Physical Geology and Geologic Time Comprehensive Study Guide

Course Overview and Administrative Structure

• Course Code and Name: GEOL 246: Physical Geology (Canadian Edition).
• Textbook Reference: Introduction to Physical Geology: The Science of Earth (Canadian Edition), published by Wiley.
• Theory Assessment Structure:
  • Theory assessment constitutes $60\%$ of the final course mark, comprising $10$ online quizzes and a cumulative final examination.
  • Quiz 1: Covers Chapters 1 and 2 ($5\%$ of overall mark), scheduled for Week 2 (Sept. 10).
  • Quiz 2: Covers Chapters 13 and 14 ($5\%$ of overall mark), scheduled for Week 3 (Sept. 14 & 17).
  • Quiz 3: Covers Chapter 3 ($5\%$ of overall mark), scheduled for Week 4 (Sept. 21 & 24).
  • Quiz 4: Covers Chapters 4, 5, and 6 ($5\%$ of overall mark), scheduled for Week 6 (Oct. 5 & 8).
  • Quiz 5: Covers Chapter 7 ($5\%$ of overall mark), scheduled for Week 7 (Oct. 15).
  • Quiz 6: Covers Chapter 8 ($5\%$ of overall mark), scheduled for Week 8 (Oct. 19 & 22).
  • Quiz 7: Covers Chapters 9 and 10 ($5\%$ of overall mark), scheduled for Week 9 (Oct. 26 & 29).
  • Quiz 8: Covers Chapters 11 and 12 ($5\%$ of overall mark), scheduled for Week 10 (Nov. 2 & 5).
  • Quiz 9: Covers Terrestrial Depositional Environments ($5\%$ of overall mark), scheduled for Week 12 (Nov. 16 & 19).
  • Quiz 10: Covers lecture material from the preceding week ($5\%$ of overall mark), scheduled for Week 13 (Nov. 23 & 26).
  • Final Exam: Cumulative examination primarily focusing on Chapters 15, 19, and 20 ($10\%$ of overall mark), scheduled for Week 16.
• Laboratory Assessment Structure:
  • Laboratory practical components account for $40\%$ of the total course mark, divided into 3 Lab Tests ($16\%$ total) and regular Lab Activities ($24\%$ total).
  • Weeks 1–2: Mineral Identification - Introduction ($3\%$ mark weight).
  • Week 3: Rock Identification - Igneous & Metamorphic Rocks ($1\%$ mark weight).
  • Week 4: Rock Identification - Sedimentary Rocks ($2\%$ mark weight).
  • Week 5: Reservoir Properties of Sedimentary Rocks ($2\%$ mark weight).
  • Week 6: Lab Test 1 ($5\%$ mark weight).
  • Week 7: Structure Contour Maps ($2\%$ mark weight).
  • Week 8: Isopach Maps ($3\%$ mark weight).
  • Week 9: Pay Maps ($3\%$ mark weight).
  • Week 10: Lab Test 2 ($5\%$ mark weight).
  • Week 11: Midterm Break (No laboratory sessions).
  • Week 12: Stratigraphic Cross-Sections ($4\%$ mark weight).
  • Week 13: Structural Cross-Sections ($4\%$ mark weight).
  • Week 14: Lab Test 3 ($6\%$ mark weight).
  • Week 15: Practical Lab Test 3 re-assessment ($5\%$ mark weight), covering Canadian Geology & Earth History.

Historical Evolution of Geochronology and Deep Time

• Early Historical Chronologies:
  • Initial estimates of Earth's age relied heavily on theological analyses. In the mid-1600s, religious scholars calculated the exact date of creation as October 23, 4004 BC.
• Development of Deep Time Concepts:
  • In the late 1700s, field observations by early geologists demonstrated that immense durations of time were required to deposit rock layers, uplift continental blocks, and carve massive landforms like deep canyons.
  • James Hutton (1788): Scottish geologist who introduced the Principle of Uniformitarianism. He proposed that natural processes operating today operated in the past at comparable rates, concluding that Earth history exhibits "no vestige of a beginning, no prospect of an end." This laid the foundation for modern Geochronology.
  • William Smith (1815): Produced the first nationwide geological map of England, Wales, and Scotland by correlating sedimentary strata using their characteristic fossil content.

• Modern Age Determination:
  • With the development of isotopic decay measurement techniques in the 20th century, geologists established the absolute age of the Earth at approximately $4.55 \times 10^9\,\text{years}$ ($4.55\,\text{billion years}$).

