Geology and Time

Historical Concepts of Geologic Time

• Early Cultural Perspectives on Earth's Age:

  • Prior to the nineteenth century, the accepted age of Earth in Western culture was largely rooted in literal biblical interpretations, placing the planet's age at approximately $6{,}000\text{ years}$.

  • By contrast, ancient Eastern philosophies (such as Chinese and Hindu traditions) viewed the Earth and cosmos as ancient beyond human comprehension, spanning cycles lasting billions of years.

• James Hutton and Modern Geologic Thought:

  • Recognized as the "Father of Geology," James Hutton was an eighteenth-century Scottish physician and farmer who revolutionized the understanding of Earth history.

  • Hutton recognized that vast, immense expanses of geologic time were necessary to account for the slow, observable physical processes shaping rock bodies and landscapes.

  • Uniformitarianism: Formulated as the fundamental tenet that "the present is the key to the past." Geologic processes currently active on Earth today operated in the same fundamental physical manner throughout the geologic past.

  • Actualism: A modernized expression of uniformitarian principles recognizing that while the fundamental physical and chemical laws of nature remain constant over time, the rates, intensities, and environmental settings of specific processes can vary.

• Categories of Geologic Time:

  • Numerical Age (Absolute Age): The measurement and specification of the actual elapsed time of a geological feature or event in concrete temporal units, such as millions or billions of years before the present.

  • Relative Time: The determination of the chronological order and sequence in which a series of geologic events took place, without reference to specific numerical dates in years.

Fundamental Principles of Relative Dating

• Principle of Original Horizontality:

  • States that sedimentary strata are naturally accumulated and deposited under the influence of gravity in essentially horizontal beds.

  • If sedimentary or volcanic rock beds are observed to be tilted, folded, or steeply dipping, they must have undergone tectonic deformation subsequent to their original deposition.

• Law of Superposition:

  • States that in an undisturbed, undeformed sequence of sedimentary or surface-deposited volcanic strata, the oldest bed is situated at the base, and layers become progressively younger toward the top.

  • Each layer is younger than the rock unit beneath it and older than the rock unit deposited above it.

• Principle of Lateral Continuity:

  • Asserts that original sedimentary layers extend outward in all horizontal directions continuously until they gradually thin out and taper at their depositional margins, or encounter an abrupt physical depositional boundary or barrier.

  • Isolated matching rock outcrops exposed on opposite sides of a canyon or valley indicate that the strata originally existed as a continuous, unbroken sheet prior to erosional carving by river incision.

• Principle of Cross-Cutting Relationships:

  • States that an igneous intrusion, fault fracture, or erosional truncation that penetrates, cuts through, or displaces another rock mass must be younger than the rock body or structural feature it alters.

  • If a fault disrupts several strata, the strata had to be deposited and lithified prior to the fault movement.

  • If an igneous dike or pluton invades host strata, the host rocks necessarily predate the melting, migration, and final crystallization of that igneous body.

• Principle of Inclusions:

  • States that any rock fragments (clasts or xenoliths) contained inside another host rock body must be older than the host rock that envelops them.

  • Granitic Xenoliths: Host granite that envelops chunks of altered country rock indicates that the country rock existed first, fragmented, and became incorporated into the molten intrusive magma.

  • Sedimentary Conglomerates: Clasts or pebbles of granite embedded inside an overlying sedimentary bed indicate that the underlying granite was emplaced, cooled, uplifted, subaerially exposed, and eroded before deposition of the younger sediment.

  • Contact Metamorphic Zones: Thermal metamorphic halos baked into the wall rocks adjacent to an igneous contact demonstrate that the intrusive pluton postdates the host wall rocks.

• Step-by-Step Reconstruction of Regional Cross-Sections:

  • Sequential Deposition and Intrusion Analysis (Reference Area Scenario):

    • Deposition of the Lutgrad Formation, followed successively by the Birkland Formation, Tarburg Formation, and Leet Junction Formation in accordance with the law of superposition.

    • Intrusion of a granitic pluton (crystallized at $540\text{ million years old}$), producing an extensive contact metamorphic zone along adjacent country rocks.

    • Tectonic uplift and regional tilting of the entire package (Lutgrad through Leet Junction formations and the granite).

    • Prolonged erosion removing the upper portions of the tilted sedimentary strata and unroofing the granitic batholith to create an irregular paleotopographic surface.

    • Submergence and horizontal deposition of the Larsonton Formation across the erosion surface.

