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Deep Time and the Age of the Earth

• Deep time narrative: Earth is incredibly old; understanding vast time scales helps explain present changes and history of Earth and the cosmos.
• Contrast between land-based telescopes and space telescopes: atmospheric distortion limits ground observations; space telescopes avoid this but require complex engineering.
• Example of ground-based limits: Keck telescopes on Mauna Loa demonstrate large-aperture optics, yet atmospheric seeing remains a challenge.

Space Telescopes: Hubble and Beyond

• 1990: NASA launched the first space telescope (Hubble Space Telescope).
  • Instruments included:
  • Fine Guidance Sensors (FGS)
  • Space Telescope Imaging Spectrograph (STIS)
  • Cosmic Origins Spectrograph (COS)
  • Near Infrared Camera and Multi-Object Spectrometer (NICMOS)
  • Advanced Camera for Surveys (ACS)
  • Wide Field Camera 3 (WFC3)
• Cost: $5 imes 10^{9}$ dollars.
• Major components: primary mirror, aperture door, secondary mirror, support systems, communication antennas, solar panels.
• Hubble initially produced blurry images due to a mis-shaped primary mirror; the error was about 1/50th the diameter of a human hair, yet sufficient to degrade image quality.

Hubble Fix and Early Triumphs

• Astronauts installed corrective optics (lenses) to correct the mirror shape.
• Post-fix, Hubble produced iconic images, including the "Pillars of Creation" showcasing star formation in the Eagle Nebula.

Designing for Risk in Scientific Discovery

• Hubble’s director Bob Williams advocated for taking calculated risks: "Scientific discovery requires risk."
• Williams personally could select 10% of observing targets; he chose to point at an extremely dark, empty patch of sky to probe the unknown.
• This bold choice faced professional pushback, with Williams offering to quit if results were unsuccessful.

The Hubble Deep Field

• A tiny dark patch near the Big Dipper was chosen for deep-field imaging; its sky area is about the size of a pinhead at arm’s length.
• Observed for 10 days to collect faint light from distant objects.
• The resulting image (Hubble Deep Field) revealed thousands of galaxies, some billions of light-years away; many stars seen in the field belong to our Milky Way, but the distant galaxies lie far beyond.
• Light from these distant objects began traveling toward Earth for billions of years.

James Webb Space Telescope (JWST)

• In 2022, JWST captured sharper images (often referred to as a follow-up to/expansion of Hubble’s deep-field capabilities).
• The JWST Deep Field provides higher resolution and greater depth in the infrared, revealing even fainter, more distant galaxies.

Interpreting Deep Field Images: Cosmic Timescales

• Deep Field images illustrate the enormous size and age of the universe.
• Observed galaxies are receding from us, indicating the universe is expanding (redshift is a key indicator).
• The broad implication: the Big Bang theory as the core explanation for the origin and expansion of the universe; the light from many galaxies has traveled for up to ~${13} imes 10^{9}$ years to reach us (roughly the age of the universe).

Age of the Universe, and the Relevance to Earth

• Emphasizes that Earth, the Sun, and our planet are vastly old, and knowing this helps interpret changes we observe on Earth.
• The scale of deep time helps frame current environmental processes and geological history.

Climate Change and Life on Earth: Contemporary Implications

• Global warming is melting Arctic sea ice; potential impacts include threatened polar bears.
• Coral reefs, such as the Great Barrier Reef, are stressed by heat, leading to bleaching and potential death.
• The Amazon rainforest is at risk from heat, drought, and logging; could dry out and possibly shift from a net CO₂ sink to a source.
• These changes are assessed within the context of Earth’s deep time, enabling perspective on significance.

Interpreting Significance: Personal and Scientific Value

• Is a given change a big deal? This is value-laden and depends on personal, social, and ecological priorities.
• Science provides the facts and concepts to help individuals decide what is significant to them.

Historical Climate and Extinction: What Science Reveals

• Science shows Earth has experienced warmer periods (e.g., 65 million years ago) and colder periods (e.g., 12,000 years ago).
• Many species have gone extinct over time (examples include Mammoth ~4,000 years ago; Dinosaurs ~65 million years ago; Chalicotherium ~1.8 million years ago).
• This historical context informs understanding of current climate change and extinction dynamics.

A Cautionary Note on Present Change

• It’s tempting to conclude climate is always changing and extinctions are constant; however, scientific evidence helps determine the magnitude and significance of current changes relative to deep time.