Economic Significance of Geologic Time

• Resource Distribution by Geologic Age:
  • Specific intervals in Earth history are associated with major accumulations of economically vital natural resources due to unique paleoclimatic, biological, and tectonic conditions.
• Major Resource Formations in Alberta (AB):
  • $370\,\text{mya}$ (Devonian): Leduc Formation (Leduc Lst) - Major carbonate reef petroleum reservoirs.
  • $250\,\text{mya}$ (Triassic): Montney Formation (Montney Sh) - Massive siltstone/shale tight gas and oil reservoir.
  • $115\,\text{mya}$ (Cretaceous): Cardium Formation (Cardium SST) - Sandstone oil reservoirs.
  • $90\,\text{mya}$ (Cretaceous): McMurray Formation (McMurray Oil Sands) - Bitumen-saturated estuarine sandstones.

Geologic Time Scale and Earth History Milestones

• Categorization of Geologic Time:
  • Geologic time is divided hierarchically into Eons, Eras, Periods, and Epochs based on major evolutionary and tectonic events.

• Precambrian SuperEon ($4.55\,\text{Ga}$ to $542\,\text{MYA}$):

  • Comprises over $88\%$ of total Earth history.
  • Hadean Eon ($4.55\,\text{Ga}$ to $4.0\,\text{Ga}$): Formation of the Earth-Moon system at ~$4.5\,\text{Ga}$; accretion of primordial crust; origin of oceans between $4.2\,\text{Ga}$ and $3.5\,\text{Ga}$.
  • Archean Eon ($4.0\,\text{Ga}$ to $2.5\,\text{Ga}$): Emergence of primitive single-celled life (prokaryotes) between $3.8\,\text{Ga}$ and $3.5\,\text{Ga}$; expansion of continental cratons reaching present sizes between $3.0\,\text{Ga}$ and $2.0\,\text{Ga}$.
  • Proterozoic Eon ($2.5\,\text{Ga}$ to $542\,\text{MYA}$): Development of single-celled eukaryotic organisms at ~$1.8\,\text{Ga}$; accumulation of an oxygen-rich atmosphere; occurrence of global extreme ice ages ("Snowball Earth" episodes).

• Phanerozoic Eon ($542\,\text{MYA}$ to Present):

  • Paleozoic Era ($542\,\text{MYA}$ to $251\,\text{MYA}$):
    • Cambrian Period ($542\,\text{MYA}$): Rapid radiation of marine life ("Cambrian Explosion"); evolution of hard-shelled organisms and trilobites; preservation of soft-bodied fauna in the Burgess Shale ($505\,\text{MYA}$).
    • Ordovician Period ($488\,\text{MYA}$): Expansion of marine invertebrate communities.
    • Silurian Period ($443\,\text{MYA}$): Appearance of the first vascular land plants.
    • Devonian Period ($416\,\text{MYA}$): Diversification of fishes; emergence of first amphibians.
    • Mississippian Period ($360\,\text{MYA}$): Dominance of seedless vascular land plants including club mosses and horsetail rushes.
    • Pennsylvanian Period ($318\,\text{MYA}$): Extensive coal-forming swamp forests.
    • Permian Period ($300\,\text{MYA}$): Diversification of reptiles; period ends with the largest mass extinction in Earth history ("The Great Dying") at $251\,\text{MYA}$.
  • Mesozoic Era ($251\,\text{MYA}$ to $65\,\text{MYA}$):
    • Triassic Period ($251\,\text{MYA}$): Emergence of dinosaurs and gymnosperms (pines, ginkgos).
    • Jurassic Period ($200\,\text{MYA}$): Dominance of dinosaurs and gymnosperm vegetation.
    • Cretaceous Period ($145\,\text{MYA}$): First flowering plants (angiosperms); ends at $65\,\text{MYA}$ with a major asteroid impact causing dinosaur extinction.
  • Cenozoic Era ($65\,\text{MYA}$ to Present):
    • Paleogene Period ($65\,\text{MYA}$): Adaptive radiation of mammals and birds.
    • Neogene Period ($23\,\text{MYA}$): Modern mammalian family expansion.
    • Quaternary Period ($2.6\,\text{MYA}$ to Present): Glacial-interglacial cycles; emergence of hominids, first appearance of Homo sapiens sapiens, and establishment of recorded human history.