    • Intrusion of a vertical igneous dike (crystallized at $78\text{ million years old}$) that cross-cuts through the granite, Birkland Formation, and Larsonton Formation.

    • Subsequent erosional beveling truncating the upper Larsonton Formation and the top of the $78\text{ million year old}$ dike.

    • Renewed horizontal sedimentation depositing the Foster City Formation, Hamlinville Formation, and Skinner Gulch Limestone.

    • Modern landscape incision by streams creating contemporary topography.

  • Hypothetical Bed Sequence (Strata A through K):

    • Deposition of sedimentary beds $A$, $B$, $C$, and $E$ in normal superpositional order.

    • Intrusion of concordant igneous sill $D$ between beds $C$ and $E$; sill $D$ is younger than both enclosing beds, corroborated by inclusions of $C$ and $E$ within the sill.

    • Intrusion of discordant igneous dike $F$, cutting across beds $A$ through $E$ and sill $D$, proving dike $F$ is younger than all cut units.

    • Tectonic tilting of beds $A$ through $E$, followed by subaerial planation and erosion to form an angular unconformity.

    • Renewed deposition of flat-lying layers $G$, $H$ (a surface lava flow adhering to superposition), $I$, $J$, and $K$.

    • Modern stream erosion incising an irregular valley into bed $K$.

Stratigraphic Correlation and Biostratigraphy

• Concepts and Methods of Stratigraphic Correlation:

  • Correlation is the practice of demonstrating chronological or stratigraphical equivalence between rock bodies, layers, or events across disparate geographic locations.

  • Physical Continuity: Tracing an uninterrupted rock layer across physical distances by walking the outcrop exposure.

  • Similarity in Rock Types (Lithologic Correlation): Matching strata based on matching mineralogy, texture, color, and vertical sequence; this method is especially dependable when an unusual or distinctive marker bed (such as a unique volcanic bentonite layer or coal seam) is involved.

  • Colorado Plateau Regional Stratigraphic Correlation: Demonstrates widespread continuity across southwestern North America from Grand Canyon National Park to Zion National Park and Bryce Canyon National Park:

    • Precambrian: Vishnu Schist.

    • Cambrian: Tapeats Sandstone, Bright Angel Shale, Muav Formation (Muav Limestone).

    • Devonian: Temple Butte Limestone.

    • Mississippian: Redwall Limestone.

    • Pennsylvanian: Supai Group / Formation.

    • Permian: Hermit Shale, Coconino Sandstone, Toroweap Formation, Kaibab Limestone (Kaibab Formation).

    • Triassic: Moenkopi Formation, Chinle Formation, Wingate Sandstone, Kayenta Formation.

    • Jurassic: Navajo Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, Winsor Formation.

    • Cretaceous: Dakota Sandstone, Tropic Shale, Straight Cliffs Sandstone, Wahweap Sandstone, Kaiparowits Formation.

    • Tertiary (Cenozoic): Wasatch Formation.

• Fossil Preservation and Faunal Succession:

  • Organisms living during sediment deposition become entombed; durable skeletal structures (bones, teeth, shells, woody material) fossilize.

  • The overwhelming majority of fossilized species preserved in ancient strata represent taxa that are now extinct.

  • Principle of Faunal Succession: Formulated by William Smith, this principle establishes that fossil organisms succeed one another vertically through rock strata in a definite, recognizable, and irreversible chronological sequence.

  • Strata in progressively deeper, older positions yield organic remains that exhibit increasingly pronounced anatomical differences from living species.

• Index Fossils and Fossil Assemblages:

  • Index Fossils: Highly diagnostic fossils characterized by short evolutionary longevity (rapid turnover), widespread global or regional geographic distribution, abundant occurrence, and ease of physical identification.

  • Fossil Assemblages: Groups of distinct, coexisting species preserved together within a discrete sedimentary stratum.

  • Dating using fossil assemblages is significantly more precise than relying on an individual taxon because the host sedimentary unit had to be deposited during the specific narrow interval of time where the life spans of all preserved species overlapped.

• Identification of Gaps Using Fossil Assemblages:

  • In an unbroken sequence (Area 1):

    • Rock $X$ contains a distinct assemblage characteristic of Time Interval $1$.

    • Rock $Y$ contains an assemblage diagnostic of Time Interval $2$.

    • Rock $Z$ contains an assemblage unique to Time Interval $3$.

  • In an adjacent section (Area 2):

    • Rock $X$ directly underlies Rock $Z$, completely missing rock layers with the diagnostic assemblage of Time Interval $2$.

