Unconformities
Summary
TLDRThis script explores the fascinating geological phenomenon of unconformities, specifically focusing on the angular unconformity at Harrow Beach in Portugal. It explains how erosional surfaces form due to weathering and erosion, leading to the truncation of rock layers and the creation of missing geological history. The video also distinguishes between four types of unconformities, emphasizing the importance of recognizing these features for historical geology and relative dating to unravel Earth's past events.
Takeaways
- 🌊 Erosional surfaces are created by the removal of rock or sediment through weathering and erosion, often caused by gravity, wind, running water, and ice.
- 📈 The formation of an erosional surface involves uplift of strata above sea level, exposure to elements, weathering, and erosion, leading to the destruction of certain layers.
- 🏞 An unconformity is a buried erosional or non-deposition surface between strata of different ages, representing a gap in geological time.
- 🔍 Erosional surfaces can be identified in the field by studying the contacts between rocks and looking for signs of weathering and erosion.
- 🌳 Plants can grow on exposed erosional surfaces, further promoting weathering and erosion, and leaving behind sedimentary structures like root traces.
- 📉 Unconformities help identify erosional surfaces by marking significant changes in rock layers or orientations, and can be of different types.
- 📚 Angular unconformities are characterized by horizontal strata overlying eroded, tilted strata, resulting from tectonic movements and subsequent erosion.
- 🪨 Nonconformities occur where sedimentary rocks lie on an erosional surface atop much older igneous or metamorphic rocks, indicating a substantial time gap.
- 🕰 Disconformities are recognized by an irregular erosional surface that truncates lower sedimentary strata, often containing signs of weathering.
- 🔬 Paraconformities are the most challenging to identify, consisting of sedimentary rocks with a surface of non-deposition and no clear evidence of missing rock or time.
- 🗺 Recognizing unconformities is crucial for historical geology, enabling the relative dating of geological events and understanding Earth's history.
Q & A
What is an unconformity in geology?
-An unconformity is a buried erosional or non-deposition surface between strata of different ages, representing a significant gap in geological record and indicating a period of time when no sediment was deposited or where existing sediment was eroded away.
Why are the strata at Harrow Beach and Portugal considered unusual?
-The strata at Harrow Beach and Portugal are unusual due to the abrupt change in layer orientation, with horizontal strata on top of nearly vertical ones, indicating an erosional surface and a spectacular example of an angular unconformity.
What are the agents of erosion that contribute to the formation of an erosional surface?
-The agents of erosion that contribute to the formation of an erosional surface include gravity, wind, running water, and ice, which break down and transport rock and sediment from their place of origin.
How does the process of weathering contribute to the formation of an erosional surface?
-Weathering contributes to the formation of an erosional surface by breaking down rocks exposed on the Earth's surface into smaller pieces, making them susceptible to erosion and transport, thus creating a surface where sedimentation can resume.
What is the significance of an angular unconformity?
-An angular unconformity signifies a period where older, tilted rock layers were eroded and then covered by younger, horizontal layers, indicating a significant geological event and a gap in the depositional record.
What are the key principles that help us understand the original deposition of strata?
-The key principles that help us understand the original deposition of strata are the Principle of Original Horizontality, which states that all strata are laid down in horizontal layers, and the Principle of Superposition, which states that the oldest layers occur at the bottom, with layers deposited in a sequential order.
How can geologists identify ancient erosional surfaces in the field?
-Geologists can identify ancient erosional surfaces in the field by studying the contacts between rocks, looking for signs of weathering and erosion such as small caves, karst-like structures, root traces, and changes in sedimentary structures and trace fossils.
What is a nonconformity and how is it identified?
-A nonconformity is a type of unconformity where sedimentary rock layers occur on an erosional surface atop much older igneous or metamorphic rocks. It is identified by recognizing the differences in rock types and the presence of an erosional surface that separates them.
What is a paraconformity and how does it differ from other types of unconformities?
-A paraconformity is a type of unconformity where both the underlying and overlying rocks are sedimentary, but there is a surface of non-deposition with no evidence of erosion. It differs from other unconformities as it lacks the evidence of an erosional surface and requires absolute age or fossil evidence to confirm the presence of a missing time interval.
Why is it important for a historical geologist to recognize and distinguish unconformities?
-Recognizing and distinguishing unconformities is important for a historical geologist because it helps in relative dating, the process of determining the sequence of geological events such as deposition, erosion, intrusion, and faulting, which is key to understanding Earth's history.
How do erosional surfaces represent missing rock in geological history?
-Erosional surfaces represent missing rock in geological history by indicating periods of time when there was no deposition of sediment or when existing rock layers were eroded away, thus creating a gap in the geological record that geologists must interpret to understand the full sequence of events.
