten magma
• Texture of rock provides insight into cooling history:
o Glassy: cools very rapidly, eg obsidian
o Aphanitic: very small mineral grains
o Phaneritic: large grains, slow cooling rate
o Poryphyritic: 2 stages of cooling ! slow (large grains) ! fast (small grains)
o Pryoclastic: broken fragments blown out of volcano ! ash fall or flow
• Major gases: water and CO2 (volatiles)
Types of igneous rocks
• Ranges from mafic (50% SiO2, 100-1200°C) to silicic magma (65-77%, <850°C)
• Silicic magmas produce granite-rhyolithe rocks and felsic minerals, eg quartz, K-feldspar
o Features developed: viscous lava, ash-flow tuff, composite volcanoes, collapse
calderas
o Mostly formed at convergent plate boundaries
• Basaltic magmas produce gabbro-bassalt rocks, eg Ca-plagioclase, pyroxene
o Basalt (most abundant extrusive igneous) either erupts from fissures to produce thin
lava flows covering broad areas, or from central vents to produce shield volcanoes
and cinder cones
o Mostly formed by mantle’s partial melting at divergent plate boundaries, and in rising
plumes
o More fluid than silicic magma
• Intrusions (plutons) cool beneath the surface, eg batholiths, stocks, dikes, sills, laccoliths
Sedimentary rocks
• Form by weathering and deposition of surface rocks, by the hydrological system
• Weathering and erosion, transportation, deposition, compaction, cementation
• Clastic rocks: warn, broken material deposited by streams, wind, glaciers, marine currents
etc
o Conglomerate (coarse), sandstone (well exposed), mudstone (fine; shale)
• Chemically precipitated rocks: limestone, gypsum
• Stratification, cross-bedding, graded bedding, ripple marks, mud cracks
• Rock layers can be grouped into formations, which are grouped into sequences that are
bound by erosion surfaces
• Tectonic activity controls sedimentary systems by shaping the land
Metamorphic rocks
• Formed from other rocks by recrystallisation in the solid state, producing new minerals,
textures and structures; occurs in deep crust and upper mantle
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Sediment’s grain size correlates with transporting medium’s energy:
o Large particles are carried by rapidly moving streams (high kinetic energy)
o Small particles are transported by slowly moving streams
Wind, glaciers, shoreline currents: activity is restricted to special climate zones
Dissolved minerals carried in solution and are ultimately precipitated as limestone or salt
Human interactions
Cross-bedding: layers in a strata which are inclined at an angle – sand grains are moved
by wind/water, forming ripples/dunes ! solidify over time
Graded bedding: produced by turbidity currents
Orogeny: mountain building; evidence of crustal uplift mostly driven by plate tectonics
o Processes producing mountain belts is called orogenesis – involves folding, thrust
faulting, metamorphism, igneous activity ! moving deep rocks to surface
Case study: Sumatra earthquake and 2004 Boxing Day tsunami
o Subduction zone – 1000km section – thrust fault seductions
o Need to research to predict next seismic events eg palaeo-tsunami records (sandmud-sand-mud) ! reconstruct history of tsunami deposition ! recurrence time, 600
years
o Integrated Ocean Drilling Program (Japan – Nankai trough) – drill and sample
directly in subduction zone to discover nature of material etc
LECTURE 5-6 – PLATE TECTONICS
• History of continental drift and development of tectonic theory (Alfred Wegener, 18801930)
• Previous theories: catastrophism, drastic changes interpreted as shrinkage effects,
upheaval etc
• Evidence for continental drift:
o Morphologic: mountain belts eg Appalachian ! Scotland ! Norway, structure and
rock type
o Palaeontologic: fossil evidence over many continents
o Palaeoclimate: evidence of glaciation in places now too warm; Wegner predicted
tropical coals, reefs, subtropical deserts and evaporites by placing Pangaea over the
south pole
o Palaeomagnetic: layered lava flows in southern ocean reveal reversals in polarity,
magnetic polar wandering curves (continents have drifted in respect to the pole,
which has roughly stayed in the same region)
o Sediment age: radiometric dating shows youngest rocks at divergent boundaries.
Deep sea drilling confirms that deep ocean sedimentation is much less than if plates
were still.
• Evidence for plate tectonics (widely accepted by 1960s-70s):
o Distinct pattern of earthquake and volcano distribution at plate boundaries
o Sea-floor topography (discovered in 1950s when hunting for subs) – trenches,
mountain ranges
o Young rocks at mid-ocean ridges (varying age of sea floor)
• The Wilson Cycle (ocean dynamics) describes birth and death of continents/oceans
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New Zealand: calculate velocities from a combination of GPS campaigns (up to 5 GPS
campaigns in any 1 region) – 8 years of data
LECTURE 7-8 – THE GREAT BARRIER REEF AND CLIMATE CHANGE
Modern reef growth window
• Growth window of reef:
o 25-29°C
o Shallow (max 150m usl), low nutrient water (25-35% salinity)
o Tropical latitude
• Distribution of modern coral reef environments:
o SE Asia, Queensland, Hawaii, Caribbean, isolated reefs in the Pacific, Brazil,
Ecuador, Red Sea Madagascar, band from India to Mauritius
o Highest concentrations Caribbean and Indo-Pacific
o Many on eastern side of continents
o Almost all limited to tropics (within ~34° N/S)
o Not near large river systems eg Amazon delta, since sediments are nutrient-rich
o Exceptions include Myrmidon Reef, GBR – 90m USL and ‘dirty reefs’ which have
much terrigenous carbonate
• Why concentration in the Pacific?
o Equatorial Pacific’s main warm currents flow in a westward direction (towards
Australia)
o Western equatorial Pacific acts as a catch for all planula larvae
o As the world’s largest tropical archipelago it contains 37% of the worlds coral reefs
and highest species diversity (581)
Major anthropogenic threats to GBR (10% already destroyed)
• Coal mining
• Ship grounding (eg Shen Neng 1, 2010), over-tourism, over fishing
• Cyanide and bombing techniques
• Runoff and turbidity due to poor mining, agriculture and forestry etc
• Direct destruction, eg Amami o shima runway built directly on top of reef (1970s)
• Climate change:
o Increased sea levels, SST (bleaching), increased temperature
o Increased dissolved CO2 ! higher rates of coral formation (CaCO3) but overall
implications are much more complex (ocean acidification)
2100 Predictions
• 40% decline in calcification, increase in bleaching frequency, not keep pace with sea
level rise, geographic range will shrink, framework more susceptible to breakdown, phase
shifts
Great Barrier Reef: 2000km long, 270,000km2
• N’ward movement over last 300mn years has allowed QLD climate to go from
subtropical ! tropical
• Highstand sea level: deltaic siliciclastic sands on coast, carbonate dominated foundation
• Lowstand sea level: floodplain (where GBR is now), siliciclastic dominated foundation
• Types of reefs: ribbon/barrier, crescent, lagoon, flat-topped, patches, high island, low
wooded, fringing, detached, deltaic
Major geological factors contributing to GBR development – feedback and interactions
o Subsistence
o Substrate
o Sea level and climate change – over last 2mn years
o Energy – orbital scale climate changes (105-104 years) – Milankovich Cycle
o Abiological (sedimentation)