    • The stratigraphic surface between $X$ and $Z$ is conclusively proven to be a disconformity representing a non-depositional or erosional gap.

Unconformities: Gaps in the Rock Record

• Nature and Significance of Unconformities:

  • Rock sequences deposited continuously without temporal breaks or pauses are termed conformable.

  • An unconformity represents a significant chronological hiatus in the geologic rock record produced by a sustained period of non-deposition, crustal uplift, and subaerial weathering/erosion, followed by the resumption of deposition.

  • The rock stratum positioned directly above an unconformity is considerably younger than the rock body situated directly beneath it.

• Angular Unconformity:

  • Characterized by tilted or folded sedimentary rock strata that are truncated by an erosional plane and subsequently overlain by flatter, younger sedimentary or volcanic strata.

  • Sequence of Development:

    • Deposition of original horizontal marine sediment beds ($A$).

    • Compressional tectonism (such as plate convergence) causing mountain building, regional uplift, and intense folding or tilting ($B$).

    • Prolonged weathering and subaerial erosion beveling the uplifted strata to an erosion surface ($C$).

    • Subsidence below sea level (or marine transgression) accompanied by the renewed deposition of flat-lying sedimentary beds ($D$).

  • Named Example: Cody, Wyoming, where younger, horizontal beds rest directly above tilted, eroded older red beds.

• Disconformity:

  • An unconformity where the contact separates parallel sedimentary rock layers above and below.

  • Because structural tilting or folding is absent, disconformities are far less visually obvious than angular unconformities and often resemble simple bedding planes.

  • They are recognized primarily through missing biozones revealed by fossil assemblages, channels cut into paleosurfaces, or weathered fossil soils (paleosols).

• Nonconformity:

  • An unconformity where younger sedimentary or surface volcanic strata rest directly on an eroded surface of much older plutonic igneous or metamorphic basement rocks.

  • Because plutonic rocks (such as granite batholiths) and regional metamorphic rocks form exclusively at profound depths under extreme heat and pressure, a nonconformity signifies vast amounts of regional crustal uplift and the deep stripping of miles of overburden before subsidence and reburial.

  • Sequence of Development:

    • Initial horizontal marine sedimentation ($A$).

    • Deep crustal burial ($B$).

    • Orogenic mountain-building episode accompanied by intense regional deformation, metamorphic foliation, and plutonic intrusion ($C$).

    • Prolonged regional uplift accompanied by extensive erosion ($D$).

    • Continued planation stripping off all overlying strata until deep crystalline rocks are exposed at the surface ($E$).

    • Subsidence and renewed deposition of sedimentary cover strata upon the crystalline basement ($F$).

• Unconformities of the Grand Canyon:

  • The Great Nonconformity: The crystalline igneous and metamorphic basement rocks of the Inner Gorge (the vertically foliated Vishnu Schist and Zoroaster Granite) are unconformably overlain by sedimentary strata of the Unkar Group and the Cambrian Tapeats Sandstone.

  • Angular Unconformity: Tilted, fault-block sedimentary strata of the Precambrian Unkar Group are beveled flat and overlain horizontally by the Cambrian Tapeats Sandstone.

  • Disconformities: Prominent disconformities occur within the Paleozoic section, noticeably separating the Devonian Temple Butte Limestone from the overlying Mississippian Redwall Limestone, as well as above the Redwall Limestone prior to the deposition of the Supai Group.

Absolute Geochronology and Radioactive Decay

• Primary Methodologies of Absolute Dating:

  • Dendrochronology: Counting annual seasonal growth rings in preserved tree trunks.

  • Varve Analysis: Counting paired couplets of fine and coarse sediment layers deposited on an annual basis in quiet glacial lakes.

  • Radioactivity: The most powerful, quantitative, and universally applicable tool for measuring deep geologic time.

• Nuclear Physics of Isotopes and Decay:

  • The atomic number of an element is determined exclusively by its quantity of nuclear protons.

  • Isotopes: Variant forms of an element possessing an identical number of protons but variable numbers of neutrons, resulting in identical chemical behavior but differing atomic mass numbers.

  • Uranium-238 ($^{238}\text{U}$): Contains $92\text{ protons}$ and $146\text{ neutrons}$ (atomic mass $= 238$).

  • Uranium-235 ($^{235}\text{U}$): Contains $92\text{ protons}$ and $143\text{ neutrons}$ (atomic mass $= 235$).

