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LEC 172 Lecture Notes & Understanding Earth Notes
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Word documents containing written up lecture notes from the lectures & associated power points. Colour coded and detailed summary notes of the relevant and accompanying chapters of Understanding Earth (main textbook for the course).

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  • May 21, 2024
  • 5
  • 2021/2022
  • Lecture notes
  • Jennie gilbert
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Understanding Earth LEC 172 Lecture 2 Reading

Ch 9; Clocks in Rocks

Reconstructing Geologic History from the Stratigraphic Record:

Absolute Age = The actual number of years elapsed from a geologic event until now.

Relative Age = The age of 1 geologic event in relation to another.

Principles of Stratigraphy:

In 1667, Danish scientist Nicolaus Steno, demonstrated that peculiar tongue stones found in Mediterranean
sedimentary rocks were identical to teeth of modern sharks. He concluded that these stones were ancient
shark teeth preserved in rocks.

He set the foundation of the modern science – Stratigraphy = The description, correlation, and classification of
strata in sedimentary rocks.

Steno set up 4 basic Principles of Stratigraphy, still used today…

Principle of Original Horizontality = A stratigraphic principle stating that sediments are deposited as
essentially horizontal beds.

Principle of Superposition = Each sedimentary rock stratum in a tectonically undisturbed sequence is younger
than the one below it and older than the one above it.

Stratigraphic Succession = A chronologically ordered set of rock strata.

There were 2 problems with Steno’s strategy – 1) there were gaps in a regions stratigraphy due to periods of
drought between floods and where tectonic uplift led to erosion of sedimentary sequences, and 2) it was
difficult to determine the relative ages of 2 formations separated by a substantial amount of space.

Fossils as Recorders of Geologic Time:

In 1793, English canal workman, William Smith determined that fossils could help geologists determine the
relative age of sedimentary rock. He observed that different layers of rock contained different sets of fossils,
distinguishable by their features. Regardless of location, he could predict stratigraphic position of any
formation based on its fossils. This stratigraphic ordering of the fossils of animals (fauna) produced faunal
succession.

The principle of faunal succession states that sedimentary strata contain fossils in a definite sequence. This
sequence can be found in other formations elsewhere, strata can be matched.

The created the modern science of Palaeontology = the historical study of ancient life-forms.

Unconformities; Gaps in the Geologic Record:

Unconformity = A surface between 2 rock layers in a stratigraphic succession that were laid down with a time
gap between them. This represents a boundary between 2 layers of sediment caused by erosion or a pause in
deposition.

Unconformities are classified according to the relationship between the layers above and below them:

1) Angular Unconformity – whereby upper sedimentary strata overlie beds that have been folded by
tectonic processes, and the plane has then been eroded to a more or less even plane.
2) Nonconformity – where upper sedimentary beds overlie metamorphic or igneous rock.
3) Disconformity – where an upper sedimentary sequence overlies the surface of a sedimentary strata
which has been deformed via erosion. Strata have the same orientation.
a. These are often created as a result of tectonic uplift or when sea levels drop.

, - Sometimes sedimentary sequences can be disturbed by other events such as tectonic faults displacing
bedding planes, dikes - in geology, tabular or sheetlike igneous body that is often oriented vertically or steeply
inclined to the bedding of pre-existing intruded rocks (https://www.britannica.com/science/dike-igneous-
rock ) - or sills - flat intrusion of igneous rock that forms between pre-existing layers of rock, which occur
parallel to the bedding of the other rocks that enclose them (https://www.britannica.com/science/sill ).

The Principle of Cross-Cutting Relationships = is a principle that states that any geologic features that cut
across strata must have formed after the rocks they cut through.

The Geologic Time Scale; Relative Ages:

In the 19th century, geologists began applying Steno and Smiths stratigraphic principles to outcrops globally,
determining the relative ages of rocks using faunal successions and cross cutting relationships, mapping a
history of geologic events = Geologic Time Scale

Intervals of Geologic Time:

Era = A division of geologic time representing one sub-division of an eon, and incl., multiple periods

Period = A division of geologic time representing one sub-division of an era.

Epoch = A division of geologic time representing one sub-division of a period.

Interval Boundaries Mark Mass Extinctions:

Major boundaries in geologic time often represent mass extinctions = A short interval in time during which a
large proportion of the species living at the time disappear from the geologic record.

Darwin’s theory of evolution explains how new species evolved afterwards. The causes of these events are
often unknown or uncertain. E.g., End of the Cretaceous period – 75% loss, likely caused by a large meteorite
impact (Mesozoic  Cenozoic Era). In contrast, the Palaeozoic-Mesozoic Boundary cause (95% loss) is still
debated.

Recent Advances in Timing Earth Systems:

Sequence Stratigraphy = Technical innovations mean that seismic waves from controlled explosions or
earthquakes can be used to construct a 3D image of stratigraphic sequences buried below the surface, rather
than just those visible at outcrops.

Chemical Stratigraphy = When sediments are buried and lithified, the minerals and chemicals that are in them
are preserved and can be used to match outcrops regionally/globally – each layer of differing sediment will be
unique.

Paleomagnetic Stratigraphy = Thermoremanently magnetized volcanic rocks record magnetic reversals which
can be detected and dated isotopically in sediment cores to date faunal successions and more recently to
measure sedimentation rates.

Clocking the Climate System – Climate changes can be charted from the isotopes contained in shelly fossils
buried in deep-sea sediments. Deep sea vessels such as JOIDES Resolution take deep sea cores globally. 14C
dating can be used to estimate when the shells formed and measure O isotopes to determine the seawater
temp.




Friday 19th November 2021 LEC 172

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