Relative Age-Dating Principles

• Definition of Relative Dating:

  • A method of determining the correct sequence of past geological events without determining their absolute age in numerical years.

• Five Major Geologic Events Used in Sequence Interpretation:

  1. Deposition: Formation of sedimentary strata or volcanic surface flows.
  2. Intrusion: Injection of molten igneous material into existing rock units.
  3. Faulting: Fracturing and displacement of rock bodies along fracture planes.
  4. Erosion: Removal of rock material by wind, water, or ice.
  5. Rock Deformation: Tilting, folding, or warping of horizontal strata due to tectonic stresses.

• The Seven Stratigraphic Principles:

  1. Principle of Superposition (Nicolas Steno, 1669): In an undeformed sedimentary sequence, the oldest rock layer lies at the base, and layers become progressively younger toward the top.
     • Example: Grand Canyon Stratigraphic Sequence (base to top): Vishnu Schist $\rightarrow$ Tapeats Sandstone $\rightarrow$ Bright Angel Shale $\rightarrow$ Muav Limestone $\rightarrow$ Redwall Limestone $\rightarrow$ Supai Group $\rightarrow$ Hermit Shale $\rightarrow$ Coconino Sandstone $\rightarrow$ Toroweap Formation $\rightarrow$ Kaibab Limestone.
  2. Principle of Original Horizontality (Nicolas Steno, 1669): Gravity causes aqueous sediments to accumulate initially in horizontal layers. Highly tilted or folded strata experienced post-depositional deformation.
  3. Principle of Lateral Continuity (Nicolas Steno, 1669): Layers of sediment extend laterally in all directions until they gradually thin and pinch out at the edge of the depositional basin, or truncate against pre-existing structural barriers.
  4. Principle of Cross-Cutting Relationships: Any geological feature (such as an igneous intrusion, fault, or erosional surface) that cuts across or disrupts another rock unit must be younger than the unit being truncated.
  5. Principle of Inclusions: Any rock fragment (xenolith or clast) enclosed within a surrounding host rock must be older than the host rock that encompasses it.
     • If granite pebbles are incorporated into an overlying sandstone, the granite is older than the sandstone.
     • If sandstone blocks are enclosed as xenoliths within an intruding granite body, the sandstone is older than the granite.
  6. Principle of Unconformities: Unconformities represent surfaces of erosion or non-deposition that indicate significant gaps in the geologic rock record.
  7. Principle of Faunal Succession (William Smith): Fossil species succeed one another in a reliable, determinable order through time. Because biological evolution is irreversible, rock strata from different regions can be correlated based on their characteristic fossil assemblages.

Unconformities and the Hiatus

• Definition of Hiatus:

  • The duration of geological time that is missing from the rock record at an unconformity surface.

• The Three Primary Types of Unconformities:

  • Disconformity: An erosional or non-depositional surface separating parallel sedimentary strata. It reflects an interval of subaerial exposure and erosion without active tectonic tilting.
  • Angular Unconformity: An erosional surface cutting across older sedimentary strata that were tilted or folded prior to the deposition of younger, overlying horizontal strata.
  • Nonconformity: An erosional surface developed on crystalline plutonic igneous or metamorphic basement rocks, directly overlain by younger sedimentary strata.

Application of Relative Age Dating in Geological Cross-Sections

• Sequential Analysis of Block Diagram (Basic Model):