Lecture Goals: What We Aim to Learn

• Gain a sense of how old the Earth is.
• Explore methods scientists use to study Earth’s history.
• Describe major changes in Earth’s recent history and the processes driving them.

Early Earth Age: From a 6,000-Year View to Deep Time

• Until the late 1700s, many scholars believed Earth was about 6,000 years old, based on a literal reading of the Bible (Genesis 5:3 suggests Adam’s descendants).
• This view contrasted with later geologic evidence for a much older Earth.

Field Evidence: Siccar Point, Scotland and Grand Canyon, USA

• Geology tour stop #1: Siccar Point (discovered by James Hutton in 1788).
  • Rock types: brown and yellow rock layers; brown layers are older, yellow layers overlie tilted brown layers.
  • Sandstones formed from river-transported sand deposited in the ocean; sandstone formation involves erosion, transport, deposition, and lithification.
• Field reasoning at Siccar Point:
  • Key question: How did light rocks end up on top of dark rocks?
  • Two explanations proposed:
    1) Light rocks formed atop dark rocks.
    2) Light rocks were thrust/tilted on top of dark rocks.
  • Observational test to distinguish hypotheses:
  • a) If light rocks formed on top, their composition should show sands that accumulated at bottom of the ocean.
  • b) If top rocks are younger than bottom rocks, they should be younger.
  • c) If top rocks fit perfectly around bumps and bulges in the bottom rocks, this supports a single, continuous deformation history rather than separate deposition.
  • Evidence at Siccar Point showed the top rocks fit around bottom bulges, supporting the idea that the top layers formed there and were not simply deposited atop later.
  • History of Siccar Point (condensed):
    1) Brown rocks laid down horizontally on the ocean floor.
    2) Brown rocks tilted nearly vertical, lifted, and eroded.
    3) Brown rocks submerged again by the sea.
    4) Yellow rocks laid down on top of the tilted brown rocks.
    5) The entire outcrop was uplifted, tilted further, and eroded.
  • Hutton’s conclusion: Rocks are very old and Earth’s history is dynamic; the deep-time perspective emerged from such observations.
  • Quote (as recorded by John Playfair): the mind grows giddy when considering the vast abyss of time.
• Geology tour stop #2: Grand Canyon.
  • Rim contains Kaibab Limestone (fossilized seashells) – deposition from calcium carbonate skeletons in the ocean.
  • Multiple sedimentary layers: Toroweap Sandstone (bottom of shallow ocean), Coconino Sandstone (ancient dunes), Hermit Shale (coastal plain with slow rivers), etc.
  • Each layer represents long, slow processes of sedimentation and erosion; together they illustrate Earth’s immense age.
  • The Grand Canyon demonstrates: 1) Earth is old; 2) The rock layers record a long sequence of environmental changes over time.
• Unconformities visible in the Grand Canyon sequence include disconformities, angular unconformities, and nonconformities (types summarized on site):
  • Disconformity: gap in time within horizontal strata due to erosion or non-deposition.
  • Angular unconformity: tilted or folded strata overlain by younger, parallel strata.
  • Nonconformity: younger sedimentary rocks overlie older igneous or metamorphic rocks.

Radioactive Dating: Confirming Deep Time

• 19th century geologists argued Earth is very old based on stratigraphic rationale; later radioactive dating provided quantitative confirmation.
• Radiometric dating example: Potassium-40 decays to Argon-40 with a half-life of $T_{1/2} = 1.3 imes 10^{9} ext{ years}.$
• Decay constant: $\boxed{\lambda = \frac{\ln 2}{T_{1/2}} = \frac{\ln 2}{1.3 \times 10^{9}\ \text{yr}}}.$
• Dating principle (simplified): If a rock initially has no Ar-40, then the amount of Ar-40 produced by radioactive decay of K-40 after time t is: $N<em>{Ar} = N</em>{K}(e^{\lambda t} - 1),$ where NK is the remaining amount of K-40 and NAr is the accumulated Ar-40.
• The age t can be inferred from the measured ratio Ar-40 to K-40: $t = \frac{1}{\lambda} \ln\left(1 + \frac{^{40}Ar}{^{40}K}\right).$
• Implications: The Earth and its rocks are about $4.5 \times 10^{9}\ ext{years}$ old, consistent with other geologic evidence.
• The clock starts when rocks first cool and crystallize; argon gas accumulates in crystals over time.

