Santhosh, Joanna Group 2 SSA Part B Durdle Door (3)
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121
Subject
Geology
Date
Dec 6, 2023
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5
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EARTH 121
Study Site Assessment Part B
November 6, 2023
SSA Part B
Group 2
Identification & Justification of Rock Type:
Durdle Door is a famous stone arch located in Dorset, England, that is mainly composed of
sedimentary rock, more specifically limestone. To justify the classification of the rock, there are
2 main characteristics to focus on.
1.
Composition
a.
Clue #1
- The visible layers of rocks compacted and cemented together show that
this site has
undergone lithification
, a process that sedimentary rocks are formed
from. Since the arch is located in the Jurassic Coast, the rocks in this area are
believed to have been deposited during the Jurassic period, as proven by the fossil
records collected. For a structure like this to form near the coast in shallow
waters, it underwent the deposition of layers of sediment over millions of years,
hardened overtime, and was uplifted to form a dome-like structure (MAT, 2023).
As a result of erosion, the heavy current and waves wore down the less resistant
layers of limestone, forming an arch. Since this rock is composed of limestone,
the cementing material that holds the sediments together would be minerals such
as calcite. All this confirms that Durdle Door has undergone the lithification
process. For visible cues, the rock contains layers of thick deposits of sediment
clumped up together, but they have been tilted.
Figure 1: Rock formation contains layers of thick sediments clumped up together. The right
side of the arch has a clearer view of the horizontal layers of sediments compacted together
and but they are slightly tilted .
b.
Clue #2
- A general identification of sedimentary rocks is finding the
presence of
fossils and individual particles of fragmented minerals cemented together
(Johnston, n.d.). The rock composition of Durdle Door is rich in fossil content.
The chemical composition of Durdle Door shows that the main mineral found is
calcium carbonate (calcite), which is typically found in limestones. This
composition is derived from the remains of marine organisms like shells and
corals (MAT, 2023). In shallow waters, the carbonate shells of dead creatures
build up to create the majority of limestones. We can also see traces of large
SSA Part B
Group 2
thrombolites in the arch. Thrombolites are clotted-like structures of carbonate that
form in saline, shallow waters where there are calcium depositing microorganisms
like cyanobateria (Hay, n.d.). Since Durdle Door is situated on the Jurassic Coast,
it overlooks the English Channel which is saline, marine water. This environment
is perfect for thrombolites to form on the rock formation.
Figure 2: The top parts of the arch have small tree holes (cyced tree fossils) that are 20 to 25
cm and surrounding them are thrombolites that are larger in diameter , around 2 meters
(West, n.d).
2.
Texture
a.
Clue #1
- The texture of the rock found in Durdle Door is mainly bioclastic. As
mentioned in Module 2c, bioclastic limestone is made from fragments of shells or
skeletal materials of marine organisms. The limestone found in Durdle Door is
formed from the accumulation and lithification of marine organism remains.
These include specimens of Micrasters which are from the echinoid family,
ostracods and more (West, n.d
)
.Researchers have also been able to find fossilized
remains of Cycad trees which are seen by the number of holes present on the top
parts of the rock (seen in Figure 2).
Unique Rock Cycle Adaptation:
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SSA Part B
Group 2
Figure 3 (Copywrite Joanna Santhosh, 2023): Unique Rock Cycle Diagram for Durdle Door
Justification for Rock Cycle:
To create a unique adaptation of the Rock Cycle for Durdle Door, we know that this rock
formation is mainly composed of limestone which is a sedimentary rock. With this knowledge, a
simple diagram can be created while interconnecting the 4 Earth Systems within this process.
The cycle begins with the decomposition of marine
sediments that contained calcium
carbonate
(calcite). Calcite is primarily found in the shells of marine organisms and corals in
oceanic environments of the
English Channel (hydrosphere)
. Over the course of millions of
years, these sediments accumulated in the ocean floors. The sediments undergo
lithification
where the accumulation of loose sediments begin to compress under high pressure and cement
together on the hydrosphere and geosphere to form limestone. Calcium carbonate also has a
chemical property of crystallizing to form limestones as well (Snyder & Russell, 2013).
The
sedimentary rock does undergo WETD
, specifically chemical erosion due to reaction with
the intense oceanic waves pushing against the rock or even weathering caused by atmospheric
conditions like rain or wind. Limestone is usually very durable but when exposed to exterior
conditions like mild acidic rain water or particles in the ocean, it begins to deteteriorate (N.D,
2016). These interactions with the atmosphere and hydrosphere further breaks down the
limestone into sediments that are transported and deposited into the ocean floors. This is clearly
visible since the result of strong erosion led to the softer layers of limestone to be eroded and
form the dome arch shape. The hydrosphere, atmosphere, and biosphere may eventually cause
the transported eroded particles to settle in sedimentary settings, where they will restart the rock
cycle again.
To extend a bit further, if subjected to high temperature, pressure or techtonic stresses, the
limestone that composed Durdle Door can
undergo metamorphism and be transformed into
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Group 2
marble (metamorphic rock
). Marble also primarily contains the mineral, calcite which if
undergone WETD, it can breakdown into sediments and when these sediments are deeply buried
and accumulated, it can form into sedimentary rocks.
Citations
1.
MAT, M. (2023, June 7).
The Durdle Door, England: Geology, formation
. Geology
Science.https://geologyscience.com/gallery/geological-wonders/the-durdle-door-england/
?amp
2.
Hays, J. (n.d.).
Stromatolites, thrombolites and the world’s oldest life forms
. Facts and
Details. https://ioa.factsanddetails.com/article/entry-309.html
3.
Snyder , K., & Russell, P. (2013, October 9).
Calcite, limestone and marble
. Earth
Sciences Museum.
https://uwaterloo.ca/earth-sciences-museum/resources/detailed-rocks-and-minerals-article
s/calcite-limestone-and-marble#:~:text=All%20limestones%20are%20formed%20when,
without%20the%20aid%20of%20organisms.
4.
King, H. M. (n.d.).
Marble
. geology. https://geology.com/rocks/marble.shtml
5.
Limestone: Characteristics, uses and problem
. GSA. (2016, October 13).
https://www.gsa.gov/real-estate/historic-preservation/historic-preservation-policy-tools/pr
eservation-tools-resources/technical-procedures/limestone-characteristics-uses-and-probl
em#:~:text=Limestone%20is%20extremely%20durable.,is%20loss%20of%20precise%20
detail
.
6.
West, I. (n.d.). Durdle Door, Dorset - Geological Field Guide - by Ian West.
https://wessexcoastgeology.soton.ac.uk/durdle.htm
7.
Johnston, J. W. (n.d.). 2c - Rock Classification. Earth 121.
https://contensis.uwaterloo.ca/sites/courses/EARTH-121/lecture-content/module2/m02c.a
spx
8.
Johnston, J. W. (n.d.). 2c1 - Three Rock Types. Earth 121.
https://contensis.uwaterloo.ca/sites/courses/EARTH-121/lecture-content/module2/m02c1.
aspx