A brief overview of earth's geology, delving into sedimentology, metamorphism and igneous petrology and the processes involved in each. Basic geological ideas are described including seismicity, structure and plate tectonics.
Answer: very small crystals in a glassy matrix only visible in thin section
6.
phaneritic
Answer: crystals visible with the naked eye
7.
equigranular
Answer: primary minerals are similar sized
8.
seriate
Answer: range of grain sizes for primary minerals
9.
hiatial;
Answer: bi-modial distribution of grain sizes of most primary minerals
10.
porphyritic;
Answer: two distinct grain sizes for primary minerals
Content preview
Dynamic Planet [1042]
T1 Earth Structure and Tectonics
Formation of the solar system and earth
T1.1
1. H, He and some Li were formed during the 'Big Bang' - most other naturally-occurring
elements were subsequently formed by different types of stellar nucleosynthesis in
which lighter elements undergo fusion to make heavier elements (Be and B are the
exceptions - formed by Cosmic ray fission).
2. Other than H, He, Be, B, and some Li, all other naturally-occurring elements originate
from stars that existed before the Solar System formed.
3. The Solar System c. 4.56 Ga ago from an accretion disc. Most of the mass is
contained in the Sun but there is an outwards progression from the terrestrial (rocky)
planets to the Jovian (gassy) planets.
4. Other objects include asteroids and comets.
5. Earth developed from the accretion of planetesimals and underwent early
differentiation into a nickel-iron core and a silicate-dominated mantle and crust.
6. The Moon was formed as a consequence of collision between Earth and a small
planet.
7. The atmosphere and oceans developed from volcanic gases.
8. Earth provides a 'Goldilocks' environment for life.
9. The Earth's orbital parameters (eccentricity, axial tilt and precession) govern some
systematic climatic variations.
10. Impact events have exerted a significant influence on Earth's history and that
influence will continue in the future. There are initiatives to map potential near-Earth
objects and consider mitigation strategies.
11. Asteroid mining is an aspirational activity for some.
Lecture notes
-During the Big Bang (13.6 billion years ago) the first elements were formed, the lightest
elements with atomic numbers 1,2,3,4,& 5. Gaseous nebulae created the first stars (due to
there being enough gravity). Stars began to create heavier elements later on. Higher mass
stars (those 10-100x the mass of our sun), supernova explosions or collisions between
neutron stars [collapsed core of a massive supergiant star] created the heaviest elements.
-There are two types of planet in our solar system;
1. terrestrial→ small, dense and rocky [Mercury, Venus, Earth, Mars]
, 2. Jovian→ larger, low density, gaseous [Jupiter, Saturn, Uranus, Neptune] Our solar
system also contains the sun (our closest star), asteroids [rock], comets [ice] and other
Kuiper/ Oort belt objects
-Nebula→swirling gas and dust leftover from the big bang and some heavier elements
leftover from stars/ the death of stars. it consists of volatile [low temperature melt/vaporise]
and refractory [high temperature] materials.
A portion of the nebula has a higher gravitational pull and pulls matter together creating a
swirling cloud of gas → gravitational pull increases creating an accretionary disc → much of
the matter is in the centre and creates a proto-sun surrounded by a proto-planetary disc → the
proto-sun becomes hot enough for nuclear fusion reactions to begin, also sending out solar
radiation and wind → most volatiles are pushed towards the outer edges of the planetary disc
and the refractory materials stay closer to the sun [because they are heavier] → the planetary
disc separates into rings where dust and ice collect to form planetesimals → a planetesimal
gets big enough to attract more of the dust in its path → this becomes a proto-planet once it is
100-1000km in diameter
Once the early Earth had enough gravity it became spherical/ even → high temperatures
melted iron which sank towards the centre due to its high density → metallic core formed and
began convection of hot rock → the earth cooled and formed a solid crust → a meteorite
impact blasted at the surface and the debris orbited around the earth → the debris collected
together and formed the moon
The earth cooled yet volcanic activity continued → early atmosphere consisted of H2O and
CO2 → earth later cooled enough to allow water to condense and form oceans → most CO2
dissolved in oceans leaving an atmosphere mainly consisting of H
-Our solar system provides a suitable environment for life
-Milankovitch cycles cause climatic variation
-impact events can be catastrophic for example causing mass extinctions → Chicxulub Crater
in Mexico from the KT mass extinction
-Asteroid mining is the hypothetical exploitation of asteroids and other minor planets and
near-earth objects to extract raw minerals mainly for researchers to study and examine them.
Due to mineral depletion on earth space based resources could be used for profit however
there are challenges faced: scarcity, economic cost, regulation, safety.
-B1612 is an NGO dedicated to planetary science and defence against near-Earth object
impacts such as asteroids. In 2012 the organisation announced it would build an asteroid
finding space observatory, the Sentinel Space telescope with an infrared detector that could
identify dangerous asteroids/ NEOs that posed risk of collision.
