EESA06 Topic 6: Earth Materials and Geologic Time

define strata

• layers of rock, particulary sedimentary, that form in basins

• i.e., sediments layered on top of each other

define stratigraphy

identifying which layers of sedimentary are layered and what they tell us about Earth’s history

what is the “basement” of the Grand Canyon made of?

Precambrian metamorphic and igneous rock

what does brittle failure of rock form?

faults

what are some types of faults?

• normal

• reversed

• strike slip

what does plastic deformation of rocks form?

folds

what are dome types of folds?

• synclines

• anticline

what is a parent isotope? give an example.

• unstable radioactive isotope that undergoes radioactive decay and transforms into a more stable isotope called a daughter isotope

• e.g., U238 → Pb

what is subsidence?

• when land depresses/goes down

• contrasts with uplift

what is uplift?

• when land goes up

• contrasts with subsidence

how does subsidence relate to the formation of strata?

basins accommodate strate when they deepen (i.e., when subsidence of crust occcurs

what is conformable succession of strata?

• constant subsidence → constant settling of sediment

• best opportunity to track Earth’s history

how long does conformable succession occur?

as long as the surface stays below the base level of erosion (usually sea level)

what are disconformities?

pauses in sedimentation within an otherwise conformable succession of strata

what are unconformities?

marks of longer episodes of uplift, erosion, and non-deposition

what is the law of superposition and what does it tell us about the Earth?

• oldest rocks are at the bottom; strata gets younger as you move upwards

• tells the relative age of strata

what are index fossils?

fossils of organisms that exist at very specific points in Earth’s history

describe index fossils

• short lived - went extinct at the same time

• evolve rapidly - i.e., species indicate specific time

what are examples of index fossils?

• trilobite

• ammonites

• crinoid

why are index fossils useful?

• evolve rapidly

• widespread

• i.e., they’ll clearly indicate what period they’re from

how can disconformities be identified?

• index fossils

• correlative layers (e.g., ash)

what do correlative layers (e.g., ash) allow us to do?

correlate across long distances and gaps

what methods can be used to do correlation?

• biostratigraphy

• lithostratigraphy

• chronostratigraphy

describe biostratigraphy

using fossils to establish the relative ages of rocks and correlate successions of sedimentary rocks

describe lithostratigraphy

• mapping rock types

• classification of rock bodies based on their observable lithological properties and their relative stratigraphic positions

what are examples of lithological properties?

• rock type

• texture

• color

describe chronostratigraphy

the branch of stratigraphy that studies the ages of rock strata in relation to time

what are fossil assemblages?

a collection of fossils found in a geological setting

how can we narrow an age range of samples (e.g., strata)?

• index fossils

• fossil assemblages

describe the law of cruss-cutting relations

if A cuts across B, then B must have existed before A

describe the law of intrusions

you can tell if A has intruded into B, therefore A must succeed B

what could granitic plutons tell us about the tectonic environment?

• granite plutons are common in orogenic zones and magmatic arcs

• granite plutons indicate that the rock formation was in these tectonic situations

what is the contact metamophosed zone?

the area immediately adjacent to igneous intrusions

what is the law of original horizontality?

• strata are originally horizontal

• deviations like tilting are due to later structural changes

what are some deviations of the law of original horizontality?

• folding

• faulting

what does the presence of erosion surface indicate?

indicates that the rock formation has been uplifed

what do the presence of tilting, uplifting, and erosion indicate?

tectonic collisions

what do tectonic collisions lead to?

• unconformities

• i.e., not disconformities

describe the law of lateral continuity

• strata originally continued laterally until the edge of the basin

• if strata are interrupted laterally, some event has to have cause that

what might lead to deviations in the law of lateral continuity?

• erosion

• faulting

• intrusion

• etc.

what does the presence of lateral layers above diagonal layers in this diagram indicate?

indicates subsidence and deposition of of new layers

describe the law of cross cutting relations (i.e., the law of intrusions)

• something that cuts across some other feature is younger

• e.g., the dike is the youngest

describe the law of inclusions

any rock fragments that are included in a rock must be older than the rock in which they are included

how can cross cutting relationships and the law of inclusions help establish a chronology?

• you can use relative dating to determine which materials are older than others based on what the intrusion goes through

• conglomerates must have formed after intrusions in order for the intrusion to be included in the conglomerate

how is a canyon created?

• uplift of a stream

• stream will continue to “carve” downwards until it meets the erosion erosion surface

what is structural geology interested in?

how rocks are…

• folded

• faulted

• fractured

• oriented

what are folds?

ductile wrinkling of rock layers

what are faults?

brittle breaks across which rock slides

what are fractures?

• cracks or breaks in rocks where there has been little to no movement

• i.e., faults with no movement

what are anticlines?

folding upwards

what are synclines?

folding downwards

why are anticlines useful?

• trap lightweight migrating fluids squeezed from rocks (e.g., hydrocarbons)

• often sought when looking for oil

where does folding happen and why?

