Introduction to Geologic Rock Dating and the Rock Cycle

Geologic Evidence and the Earth's Crust

• The Search for Evidence: To determine the age of the Earth, rocks, or fossils, scientists look for evidence within the Earth's crust.

• Exploration Limits: Human drilling has not reached the Earth's mantle or core; consequently, all available physical samples originate from the crust.

• Crustal Classifications:

  • Continental Crust: Characteristics include being significantly thicker but less dense.

  • Oceanic Crust: Characteristics include being thinner but more dense.

The Molecular and Elemental Composition of Rocks

• Atomic Structure: Atoms are the smallest particles composing matter on Earth, consisting of:

  • Electrons: Particles with a negative charge.

  • Protons: Particles with a positive charge.

  • Neutrons: Particles with no charge (neutral).

• Ions: Positively or negatively charged atoms resulting from an imbalance between protons in the nucleus and electrons.

• Isotopes: A specific variation of an element where the number of neutrons differs, but the number of protons remains the same. This is the primary component used by scientists for rock dating.

  • Hydrogen Isotope Example:

    • Hydrogen: $1$ proton, $0$ neutrons.

    • Deuterium: $1$ proton, $1$ neutron.

    • Tritium: $1$ proton, $2$ neutrons.

• Elements in the Earth's Crust: Elements are classified by unique combinations of subatomic particles. The most abundant elements by weight in the crust include:

  • Oxygen: $46.6\%$

  • Silicon: $27.7\%$

  • Aluminum, Iron, Calcium, Sodium, Potassium: Grouped together as major elements.

  • Other Elements: All other elements combined make up only $1.5\%$

The Hierarchy of Matter: From Atoms to Rocks

• Structural Chain: Elements combine to form minerals, which subsequently join together to form rocks.

• Minerals: Unique combinations of elements that result in characteristic hardness, color, and density.

  • Silicate Minerals: These are the most widespread minerals on Earth, composing $95\%$ of the Earth's crust.

• Rock Definition: Rocks can be mixtures of minerals (e.g., granite), a mass of a single mineral (e.g., rock salt/halite), undifferentiated material (e.g., obsidian/volcanic glass), or organic matter (e.g., coal).

The Rock Cycle and Uniformitarianism

• The Rock Cycle: A continuous process where rocks in the crust are moved, recycled, destroyed, and created. This is driven by:

  • Internal Processes: Driven by plate tectonics and conductive currents.

  • External Processes: Driven by wind, water, and ice on the Earth's surface.

• Uniformitarianism: A fundamental scientific principle stating that the physical processes acting in the environment today have also been active throughout geologic time.

  • Example: If gravity causes a rock to fall today or water carves a valley today, it is assumed these processes functioned identically in the past.

Igneous Rock Classification

• Formation: Igneous rocks only form when rock is melted, cooled, and re-hardened. Approximately $90\%$ of the Earth's crust is igneous.

• Locational Categories:

  • Extrusive (Volcanic): Molten rock that exits the surface and cools. Example: Basalt.

  • Intrusive (Plutonic): Molten rock that cools and hardens inside the Earth's crust. Example: Granite.

• Compositional Categories:

  • Mafic: Often extrusive, dark-colored, fast-cooling, and fine-grained.

  • Felsic: Often intrusive, light-colored, slow-cooling, and coarse-grained.

• Intrusive Structures: These include sills, dikes, laccoliths, and batholiths.

Sedimentary Rock Classification

• Formation: Derived from "rock bits" or organic matter that are disintegrated via weathering, transported by erosion (wind, water, ice), and eventually deposited and cemented together.

• Law of Original Horizontality: Based on uniformitarianism and gravity, sedimentary rocks are deposited in horizontal layers.

• Sub-categories of Sedimentary Rocks:

  • Clastic: Composed of physical rock fragments. Examples: Sandstone (sand bits), Shale (compacted mud).

  • Chemical: Formed by precipitation from evaporated fluids. Examples: Halite, Gypsum.

