Unconformities

Summary

This 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

00:00

🌊 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.

05:06

🔍 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.

10:08

🕰️ 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.

Keywords

💡Strata

Strata refer to layers of rock that are formed by the accumulation of sediment or volcanic material. In the context of the video, strata are crucial for understanding geological history as they represent different periods of deposition. The video discusses how the orientation of these strata can change dramatically at an unconformity, such as the vertical strata below horizontal ones, indicating significant geological events.

💡Metamorphic Rocks

Metamorphic rocks are formed when existing rock types undergo changes in their mineralogy and texture due to heat, pressure, or mineral exchange. The script mentions B's strata of metamorphic rocks, emphasizing their presence alongside sedimentary rocks, which is important for illustrating the geological processes that have shaped the landscape.

💡Sedimentary Rocks

Sedimentary rocks are formed from the accumulation and cementation of mineral and organic particles. They often form in layers and can provide a record of the Earth's history. In the video, the script describes the unusual arrangement of sedimentary rock layers, which are key to identifying the presence of an unconformity.

💡Unconformity

An unconformity is a surface of erosion that separates two groups of strata of different ages. It signifies a significant gap in the geological record. The video uses the example of an abrupt change in rock layers to explain the concept of an unconformity, highlighting its importance in understanding geological time and processes.

💡Erosional Surface

An erosional surface is a type of unconformity that results from the erosion of the Earth's surface, leading to the removal of rock or sediment. The script describes the formation of an erosional surface through a sequence of events, including uplift, weathering, and erosion, which ultimately leads to the creation of an unconformity.

💡Deposition

Deposition is the process by which sediment, soil, and rocks are added to a landform or landmass. In the video, deposition is a key process in the formation of strata, which are later subject to uplift and erosion, setting the stage for the development of an unconformity.

💡Weathering

Weathering is the process of breaking down rocks, soil, and minerals through contact with the Earth's atmosphere, water, and biological organisms. The script explains how weathering contributes to the formation of an erosional surface, which is a prerequisite for an unconformity.

💡Angular Unconformity

An angular unconformity is a type of unconformity where younger, horizontal strata rest on older, tilted strata. The video script uses the example of an angular unconformity at Tell Hero Beach to illustrate how geological processes can result in significant changes in the orientation of rock layers.

💡Nonconformity

A nonconformity is an unconformity where sedimentary rocks overlie igneous or metamorphic rocks. The script explains that nonconformities indicate a significant gap in geological time and processes, as they represent a transition from one type of rock formation to another.

💡Disconformity

A disconformity is an unconformity characterized by parallel layers of rock with a significant time gap between them. The script suggests that disconformities are identified by the presence of an erosional surface that truncates the lower sedimentary strata, indicating a period of non-deposition or erosion.

💡Paraconformity

A paraconformity is an unconformity where there is a time gap between sedimentary layers, but without significant erosion or change in depositional environment. The video script points out that paraconformities are difficult to identify because they lack the obvious signs of erosion found in other types of unconformities.

💡Relative Dating

Relative dating is the process of determining the sequence of geological events without necessarily determining their absolute ages. The script emphasizes the importance of recognizing unconformities for relative dating, which allows geologists to reconstruct the history of geological events and understand the Earth's past.

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