1.  Deposition of Conglomerate layer $A$ in a basin.
2.  Deposition of Sandstone layer $B$ on top of $A$.
3.  Deposition of Shale layer $C$ on top of $B$.
4.  Deposition of Limestone layer $D$ on top of $C$ under marine conditions.
5.  Intrusion of horizontal igneous Sill $E$ into Shale $C$ and Limestone $D$ ($E$ is younger than $C$ and $D$).
6.  Intrusion of igneous Dike $F$ cutting diagonally through layers $A$, $B$, $C$, $D$, and Sill $E$ ($F$ is younger than $A$ through $E$).
7.  Tectonic tilting of units $A$ through $F$, followed by uplift and erosion, generating Angular Unconformity $G$.
8.  Subsequence deposition of Conglomerate $H$ over unconformity surface G$.\n    9.  Deposition of Sandstone I$over$H$.
10. Deposition of Limestone $J$ over I$.\n    11. Fluvial incision carving a river valley through upper units J$,$I$, and$H.\n\n*   **Complex Cross-Section Analysis**:\n    *   In complex geologic cross-sections featuring folded metamorphic units, faulted intrusive bodies, and multiple generations of cross-cutting dikes, the relative sequence of events from oldest to youngest is derived by systematically applying cross-cutting rules and superposition:\n    \text{Oldest: } i \rightarrow j \rightarrow f \rightarrow g \rightarrow k \rightarrow e \rightarrow d \rightarrow c \rightarrow b \rightarrow a \rightarrow h \text{ :Youngest}\n\n![Complex Geological Cross Section](https://assets.knowt.com/pdf-flow-prod/0abbeed8-160d-49db-a0bd-2bbd3211ae09-figures/91.jpg)\n\n# Stratigraphic Correlation and Cretaceous Paleogeography\n\n*   **Stratigraphic Correlation**:\n    *   The practice of matching specific rock strata exposed at one geographical location with corresponding strata exposed at other distant locations based on lithological similarities and fossil assemblages.\n\n*   **Cretaceous Interior Seaways of North America**:\n    *   During the Cretaceous Period, global high eustatic sea levels combined with tectonic downwarping created sea flooding across North America.\n    *   **Western Interior Seaway**: Submerged the central continent, connecting the Arctic Ocean with the Gulf of Mexico.\n    *   **Hudson Seaway**: Covered central Canada around present-day Hudson Bay.\n    *   **Labrador Seaway**: Submerged eastern coastal margins.\n\n![Cretaceous Seaways Map](https://assets.knowt.com/pdf-flow-prod/0abbeed8-160d-49db-a0bd-2bbd3211ae09-figures/130.jpg)\n\n# Absolute Age-Dating and Radiometric Principles\n\n*   **Isotopic Decay Physics**:\n    *   Unstable radioactive parent isotopes undergo spontaneous nuclear transformation into stable daughter isotopes at a mathematically constant rate unaffected by ambient pressure, temperature, or chemical bonding environment.\n\n*   **Mechanisms of Radioactive Decay**:\n    *   **Alpha Decay**: Emission of an alpha particle (^4_2\text{He}$nucleus). Atomic number decreases by$2$; mass number decreases by$4$.
*   **Beta Decay**: Emission of a beta particle ($^0_{-1}e$ electron) when a neutron decays into a proton. Atomic number increases by $1$; mass number remains unchanged ($0$).
*   **Electron Capture**: An orbital electron is captured by a proton, converting it into a neutron. Atomic number decreases by $1$; mass number remains unchanged ($0$).

• Mathematical Concept of Half-Life ($t_{1/2}$):
  • The half-life ($t_{1/2}$) is the time required for half of the initial quantity of parent radioisotope atoms in a closed system to decay into daughter products.

*   **Half-Life System Ratios**:
    *   $0\text{ half-lives}$: Parent fraction = $1$ ($100\%$); Daughter fraction = $0$ ($0\%$); Parent:Daughter ratio = $1:0$.
    *   $1\text{ half-life}$: Parent fraction = $\frac{1}{2}$ ($50\%$); Daughter fraction = $\frac{1}{2}$ ($50\%$); Parent:Daughter ratio = $1:1$.
    *   $2\text{ half-lives}$: Parent fraction = $\frac{1}{4}$ ($25\%$); Daughter fraction = $\frac{3}{4}$ ($75\%$); Parent:Daughter ratio = $1:3$.
    *   $3\text{ half-lives}$: Parent fraction = $\frac{1}{8}$ ($12.5\%$); Daughter fraction = $\frac{7}{8}$ ($87.5\%$); Parent:Daughter ratio = $1:7$.
    *   $4\text{ half-lives}$: Parent fraction = $\frac{1}{16}$ ($6.25\%$); Daughter fraction = $\frac{15}{16}$ ($93.75\%$); Parent:Daughter ratio = $1:15$.
    *   $5\text{ half-lives}$: Parent fraction = $\frac{1}{32}$ ($3.125\%$); Daughter fraction = $\frac{31}{32}$ ($96.875\%$); Parent:Daughter ratio = $1:31$.