  • Biological: Formed from compacted organic remains. Examples: Limestone (bones and shells), Coal (dead plant matter).

Metamorphic Rock Classification

• Formation: Created when igneous or sedimentary rocks are subjected to incredible heat and pressure in the subsurface (often near subduction zones) without melting.

• Characteristics: These are the strongest rocks on Earth and are highly resistant to weathering and erosion.

• Structural Categories:

  • Foliated: Minerals are flattened into parallel layers resembling pages in a book. Example: Slate.

  • Non-foliated: Lacks visible layers or banding. Example: Quartzite.

Radiometric Dating and the Atomic Clock

• Mechanism: Scientists use radioactive isotopes with unstable nuclei. As the nucleus disintegrates, it emits radiation and decays into a more stable end product (different element).

• Radioactive Decay: The process of emitting charged particles at a specific, reliable rate.

• Proportion Measurement: Scientists compare the ratio of the original "Parent" isotope to the "Daughter" decayed product.

• Mass Spectrometry: An instrument (based on designs by Rutherford) that separates isotopes by mass to count the relative proportions of elements.

• Half-Life: The amount of time required for $50\%$ of the original radioactive isotope to decay away.

  • Decay Curve: The relationship is non-linear. As the parent atom percentage decreases (green curve), the daughter atom percentage increases (blue curve).

• Common Dating Clocks:

  • Uranium to Lead: Used for exceptionally old rocks; has a half-life of roughly $4.5 \times 10^9$ years.

  • Potassium to Argon: Common radiometric clock.

  • Carbon-14 to Nitrogen-14: Used for organic materials.

Case Study: Geological Dating in New Zealand

• Speaker: Hamish Campbell, Geologist at the Institute of Geological and Nuclear Sciences.

• Location: Wellington Harbor and the Hutt Valley, observing the Wellington Fault scarp.

• Fossil Dating:

  • Trace Fossils: Burrows from organisms on the seafloor dated to approximately $65,000,000$ years old.

  • Bellumites: Remains of a type of squid dated to approximately $150,000,000$ years old.

  • Fossil Sequence: Used to date the last $550,000,000$ years of Earth's history.

• Rock Processing: A sample of Graywacke (New Zealand's dominant rock type) is crushed using a hydraulic ram into gravel, then into a sugar-consistency powder using a tungsten steel mill called a "T-mite" to extract Zircon minerals for Uranium-Lead dating.

Radiocarbon Dating for Organics

• Carbon-14 Formation: High in the atmosphere, cosmic rays hit nitrogen atoms to create Carbon-$14$ (unstable), which eventually enters the food chain.

• Biological Clock: Living organisms maintain a constant ratio of Carbon-$12$ (stable) and Carbon-$14$. Upon death, Carbon-$14$ begins to decay into Nitrogen.

• Numerical Constraints:

  • Half-life: The average half-life of Carbon-$14$ is $5,730$ years.

  • Applicability: Radiocarbon dating is only effective for fossils up to approximately $60,000$ years old.

Geologic Time Scale and Oldest Earth Rocks

• Stratigraphy: The study of rock layering where fossils appear and disappear in the record.

• Human Placement: Humans live in the Holocene epoch, Quaternary period, Cenozoic era.

• Oldest Terrestrial Rock: Found in the Jack Hills region of Australia.

  • Age: Dated using Zircon minerals to be $4,375,000,000$ years old (approximately $4.375$ billion years).

  • Composition: Water-rich, granite-like rock.

Questions & Discussion

• Inquiry on Origin Knowledge: Professor Preciado asks how we know the oldest/youngest places on Earth. This is answered by dating crustal rocks using isotopes.

• Inquiry on Rock Composition: Rocks are composed of minerals, which are composed of elements, which are composed of atoms.

• Video Discussion: The video with Hamish Campbell explains the specific use of mass spectrometers as a "sheep race" to separate isotopes like Uranium from Lead based on mass.