Perspective: Earth’s Age in Context

• 4.5 billion years is difficult to grasp; it helps to compare to human milestones and natural events:
  • Human civilization: pyramids built about $4.5 \times 10^{3}\ ext{years}$ ago.
  • Dinosaur extinction: about $6.5 \times 10^{7}\ ext{years}$ ago.
  • End of the last ice age: about $1.2 \times 10^{4}\ ext{years}$ ago.
• Math facts to recall:
  • There are 1000 thousands in a million.
  • There are 1000 millions in a billion.
  • There are 1000 billions in a trillion.
• Perspective on scale: If Earth’s history were condensed into one year, dinosaurs would appear around late December (with modern humans arriving extremely recently in the calendar year).
• Example: If the year starts on January 1, dinosaurs go extinct on roughly December 26; modern humans appear in the last small sliver of the year, illustrating how young human history is relative to Earth’s history.

Timeline Visualization: Pyramids, Dinosaurs, and Deep Time

• Question prompts (i-clicker style) illustrate that pyramids built around $4.5 imes 10^{3}\ ext{years}$ ago is a tiny fraction of Earth’s age.
• Correct interpretation: The Earth is so old that 4500 years is not visible on the timeline; the pyramids are roughly at the 1/1000 to 1/1000000 scale depending on how the axis is drawn.
• Quantitative takeaway: The pyramids are about 1/1{,}000{,}000 as old as Earth; more generally, many human events occupy a negligible fraction of Earth’s 4.5-billion-year history.
• The takeaway: Earth’s history is dominated by geologic processes that span hundreds of millions to billions of years, far exceeding human timescales.

Human Impacts in the Context of Deep Time

• The age and processes of Earth provide a backdrop for assessing contemporary environmental changes.
• The science of climate change and extinction requires careful interpretation against the long arc of Earth’s history.
• The study guide encapsulates the key facts and concepts needed to reason about Earth’s age, dating methods, and geological history:
  • Age of the Earth: $4.5 \times 10^{9}\ ext{yr}$.
  • Dinosaur extinction: $6.5 \times 10^{7}\ ext{yr}$ ago.
  • End of last ice age: $1.2 \times 10^{4}\ ext{yr}$ ago.
  • Formation processes: limestone forms from skeletons of plankton in oceans; sandstone from eroded sand; mudstone from compressed mud and silt.
  • Plate tectonics: continents have moved and reconfigured over Earth's history; Pangaea formed and broke apart around $2.0 \times 10^{8}\ ext{yr}$ ago.

Study Guide Highlights

• Reading: Geology: the Physical Basis for Environmental Science (textbook; Ch. 2 / 4th ed.).
• Facts to memorize:
  • Age of Earth: $4.5 \times 10^{9}\ ext{yr}$.
  • Dinosaur extinction: $6.5 \times 10^{7}\ ext{yr}$ ago.
  • End of last ice age: $1.2 \times 10^{4}\ ext{yr}$ ago.
  • Great Pyramid built: $4.5 \times 10^{3}\ ext{yr}$ ago.
  • Know how many millions there are in a billion.
  • Be able to describe how limestone, sandstone, and mudstone are formed.
  • Be able to describe evidence for the great age of the Earth.
  • Be able to describe the history of a location from its geology.
  • Be able to answer timeline questions presented in lectures.

Quick Reference: Core Concepts and Formulas

• Age interpretations rely on deep time scales and cross-cutting evidence from multiple sources (rock records, fossil records, radiometric dating).
• Key equations:
  • Radiometric decay constant: $\lambda = \frac{\ln 2}{T<em>{1/2}}$ with $T</em>{1/2} = 1.3 \times 10^{9}\ \text{yr}$ for $^{40}\mathrm{K} \rightarrow ^{40}\mathrm{Ar}$.
  • Dating relation: $t = \frac{1}{\lambda} \ln\left(1 + \frac{^{40}\mathrm{Ar}}{^{40}\mathrm{K}}\right)$.
• Important age landmarks:
  • Earth: $4.5 \times 10^{9}\ \text{yr}$.
  • Dinosaurs extinct: $6.5 \times 10^{7}\ \text{yr}$ ago.
  • End of last ice age: $1.2 \times 10^{4}\ \text{yr}$ ago.
  • Pangaea breakup: ~$2.0 \times 10^{8}\ \text{yr}$ ago.
• Unconformities in sedimentary records illustrate gaps in time and changes in deposition/erosion history (disconformity, angular unconformity, nonconformity).
• Deep-field astronomy demonstrates the scale of the cosmos and the cumulative time required for light to reach Earth, reinforcing the concept of a very old universe and planet.

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