Common Radioisotopes Used in Isotopic Geochronology

• Radioisotopic Systems and Half-Lives:
  • Samarium-147 ($^{147}\text{Sm}$) to Neodymium-143 ($^{143}\text{Nd}$): Half-life = $106 \times 10^9\,\text{years}$ ($106\,\text{billion years}$).
  • Rubidium-87 ($^{87}\text{Rb}$) to Strontium-87 ($^{87}\text{Sr}$): Half-life = $48.8 \times 10^9\,\text{years}$ ($48.8\,\text{billion years}$).
  • Thorium-232 ($^{232}\text{Th}$) to Lead-208 ($^{208}\text{Pb}$): Half-life = $14 \times 10^9\,\text{years}$ ($14\,\text{billion years}$).
  • Uranium-238 ($^{238}\text{U}$) to Lead-206 ($^{206}\text{Pb}$): Half-life = $4.5 \times 10^9\,\text{years}$ ($4.5\,\text{billion years}$).
  • Potassium-40 ($^{40}\text{K}$) to Argon-40 ($^{40}\text{Ar}$): Half-life = $1.25 \times 10^9\,\text{years}$ ($1.25\,\text{billion years}$).
  • Uranium-235 ($^{235}\text{U}$) to Lead-207 ($^{207}\text{Pb}$): Half-life = $0.7 \times 10^9\,\text{years}$ ($700\,\text{million years}$).
  • Beryllium-10 ($^{10}\text{Be}$) to Boron-10 ($^{10}\text{B}$): Half-life = $1.52 \times 10^6\,\text{years}$ ($1.52\,\text{million years}$).
  • Chlorine-36 ($^{36}\text{Cl}$) to Argon-36 ($^{36}\text{Ar}$): Half-life = $300,000\,\text{years}$.
  • Uranium-234 ($^{234}\text{U}$) to Thorium-230 ($^{230}\text{Th}$): Half-life = $248,000\,\text{years}$.
  • Thorium-230 ($^{230}\text{Th}$) to Radium-226 ($^{226}\text{Ra}$): Half-life = $75,400\,\text{years}$.
  • Carbon-14 ($^{14}\text{C}$) to Nitrogen-14 ($^{14}\text{N}$): Half-life = $5,730\,\text{years}$.

Isotopic Clocks, Decay Chains, and Environmental Hazards

• Uranium-238 Decay Chain:

  • $^{238}\text{U}$ decays through a multi-step cascade involving both alpha ($\alpha$) and beta ($\beta$) emissions before reaching stable $^{206}\text{Pb}$.
  • Sequence: $^{238}\text{U}$ ($t_{1/2} = 4.5 \times 10^9\,\text{yr}$, $\alpha$) $\rightarrow$ $^{234}\text{Th}$ ($24.5\,\text{days}$, $\beta$) $\rightarrow$ $^{234}\text{Pa}$ ($1.14\,\text{min}$, $\beta$) $\rightarrow$ $^{234}\text{U}$ ($2.33 \times 10^5\,\text{yr}$, $\alpha$) $\rightarrow$ $^{230}\text{Th}$ ($8.3 \times 10^4\,\text{yr}$, $\alpha$) $\rightarrow$ $^{226}\text{Ra}$ ($1590\,\text{yr}$, $\alpha$) $\rightarrow$ $^{222}\text{Rn}$ ($3.825\,\text{days}$, $\alpha$) $\rightarrow$ $^{218}\text{Po}$ ($3.05\,\text{min}$, $\alpha$) $\rightarrow$ $^{214}\text{Pb}$ ($26.8\,\text{min}$, $\beta$) $\rightarrow$ $^{214}\text{Bi}$ ($19.7\,\text{min}$, $\beta$) $\rightarrow$ $^{214}\text{Po}$ ($1.5 \times 10^{-4}\,\text{sec}$, $\alpha$) $\rightarrow$ $^{210}\text{Pb}$ ($22\,\text{yr}$, $\beta$) $\rightarrow$ $^{210}\text{Bi}$ ($5\,\text{days}$, $\beta$) $\rightarrow$ $^{210}\text{Po}$ ($140\,\text{days}$, $\alpha$) $\rightarrow$ $^{206}\text{Pb}$ (stable).

• Radon Gas Environmental Hazard:

  • Radon-222 ($^{222}\text{Rn}$) is an intermediate gaseous daughter product formed during $^{238}\text{U}$ decay.
  • Because radon is an inert, odorless, colorless gas, it migrates easily from uranium-bearing bedrock, fractured rock, and surrounding soil through sump pits, basement wall cracks, floor drains, and well-water lines into residential buildings.
  • Inhalation of radon gas and its alpha-emitting decay progeny is the second leading cause of lung cancer worldwide.