Outlines
🌊 Erosional Surfaces and Unconformities
This paragraph delves into the concept of erosional surfaces and unconformities in geology. It describes an unusual rock formation at Harrow beach and Portugal, characterized by a stark difference in the orientation of rock strata. The text explains that this represents an erosional surface, which is a record of past geological activity. Unconformities are introduced as buried surfaces between different aged strata, often formed by the processes of weathering and erosion. The paragraph outlines the sequence of events leading to the formation of an erosional surface, including deposition, uplift, weathering, and new sedimentation. It also touches on the identification of these surfaces in the field, including the presence of root traces and cross-cutting relationships.
🔍 Identifying Erosional Surfaces and Types of Unconformities
The second paragraph focuses on the methods geologists use to identify ancient erosional surfaces through the examination of rock contacts. It mentions the presence of karst-like structures and root traces as indicators of these surfaces. The paragraph also discusses the concept of unconformities, which are significant gaps in the geological record, and outlines four types of unconformities: angular, nonconformity, disconformity, and paraconformity. Each type is characterized by different geological features and processes. Angular unconformities are distinguished by horizontal strata overlying eroded, tilted layers, while nonconformities involve sedimentary layers on top of much older igneous or metamorphic rocks. Disconformities and paraconformities are more challenging to identify, with the former involving an erosional surface and the latter a surface of non-deposition without clear evidence of missing rock.
🕰️ Understanding Unconformities for Geological Time Travel
The final paragraph emphasizes the importance of recognizing unconformities for historical geology. It explains that the ability to distinguish between different types of unconformities is crucial for understanding the sequence of geological events, a process known as relative dating. This scientific method allows geologists to travel backward through Earth's history by determining the order of events such as deposition, erosion, intrusion, and faulting. The paragraph highlights the significance of absolute ages and fossil evidence in identifying paraconformities, where there is no clear physical evidence of an erosional surface but a gap in the rock record is indicated by the stratigraphy.
Mindmap
Keywords
💡Strata
💡Metamorphic Rocks
💡Sedimentary Rocks
💡Unconformity
💡Erosional Surface
💡Deposition
💡Weathering
💡Angular Unconformity
💡Nonconformity
💡Disconformity
💡Paraconformity
💡Relative Dating
Highlights
The unusual strata of metamorphic and sedimentary rocks at Harrow beach and Portugal are introduced.
An abrupt change in rock layers signifies an erosional surface and an example of an unconformity.
Unconformities are buried erosional or non-deposition surfaces between strata of different ages.
Erosional surfaces are produced by the removal of rock or sediment through weathering and erosion.
The sequence of events leading to the formation of an erosional surface is detailed, including deposition, uplift, and weathering.
Stratum C is completely destroyed, and part of stratum B becomes truncated, leading to the formation of an erosional surface.
New deposition on top of truncated layers creates an erosional surface, marking the absence of certain rock layers.
Erosional surfaces are illustrated in geologic sections with wavy, irregular lines, indicating missing rock and history.
Geologists can identify ancient erosional surfaces through the presence of caves, karst structures, and root traces.
Unconformities help identify erosional surfaces, where rocks do not conform across the line of erosion.
Angular unconformities are characterized by horizontal strata atop tilted and eroded beds.
The process of plate tectonics and erosion leads to the formation of angular unconformities.
Nonconformities occur where sedimentary rock layers rest on an erosional surface atop older igneous or metamorphic rock.
Disconformities are recognized by an irregular erosional surface that truncates lower sedimentary strata.
Paraconformities are the most challenging unconformities to identify, consisting of sedimentary rock with a surface of non-deposition.
Relative dating is the process of determining the sequence of geological events, such as deposition, erosion, and intrusion.
Recognizing unconformities is crucial for historical geologists to understand Earth's history and perform relative dating.