• Mechanisms of Radioactive Decay:

  • Radioactivity represents the spontaneous, natural disintegration of an unstable parent nucleus into a more stable daughter configuration, accompanied by the release of nuclear energy and subatomic particles (detectable with radiation detectors like Geiger counters).

  • 1. Alpha Decay ($\alpha$):

    • An unstable nucleus expels an alpha particle consisting of $2\text{ protons}$ and $2\text{ neutrons}$ (a Helium nucleus, $^4\text{He}$).

    • Mass Effect: The atomic mass number decreases by exactly $4$.

    • Atomic Effect: The atomic number decreases by exactly $2$, transforming the atom into an entirely different element.

  • 2. Beta Decay ($\beta$):

    • A neutron within the nucleus spontaneously ejects a high-energy electron (beta particle) and is converted into a proton.

    • Mass Effect: The atomic mass number remains unchanged.

    • Atomic Effect: The atomic number increases by exactly $1$, creating a new element.

  • 3. Electron Capture:

    • A nuclear proton captures an inner orbital electron, neutralizing its positive charge and transforming into a neutron.

    • Mass Effect: The atomic mass number remains unchanged.

    • Atomic Effect: The atomic number decreases by exactly $1$, creating a new element.

• Key Decay Chains:

  • Uranium-238 to Lead-206 ($^{238}\text{U} \rightarrow {}^{206}\text{Pb}$):

    • Parent nucleus $^{238}\text{U}$ ($92\text{ protons}$, $146\text{ neutrons}$) undergoes a multi-step decay series to reach stable daughter $^{206}\text{Pb}$ ($82\text{ protons}$, $124\text{ neutrons}$).

    • Net loss throughout cascade: $10\text{ protons}$ and $22\text{ neutrons}$ ($32\text{ total nucleons}$ expelled).

    • Cascade intermediates: $^{238}\text{U} \rightarrow {}^{234}\text{Th} \rightarrow {}^{234}\text{Pa} \rightarrow {}^{234}\text{U} \rightarrow {}^{230}\text{Th} \rightarrow {}^{226}\text{Ra} \rightarrow {}^{222}\text{Rn} \rightarrow {}^{218}\text{Po} \rightarrow {}^{214}\text{Pb} \rightarrow {}^{214}\text{Bi} \rightarrow {}^{214}\text{Po} \rightarrow {}^{210}\text{Pb} \rightarrow {}^{210}\text{Bi} \rightarrow {}^{210}\text{Po} \rightarrow {}^{206}\text{Pb}$.

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

    • Unstable parent $^{40}\text{K}$ ($19\text{ protons}$) undergoes electron capture, absorbing an orbiting electron to neutralize a proton into a neutron.

    • Produces the noble gas daughter $^{40}\text{Ar}$ ($18\text{ protons}$), reducing the atomic number from $19$ to $18$.

The Half-Life Concept and Radiometric Systems

• Definition and Mathematics of Half-Life:

  • The half-life ($t_{1/2}$) is the precise duration of time required for exactly half ($50\%$) of the unstable parent radioisotope nuclei in a closed system to decay into stable daughter products.

  • Radioactive decay rates are physically constant, impervious to changes in crustal temperature, tectonic pressure, chemical bonding, or gravitational fields.

  • Geometric progression of remaining parent isotope from an initial pure sample (e.g., $1\text{ mg}$ of $^{40}\text{K}$):

    • After 1 half-life: $\frac{1}{2}\,(50\%)$ parent remaining, $\frac{1}{2}\,(50\%)$ daughter accumulated.

    • After 2 half-lives: $\frac{1}{4}\,(25\%)$ parent remaining, $\frac{3}{4}\,(75\%)$ daughter accumulated.

    • After 3 half-lives: $\frac{1}{8}\,(12.5\%)$ parent remaining, $\frac{7}{8}\,(87.5\%)$ daughter accumulated.

    • After 4 half-lives: $\frac{1}{16}\,(6.25\%)$ parent remaining, $\frac{15}{16}\,(93.75\%)$ daughter accumulated.

    • After 5 half-lives: $\frac{1}{32}\,(3.125\%)$ parent remaining, $\frac{31}{32}\,(96.875\%)$ daughter accumulated.

• Common Geochronological Isotope Systems:

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

    • Half-life: $1.3\text{ billion years}$.

    • Effective dating range: $100{,}000\text{ to } 4.6\text{ billion years}$.

  • Uranium-238 to Lead-206 ($^{238}\text{U} \rightarrow {}^{206}\text{Pb}$):

    • Half-life: $4.5\text{ billion years}$.