• Potassium-Argon ($^{40}\text{K} \rightarrow ^{40}\text{Ar}$) Dating:

  • Potassium is abundant in common rock-forming silicates such as potassium feldspar, muscovite, biotite, and hornblende.
  • $^{40}\text{K}$ decays to $^{40}\text{Ca}$ and $^{40}\text{Ar}$ ($t_{1/2} = 1.25 \times 10^9\,\text{years}$).
  • Used extensively to date igneous volcanic and plutonic rocks. Trapped atmospheric $^{40}\text{Ar}$ must be corrected for during mass spectrometry analysis.

• Radiocarbon ($^{14}\text{C}$) Dating:

  • Cosmic ray neutrons convert atmospheric nitrogen into radiocarbon: $^1_0n + ^{14}_7\text{N} \rightarrow ^{14}_6\text{C} + ^1_1p$
  • Living plants and animals incorporate $^{14}\text{C}$ in equilibrium with the atmosphere via photosynthetically fixed $\text{CO}_2$ and the food chain.
  • Upon death, $^{14}\text{C}$ uptake ceases and decays back to $^{14}\text{N}$ ($t_{1/2} = 5,730\,\text{years}$).
  • Applicable to organic matter up to ~$50,000\,\text{years}$ old. Radiocarbon dates are calibrated against independent incremental clocks including tree rings (dendrochronology), annual lake varves, and deep-sea coral growth bands.

Determination of the Absolute Age of Earth and Solar System Materials

• Evidence from Terrestrial Crustal Rocks and Minerals:

  • Oldest Dated Mineral: Zircon ($\text{ZrSiO}_4$) grains extracted from sandstone within the Narryer Gneiss in Jack Hills, Western Australia, dated radiometrically up to $4.40 \times 10^9\,\text{years}$ ($4.40\,\text{Ga}$).
  • Oldest Dated Whole Rock: Garnet-bearing gneiss of the Nuvvuagittuq greenstone belt located on the eastern shore of Hudson Bay in Québec, Canada, dated at ~$4.28 \times 10^9\,\text{years}$ ($4.28\,\text{Ga}$).

• Lead Isotope Evolution in Primitive Meteorites (Chondrites):

  • Chondrites are primitive stony meteorites representing unmodified solar nebula material.
  • Chondritic meteorites contain lead isotopes ($^{204}\text{Pb}$, $^{206}\text{Pb}$, $^{207}\text{Pb}$, $^{208}\text{Pb}$) but virtually zero original Uranium or Thorium.
  • $^{204}\text{Pb}$ is non-radiogenic, remaining constant over time.
  • $^{206}\text{Pb}$, $^{207}\text{Pb}$, and $^{208}\text{Pb}$ accumulate over time across planetary bodies due to $\text{U}$ and $\text{Th}$ decay.
  • Comparing Earth's present isotopic lead ratios with primitive chondritic ratios establishes Earth's bulk accretion age at $4.55 \times 10^9\,\text{years}$ ($4.55\,\text{Ga}$).

• Lunar Rock Evidence (Apollo Missions):

Questions and Analytical Scenarios

• Question: Why are rocks older than $4.0\,\text{Ga}$ extremely rare on Earth?

  • Answer: Earth's dynamic plate tectonics, continuous mantle convection, subduction recycling, and active surface hydrologic erosion destroy pre-existing continental crust, whereas airless bodies like the Moon lack liquid water and active plate tectonics, preserving primitive crust.

• Question: Do detrital zircon grains dated at $4.40\,\text{Ga}$ inside a sedimentary sandstone indicate the age of the sandstone itself?

  • Answer: No. Detrital zircons reflect the radiometric cooling age of the pre-existing igneous or metamorphic parent rock from which the grains were eroded. The host sandstone must be younger than the included zircon grains according to the Principle of Inclusions.

• Question: Which radioisotope pairs are best suited to date geological samples that are approximately $1,000,000\,\text{years}$ ($1\,\text{Ma}$) old?

  • Answer: Beryllium-10 ($^{10}\text{Be}$ to $^{10}\text{B}$, $t_{1/2} = 1.52\,\text{Ma}$) or Chlorine-36 ($^{36}\text{Cl}$ to $^{36}\text{Ar}$, $t_{1/2} = 300,000\,\text{yr}$) are ideal due to their intermediate half-life ranges.