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Lecture 8

EESA06H3 Lecture 8: EESA06 Lecture 8-10


Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles
Lecture
8

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EESA06 Notes
Lecture 8: Geologic Time- How do we date the past?
Age-dating rocks: provide info about absolute age
Relative Age-dating: age of a rock layer compared to other layers (determined by looking at the position of rock
layers)
Absolute-age: determined using radiometric dating; age of a rock in years.
Strata : layers of rock
Stratigraphy 地层学: study of Earth history as revealed by strata; how old rocks are
Accommodation: to have rocks preserved 保护, we need areas of the crust that are subsiding 下沉 so it can flood
w/ water and we can accumulate 累积 sediment 沉淀物; it is the space/area for sediment to be deposited between
sedimentary surface and sea level
Conformable succession 演替: can easily tell the oldest and youngest rocks; no disturbances 动乱 or breaks;
younger rocks lie above older rocks - continuous
Unconformable succession: major gaps of years between rocks, separated by long period of inactivity; period of
erosion 腐蚀; like going from ch 2 to ch 8.
- Unconformities 不一致 mark episodes of uplift, erosion and non-deposition
Index fossils 标准化石 + faunal succession 动物区系演替: fossils = remains of an organism; index organisms that
evolved quickly. Can be specific about the time range they lived in. Faunal succession is a tool of stratigraphy and
comprises the basis for the geologic time scale.
Brittle 易碎的 failure of rocks to form faults: faults occur when brittle rocks fracture and there is an off-set. Brittle
failure is like breaking a ruler and it snaps; these processes occur @ shallow depths of the earth’s crust
- Plastic deformation of rocks to form folds
Dip and strike: orientation of a geologic feature. The strike line of a fault is a line representing the intersection of
that feature with a horizontal plane.
- Rocks dip to the west
- If a rock is flat, its dip is 0 degrees. If vertical upright, 90 degrees
- Strike = the edge of any rock layer (north-south)
Isotope 同位素 dating with unstable radioactive isotopes: Parent isotopes (i.e. uranium daughter product = lead)
Half-life: time it takes for 50% of the radioactive
(parent isotope) to decay into daughter isotopes (i.e.
uranium to lead)
Geological timescale: a system of chronological dating
that relates geological strata to time.
Uplift: opposite of subsidence; results in non-
accommodation
erosion and removal of sediment
rock; increase in elevation
Subsidence 下沉: motion of a surface as it shifts
downward relative to a datum 基面 i.e. sea level
The older material at the bottom is not flat, it has
been tilted to get deposited leads to plutons 深层岩
intruded as molten magma into rocks.

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EESA06 Notes
Accommodation sediment goes into water, accumulates layers subsidence of crust allows to preserve
sediment material
A disconformity 不一致地层 is a mini unconformity (i.e. someone rips out pages of a book) when age-dating
rocks, the amount of time we think it is, is more than what’s actually recorded in the rock. We can only apply dates
of the rocks from the surface but in reality, the reserved rocks took time to be deposited (for which that time may
not have been recorded)
The use of ‘index fossils’
标准化石
for identifying relative age of strata + gaps (disconformities)
Best index fossils = SHORT LIVED and rare.
Grand Canyon 大峡谷 has rocks with complete succession 演替
Volcanic ash based on volcanic ash horizons, we can date them and be
exact about matching rocks
Law of faunal succession 动物区系演替: observing that sedimentary
rock strata contain fossilized 石化的 flora 植物群 and fauna 动物群, the
fossils succeed each other vertically in a specific order and can be identified
over wife horizontal distances. (see image above)
Types of Index Fossils
Trilobite 三叶虫: ancient arthropods 节肢动物
Crinoid 海百合: lily or kelp-like, live in both shallow and deep water
Ammonites 菊石: looks round like a conch 贝壳, or tires but with diff patterns
Pluton 深层岩体 intrusion 侵入
Rocks become tilted and eroded 被侵蚀 when intruded by a pluton
Some parts of rocks get removed above the erosion surface
Contact metamorphism 接触变质作用: metamorphism 变质 due to contact with or proximity 接近 to an
igneous intrusion
Polyphase/multiphase 多相的: multiple phases of deformation 变形, its effects are superimposed 叠加
on previous rocks

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EESA06 Notes
Folded Rocks
Synclin 向斜: rock is U shape; rock dips towards centre
Anticline 背斜: where rocks are folded UPWARDS (upside down U)
When we used fossil fuels, oil + gas accumulated in anticlines called “anticline trap 背斜型圈闭 bc after
reaching great depth, they try to migrate back up but reach an impermeable 不渗透度 cap where oil cannot go
through.
Recumbent fold 伏卧褶皱: has a horizontal axial plane, 2 limbs of a fold are PARALLEL to each other
Way up criteria: a characteristic relationship observes in rocks to determine if
they are the right way up
Hanging wall block 顶壁断块: when mining, rocks hang above you; it’s a block of
rock positioned over the fault or ore body
TYPES OF FAULTS** We use faults 产生断层 to know if rocks pull apart or shove
together
Normal fault: extended i.e. Iceland, East Africa
Reverse fault: rocks pushed together, footwall = rock block is below i.e.
Himalayas, Mountain zones
Strike-slip fault: slide past each other i.e. San Andreas
Graben 地堑: form narrow rift valleys
The most dangerous reverse fault in Canada is the Cascadia subduction
zone below Vancouver (pacific plate is being pushed below north american
plate)
Important people:
Georgius Agricola: “fatherof mining geology” recognizes that fossils are
remains of marine organisms in 1531
Nicholas Steno: showed by reasoning that fossils and crystals must’ve
solidified before the host rock that contained them was formed. “mountains can be raised and lowered, land can be
conveyed from one place to another”
William Strata Smith: a canal engineer; developed first idea of stratigraphy, made a geology map showing rock
layers with unconformities. Significant for students in 1815, tells Earth had a history
Sir William Logan: had a geological map of Upper Canada 1865; The Scottish Enlightenment 1730-1830
Joseph Black: pioneer 先锋 of chemistry
James Hutton: oil shells; he burned oil
James Watt: invented the steam engine
John Playfair; geologist; the concept of ‘deep time’; wrote the book huttonian theory of earth
Archbishop Ussher: he was precise saying earth formed at 9am on Oct 26th 4004 BC based on counting generations in
the bible
Arthur Holmes: pioneer of absolute isotopic dating
The Industrial Revolusion
Watt’s Steam Engine: a form of pump used to remove water from mines
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