    • Effective dating range: $10\text{ million to } 4.6\text{ billion years}$.

  • Uranium-235 to Lead-207 ($^{235}\text{U} \rightarrow {}^{207}\text{Pb}$):

    • Half-life: $703\text{ million years}$ (or $713\text{ million years}$).

    • Effective dating range: $10\text{ million to } 4.6\text{ billion years}$.

  • Thorium-232 to Lead-208 ($^{232}\text{Th} \rightarrow {}^{208}\text{Pb}$):

    • Half-life: $14.1\text{ billion years}$.

    • Effective dating range: $10\text{ million to } 4.6\text{ billion years}$.

  • Rubidium-87 to Strontium-87 ($^{87}\text{Rb} \rightarrow {}^{87}\text{Sr}$):

    • Half-life: $49\text{ billion years}$.

    • Effective dating range: $10\text{ million to } 4.6\text{ billion years}$.

  • Carbon-14 to Nitrogen-14 ($^{14}\text{C} \rightarrow {}^{14}\text{N}$):

    • Half-life: $5{,}730\text{ years}$.

    • Effective dating range: $100\text{ to } 40{,}000\text{ years}$.

• Analytical Procedures and Practical Execution:

  • Quantitative isotopic measurement requires highly trained specialists and precision mass spectrometers.

  • Laboratory Procedure:

    • Quantify the remaining parent isotope concentration ($\text{e.g., } {}^{40}\text{K}$) via chemical analysis.

    • Quantify the accumulated daughter product concentration ($\text{e.g., } {}^{40}\text{Ar}$).

    • Calculate the initial quantity of parent present at the moment of mineral formation: $\text{Original } {}^{40}\text{K} = \text{Remaining } {}^{40}\text{K} + \text{Accumulated } {}^{40}\text{Ar}$.

    • Determine the number of elapsed half-lives from the parent-to-daughter ratio and multiply by $t_{1/2}$ to yield the numerical age.

• Methodological Limitations and Rock Types:

  • Igneous Rocks: Most successfully dated because crystallization from magma locks parent isotopes into crystal lattices while excluding daughters, resetting the radioactive clock to zero.

  • Sedimentary Rocks: Rarely datable directly by radiometric methods. Dating mineral clasts (such as quartz or feldspar grains) reveals the crystallization age of the distant parent igneous/metamorphic rock source, not the time of sediment deposition.

  • Metamorphic Rocks: Thermal metamorphism often reopens crystal systems, permitting daughter isotopes to escape or parents to migrate; this yields dates reflecting metamorphic heating rather than original protolith age.

  • Chemically Weathered Rocks: Weathering leaches mobile ions and alters isotopic ratios, producing entirely erroneous dates.

  • Specific System Constraints:

    • K-Ar Limitations: Though exceptionally versatile across deep time, $^{40}\text{Ar}$ is an unreactive gas. Heating events can cause argon leakage from minerals, leading to an artificially young measured age.

    • U-Pb Concordance: Highly reliable because two completely separate isotopes of uranium ($^{238}\text{U}$ and $^{235}\text{U}$) decay to two separate isotopes of lead ($^{206}\text{Pb}$ and $^{207}\text{Pb}$) within the same crystal (e.g., zircon), providing a built-in cross-check.

• Environmental Geology Hazard: Radon Gas:

  • Radon-222 ($^{222}\text{Rn}$) is an intermediate, gaseous radioactive daughter formed during the decay of Radium ($^{226}\text{Ra}$) in the $^{238}\text{U} \rightarrow {}^{206}\text{Pb}$ series.

  • Possesses a half-life of $3.8\text{ days}$.

  • Health hazard: Being a gas, it diffuses through soil and fractures into indoor air; as an alpha emitter, it damages alveolar tissue upon inhalation, causing between $5{,}000\text{ and } 30{,}000\text{ lung cancer deaths annually in the United States}$.

  • Commonly concentrated in bedrocks composed of granite, gneiss, limestone, black shale, and phosphorus-rich soils.

  • Migrates into structures with unsealed basements and crawl spaces.

Integrating Relative and Absolute Dating

• Age Bracketing of Sedimentary Strata:

  • Sedimentary rocks containing fossils are correlated relatively, but cannot typically be dated isotopically.

  • Numerical ages are assigned by bracketing sedimentary layers between cross-cutting or interbedded igneous bodies of known radiometric age.