• mostly 10+ km below ground

• rock is more ductile

what is the dip of a layer?

• how “far away” from flat

• i.e., tilt

• doesn’t tell much about orientation

what is the strike of a layer?

• measures orientation

• the intersection of a horizontal plane and an inclined surface

what are the elements of this diagram? describe each.

1. hanging-wall block - block of rock above the fault plane

   • moves down relative to footwall in normal fault

   • moves up relative to footwall in reverse fault

2. footwall block - block of rock below the fault plane

   • where miners are positioned

3. ore body - ore deposits

   • often form along faults because the fractures create pathways for mineral-rich fluids

list the different types of faults

• normal fault

• reverse fault

• strike-slip fault

describe a normal fault

• slippage involving extension

• footwall block moves up

• hanging wall moves down

describe a reverse fault

• “thrust fault”

• hanging wall block pushed up relative to footwall block

• compression of 2 blocks

what is an example of strike-slip faults

• transform margins

• e.g., San Andreas Fault

describe grabens

the hanging wall block drops down between 2 footwall blocks

what is the oldest know geological map?

• Ancient Engyptian

• looked for quartz veins to find gold

• for Ramesses IV

what role did the Industrial Revolution (1790-1820) have in mineral extraction?

increased demand for

• resources - e.g., coal

• geologists

describe the No Coal Problem

• 1840 - Province of Canada

• there was a growing domestic market based on manufacturing

• no coal to power it

who was William Logan?

• provincial geologist

• first director of the Geologic Survey of Canada (GSC)

what resource was rich in the Province of Canada? was this resource useful?

• oil shale (Georgian Bay)

• poor substitute for coal

describe the significance of Petrolia’s resources

• first “oil boom” in North America (1861)

• free-flowing oil

how old did Archbishop Usher (1581-1656) suggest Earth was? on what did he base this calculation?

• ~ 6000 years

• Earth formed at 9am on OCt 26, 4004 BC

• based on counting generations in the Bible

what was James Hutton’s (1726-1797) contribution to modern geology?

• proposed uniformatarianism

  • i.e., past is key to present

  • believed there was not point to aging the Earth

• recognized

  • unconformities within rock layers

  • igneous origin of granite

what ideas/people did James Hutton (1726-1797) come in conflict with?

• Diluvialism

• Catastrophism

• i.e., with people like Abram Werner

define diluvialism

specific surface features provided evidence of a worldwide flood which had followed earlier geological eras

define catastrophism

changes in the earth's crust during geological history have resulted chiefly from sudden violent and unusual events

define creationism

belief that the universe, Earth, life, and humans originated through supernatural acts of divine creation

define uniformatatianism

theory that changes in the earth's crust during geological history have resulted from the action of continuous and uniform processes

define evolution

the process by which living organisms change over time through changes in the genome

how many years did Charles Lyell (1830) estimate was needed to carve the Niagara Gorge? why?

• at least 12 000

• total erosion: 12 km

• rate of erosion: 1m/year (before stabilization measures)

how old did William Tohmson, Lord Kelvin (1824-1907) suggest Earth was? on what did he base this calculation?

• 24-40 million years

• based on: cooling rate of molten Earth through

  • conduction

  • radiation

NOTE: he didn’t know about the structure of the Earth, mantle convection, and radiation of minerals

how old did John Joly (1857-1933) suggest Earth was? on what did he base this calculation?

• 90-100 million years

• based on: rate of delivery of salt to the ocean

  • total salts in ocean / rate of addition = age

how did Arthur Holmes (1890-1965) contribute to contribute to calculating Earth’s age?

• pioneered absolute dating/radiometric dating

• published The Age of the Earth (1913)

what is an isotope?

different versions of an element having

• same number of protons

• different number of neutrons

describe radioactive decay

• unstable isotopes breakdown in various ways

• e.g., losing protons and neutrons

• produces daughter products with different atomic numbers

define half-life

• time needed to reduce the original amount of a radioactive isotope by half

• result of radioactive decay

describe radiocarbon age dating

• used to date organic material 75 000 years/75 ka

• corrections needed for

  • changing solar flux

  • amount of C-14 in the atmosphere

why is there a limit to radiocarbon age dating?

• 75 ka limit

• because C-14 has a relatively short half-life

what is the oldest rock in Canada?

• Acasta gneiss from NW Territories

• 4 billion years

what is the purpose of relative dating over radiometric dating?

not all rocks lend themselves to radiometric dating

what is the geologic column?

• diagram that represents the sequence of rock formations and their relationship to geologic time

• oldest rocks at the bottom and youngest at the top

how was the geologic column put together?

• from sites around the world

• based on relative ages established from fossils

what are golden spike?

places that mark an agreed upon age boundary

what is the proportion of Precambrian and Phanerozoic time on the Geologic Time Scale?

• 87% Precambrain

• 13% Phanerozoic