Transcripts
do you want to see a really cool rock
B's strata of metamorphic and
sedimentary rocks are located on tell
Harrow beach and portugal do you see
what makes them so unusual take a closer
look for a moment
don't the strata in the lower part of
the image seem odd to you notice how
there is an abrupt change in layers
going from the bottom to the top of the
photograph the strata in the top are
pretty much horizontal but below them
the strata are practically vertical this
boundary is an erosional surface and it
is a spectacular example of an
unconformity an unconformity is a buried
erosional or non depositional surface
between strata of different ages but
what does this mean
how do on conformities form and are
there different types to begin to answer
these questions
let's review erosional surfaces
erosional surfaces are surfaces produced
by the removal of rock or sediment they
were produced in the past by weathering
and erosion rocks exposed on the surface
of our planet are constantly being
broken down into smaller pieces through
the process of weathering and
transported from their place of origin
by the process of erosion the agents of
erosion are gravity wind running water
and ice
this image shows you the sequence of
events leading to the formation of an
erosional surface let's walk through it
together at time 1 there is deposition
of strata
there's a B and C these layers are
deposited under water through
sedimentation in the ocean during time -
these layers are uplifted the strata
rise above sea level so they are no
longer located under water because they
are above sea level now there is no more
deposition or sedimentation they are
exposed to the elements gravity wind
running water and ice during time 3
these agents cause weathering and
erosion of rock stratum C is completely
destroyed and so is part of stratum B
stratum B becomes truncated finally
during time for the strata are subsided
the layer sink back beneath the ocean
and there is new deposition and
sedimentation because stratum C was
completely destroyed the new sediment
stratum D is deposited directly on top
of what's left of a layer B we called
the contact boundary between stratum B
and stratum D and erosional surface as
it records when and where the layers
were exposed above water and rock layers
like syrup the stratum C were destroyed
another way of thinking about the
erosional surface is that it represents
missing rock rock corresponding to time
that we can't study erosional surfaces
formed during periods of time when there
is no deposition of sediment not to
mention the strata beneath them tend to
be missing and truncated rock layers are
missing you aren't seeing the entire
history of deposition and sedimentation
erosional surfaces are generally
illustrated in geologic sections as wavy
irregular on
relating lines often enough you will see
abrupt changes in lethality across lines
of erosion as well as the termination of
fault lines and intrusive rocks in this
example notice how the intrusive rock R
was eroded away along with the
sedimentary rock layer X once
sedimentation resumed the new rock be
formed on top of both R and X we call
this a cross-cutting relationship we can
identify erosional surfaces in the field
by carefully studying the contacts
between rocks geologists can identify
ancient erosional surfaces from a
variety of observation including the
presence of small cave and karst like
structures which form through
dissolution of rock they can also
recognize with illogical and
mineralogical patterns that evolved due
to the exposure of rock to the chemical
reactions involved in weathering
sedimentary structures and trace fossils
produced by plant roots can also help
you to identify erosional surfaces when
surfaces are exposed and experiencing
erosion they may be overgrown by plants
plants will grow on the rock or sediment
further promoting weathering and erosion
in the process they produce sedimentary
structures called root traces when the
rock or sediment is later buried these
root traces may remain marking the
location of the erosional surface of
course unconformity x' can also help you
to identify erosional surfaces
unconformity x' rocks do not conform
with each other across the line of
erosion the line may mark a significant
change in the lethargy of rock for
example there may be igneous rock below
the erosional surface and sedimentary
rock above it alternatively the
erosional surface may mark a big change
in the orientations of the strata like
the erosional surface at talero beach in
portugal there are actually four types
of unconformity x' i find that the best
way to learn about unconformity x' is to
learn how to distinguish these four
types the erosional surface a tell hero
beach marks an angular unconformity
angular unconformity z' are easy to
recognize at angular unconformity x'
horizontal strata lie on top of tilted
and eroded beds in these cases the lower
strata were first tilted as a result of
plate tectonics and then truncated by
erosion before the upper strata were
laid down above and parallel to the
erosional surface let's take a moment
and walk through this process step by
step the principle of original
horizontality tells us that all strata
are laid down in horizontal layers and
the principle of superposition tells us
that the oldest layers occur near the
bottom so strata are laid down in order
one after another after another and so
on and so on
sooner or later however sedimentation
may stop no more layers may be deposited
for quite some time during this hiatus
in sedimentation tectonic forces on our
planet may fold distort or simply tilt
the layers
like all other rocks on the surface of
our planet
these tilted strata are subject to
weathering and erosion the processes
would destroy and truncate the rocks
producing an erosional surface
eventually sedimentation would resume at
this location and sediment will be
deposited on top of the erosional
surface above the truncated layers again
the layers are laid down one after
another after another and so forth
overall this is the natural process that
produces an angular unconformity it's a
long process it can take tens or
hundreds of millions of years or longer
another type of unconformity is called a
nonconformity non-conformities are
generally easy to identify if you can
recognize the differences between
igneous metamorphic and sedimentary
rocks at non-conformities sedimentary
rock layers occur on an erosional
surface located at the top of much older
igneous or metamorphic rock how do you
know that there is an erosional surface
here igneous metamorphic and sedimentary
rocks formed through very different
processes and under very different
circumstances there must have been
enough time for erosion as well as the
changing conditions between these rocks
being formed this conformities are
harder to identify in these cases
sedimentary rocks occur on both sides of
the erosional surfaces the best way to
recognize a dis Conformity is to attempt
to identify the erosional surface this
Rochelle surface truncates the lower
sedimentary strata and it is usually
quite irregular it may contain cave or
karst like structures root traces and
other signs of weathering and erosion of
all the types of unconformity 'z however
para conformities are the hardest to see
and recognize like dis conformities para
conformities consists entirely of
sedimentary rock however instead of an
erosional surface there is only a
surface of non deposition there is no
evidence of weathering or erosion the
sedimentary strata on opposite sides of
the surface seemed to be conformable
there is no obvious evidence of missing
rock the boundary is regular you usually
need to be absolute ages of the strata
or fossil evidence to show that there is
missing time or missing rock in your
sequence learning to recognize and
distinguish unconformity x' are
important steps in becoming a historical
geologist with this knowledge you can
begin to look at geologic cross-sections
and determine the sequence of events
that led to the structures you find we
call this process relative dating it is
the science of determining the relative
order of past events such as deposition
erosion intrusion and faulting it is the
key to traveling backward through time
through earth history
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