  • Application to Complex Cross-Sections: If horizontal sedimentary formations (such as the Larsonton, Foster City, and Hamlinville formations) are deposited upon an eroded pluton dated to $540\text{ million years}$, and are subsequently cross-cut by an igneous dike dated to $78\text{ million years}$, their deposition is securely bracketed between $540\text{ and } 78\text{ million years ago}$.

The Standard Geologic Time Scale

• Structure and Hierarchy:

  • A globally unified relative time framework established originally via fossil assemblages and law of superposition, calibrated with radiometric ages.

  • Hierarchical classification: Eon $\rightarrow$ Era $\rightarrow$ Period $\rightarrow$ Epoch.

• Precambrian Time (Informal Eon/Supereon):

  • Encompasses approximately $88\%$ of total Earth history, extending from Earth's origin ($4{,}550\text{ million years ago}$) to the base of the Cambrian period ($541\text{ million years ago}$).

  • Hadean Eon: $4{,}550\text{ Ma to } 4{,}000\text{ Ma}$; planetary accretion and differentiation.

  • Archean Eon: $4{,}000\text{ Ma to } 2{,}500\text{ Ma}$; origin of earliest primitive life.

  • Proterozoic Eon: $2{,}500\text{ Ma to } 541\text{ Ma}$; oxygenation of atmosphere and appearance of eukaryotic and multicellular life.

• Phanerozoic Eon ("Visible Life"):

  • Spans from $541\text{ million years ago}$ to the present day, subdivided into three major eras:

  • 1. Paleozoic Era ("Old Life"):

    • Duration: $541\text{ Ma to } 252\text{ Ma}$.

    • Characterized by an explosion of diverse, abundant, complex marine invertebrates, followed by the evolution of fishes, amphibians, and early land plants.

    • Cambrian Period ($\text{-C-}$ or $\in$): $541\text{ to } 488\text{ Ma}$.

    • Ordovician Period ($\text{O}$): $488\text{ to } 444\text{ Ma}$.

    • Silurian Period ($\text{S}$): $444\text{ to } 416\text{ Ma}$.

    • Devonian Period ($\text{D}$): $416\text{ to } 359\text{ Ma}$.

    • Carboniferous System: Subdivided into:

      • Mississippian Period ($\text{M}$): $359\text{ to } 318\text{ Ma}$.

      • Pennsylvanian Period ($\mathbb{P}$): $318\text{ to } 299\text{ Ma}$.

    • Permian Period ($\text{P}$): $299\text{ to } 252\text{ Ma}$.

  • 2. Mesozoic Era ("Middle Life"):

    • Duration: $252\text{ Ma to } 66\text{ Ma}$.

    • Characterized by the radiation and dominance of reptiles and dinosaurs, as well as the appearance of early mammals and birds.

    • Triassic Period ($\mathbb{R}$): $252\text{ to } 200\text{ Ma}$; emergence of dinosaurs at approximately $230\text{ million years ago}$.

    • Jurassic Period ($\text{J}$): $200\text{ to } 145\text{ Ma}$.

    • Cretaceous Period ($\text{K}$): $145\text{ to } 66\text{ Ma}$; concluded by the mass extinction of dinosaurs at approximately $65\text{ to } 66\text{ million years ago}$.

  • 3. Cenozoic Era ("New Life"):

    • Duration: $66\text{ Ma to Present}$.

    • The age of mammals, flowering plants, and hominid evolution; historically divided into the Tertiary and Quaternary periods.

    • Paleogene Period ($\text{PG}$): $66\text{ to } 23\text{ Ma}$.

      • Paleocene Epoch: $66\text{ to } 56\text{ Ma}$.

      • Eocene Epoch: $56\text{ to } 33.9\text{ Ma}$.

      • Oligocene Epoch: $33.9\text{ to } 23\text{ Ma}$.

    • Neogene Period ($\text{N}$): $23\text{ to } 2.6\text{ Ma}$.

      • Miocene Epoch: $23\text{ to } 5.3\text{ Ma}$.

      • Pliocene Epoch: $5.3\text{ to } 2.6\text{ Ma}$.

    • Quaternary Period ($\text{Q}$): $2.6\text{ Ma (or } 1.6\text{ Ma) to Present}$.

      • Pleistocene Epoch: $2.6\text{ Ma to } 0.01\text{ Ma}$; extensive continental glaciation and modern human evolution.

      • Holocene (Recent) Epoch: $0.01\text{ Ma } (10{,}000\text{ years ago})\text{ to Present}$; post-glacial civilization.