Earth's Morphology
T1.2
, 1. Earth's solid surface has a bimodal elevation distribution with maxima of 797 m
(corresponding to the continents) and -3686 m (corresponding to the oceanic abyssal
plains).
2. The highest elevation is Mount Everest (8848 m) and lowest elevation is the
Marianas Trench (-10,911 m).
3. The first-order control on the variation in elevation is the thickness and composition
of the continental and oceanic lithosphere. The continental lithosphere is thicker, and
the continental crust is thicker and lower density, than the oceanic lithosphere and
oceanic crust.
4. The principle of isostasy is based on buoyancy forces acting on the lithosphere -
meaning that it 'floats' on the underlying asthenosphere. A column of thicker and
lower mean density continental lithosphere will have a higher elevation than a
column of thinner and higher mean density oceanic lithosphere.
5. Second-order controls are primarily due to plate tectonic interactions influencing
lithospheric thickness, e.g. in zones of continent-continent collision, the lithosphere
is thickened to create mountains with higher mean elevation; in zones where oceanic
lithosphere is subducted below continental lithosphere, oceanic trenches are
generated.
6. There are substantial areas of continental shelf covered by shallow seas (<200 m).
These are underlain by continental, rather than oceanic lithosphere.
7. On a cross-section from the Pacific Ocean, through South America and the Atlantic
Ocean, to the west African coast, you should be able to identify the following major
topographical features: oceanic trench, mountain chain (Andes), continental
platform, continental shelf, continental slope, continental rise, ocean floor (abyssal
plain), mid-ocean ridge.
8. Be aware of vertical exaggeration on topographical cross-sections (vertical scale is
much larger than the the horizontal scale).
9. Human influence is recognisable in changing land-use (decreased forest and
increased urbanisation, cropland, pasture).
Lecture notes
-A high proportion of earths land is at sea level or forms parts of the Abyssal plains
-There are very few points very high or very low and the majority of earths surface is two
levels; oceanic low elevation and continental high elevation. The average elevation of
continental crust is much greater as continental plates do not subduct.
-Variations in sea level [over time or space] expose or cover the continental shelf ie more or
less of the continent → this is climate controlled
-Tectonics can also control sea level and the variation in land level
-Variations in topography;
, variations in crustal thickness → isostasy means the lithosphere will float on the
underlying asthenosphere as it is buoyant. Different thicknesses of crust means there
will be different levels of crust
oceanic and continental crust → slight variation in density and thickness. Oceanic is
thinner and denser and will sink below continental.
-Tectonic plate movement creates large scale topographical features
Major morphological features;
continents; continental platforms, mountain belts, rift valleys, continental shelves.
oceans; oceanic ridges, ocean floor [abyssal plain], oceanic islands, oceanic plateaus,
oceanic trenches.
-Isostasy can explain changes in elevation between continents and oceans. Thicker,
lighter continental crust has a higher elevation than thinner, higher density oceanic.
Minerals, rocks and geological structures and the rock cycle
T1.3
1. Minerals are formed from either native elements (rare) or chemical compounds
resulting from the combination of two or more elements. They have a specific crystal
structure, well-defined chemical composition (fixed or varying between fixed limits)
and occur naturally.
2. There are presently (2020) about 5500 internationally-recognised and 'approved'
minerals but many of them are exceptionally rare. A knowledge of about 50 common
'rock-forming' and 'ore' minerals will get you a long way.
3. The Earth's crust is dominated by the elements O, Si, Al, Fe, Ca, Na, K and Mg. Of
these, O and S are most important and, consequently, silicate minerals are dominant.
4. It is typically non-silicate mineral groups, such as sulphides, oxides and carbonates,
that are of interest for mineral extraction because: (a) they have a higher content of
metals of interest, and (b) it is commonly easier to separate these metals out of
these mineral groups than silicates (i.e. less use of energy and reagents).
5. Potential adverse environmental impacts of mineral extraction also vary between
mineral groups. Sulphides, in particular, can be associated with the development of
acid mine drainage / acid rock drainage that is a major global issue.
6. Igneous rocks form from the cooling and crystallisation of a silicate-dominated melt
(exceptionally rarely other melt types such as carbonate).
7. Magma includes melt plus minerals / gases crystallised / exsolved from the melt and,
commonly, inclusions of rock from the source melting region and/or other rocks
through which the magma has passed.
8. Intrusive igneous rocks result from magma being injected (emplaced) below the
Earth's surface; they include intrusive sheets (tabular bodies with high length to
thickness ratio) such as dykes (cut across host rock layering) and sills (parallel to host
rock layering) that are developed when magma is emplaced into fractures. Plutons
are large, non-tabular, intrusions; multiple plutons can coalesce into a batholith.
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