Geologic Time and the Geologic Time Scale

Incomplete Rock Records and Unconformities

• Completeness of the Rock Record:

  • Earth's rock layers do not contain a complete record of Earth history due to continuous geological processes that erase rock material.

  • After rock layers form, geological forces often uplift and expose them at Earth's surface.

  • Once exposed at the surface, environmental forces such as wind and rain cause weathering and erosion, removing significant amounts of material from the rock record.

  • Eroded areas create structural gaps in the geological record.

• Definition and Nature of Unconformities:

  • Unconformity (un kun FOR muh tee): A surface where rock has eroded away, producing a break, or gap, in the rock record.

  • Physical distinction: An unconformity is not a hole or a physical space in the rock. It is an eroded surface on top of older rocks with younger rocks deposited on top.

  • Temporal representation: An unconformity represents a gap in time, which can span a few hundred years, a million years, or billions of years.

• The Three Major Types of Unconformities:

  • Disconformity: Occurs when younger sedimentary layers are deposited on top of older, horizontal sedimentary layers that have been eroded.

  • Angular Unconformity: Occurs when sedimentary layers are deposited on top of older sedimentary layers that have been tilted or folded prior to erosion.

  • Nonconformity: Occurs when younger sedimentary layers are deposited on top of older, eroded igneous or metamorphic rock layers.

Strategies for Filling Gaps: Correlation and Index Fossils

• Concept of Correlation:

  • Correlation (kor uh LAY shun): The geological process of matching rocks and fossils from separate locations to fill gaps in the rock record and build a comprehensive geological history.

  • Methods of correlating rock layers:

  • Direct physical observation: Connecting rock layers by physically walking along exposed rock formations and observing visual and structural similarities.

  • Matching exposed layers: When soil covers rocks or erosion has occurred, geologists correlate rock units by matching exposed rock layers in different geographic locations.

• Regional vs. Intercontinental Correlation:

  • Regional Correlation: Rock formations located within a few hundred kilometers of one another (such as across national parks) are correlated based on:

  • Similarities in rock type (lithology).

  • Geological rock structures.

  • Fossil evidence.

  • Intercontinental / Long-Distance Correlation: When determining the relative ages of rock formations located very far apart or on different continents, geologists rely primarily on fossils.

  • Fundamental inference: If two or more widely separated rock formations contain fossils of about the same age, geologists infer that the rock formations are also about the same age.

• Index Fossils:

  • Limitations of general fossils: Species that lived on Earth for hundreds of millions of years are not helpful for relative-age dating because they represent time spans that are too long.

  • Index Fossils: Fossils of species used to determine relative geological age. To qualify as an index fossil, an organism must satisfy three essential criteria:

  1. Existed on Earth for a relatively short length of time.

  2. Was abundant (existed in large quantities).

  3. Inhabited many widely distributed geographic locations across Earth.

  • Trilobites: Certain species of trilobites serve as key index fossils because they existed for brief time intervals across many different global regions.

  • Stratigraphic application: When an index fossil is found in rock layers at separate locations, geologists infer that those rock layers are of similar age.

Evidence of Major Geologic Events and Key Beds

• Physical Clues of Major Geologic Events:

  • Most major geological events leave distinct physical evidence in the rock record:

  • Volcanic Eruptions: Preserved in rock strata as layers of volcanic ash.

  • Glacial Movement: Recorded by gouged and scarred rock surfaces.

  • Mountain Uplift and Erosion: Evidenced by uplifted mountain formations that have undergone severe weathering and erosion.

  • Ocean Inundations: Evidenced by rock sequences showing marine transgression (oceans flooding the land) and regression (oceans retreating) repeatedly over geological time.

• Key Beds:

  • Key Bed: Rocks or sediment layers used as a distinct marker to correlate rock layers across different geographic regions where layers are exposed.

  • Origin: Formed by sudden, catastrophic global or regional events such as large meteorite strikes or volcanic eruptions that leave a unique, easily recognizable layer.

  • Stratigraphic application: Based on the principle of superposition:

  • Rock layers situated above a key bed are younger than the key bed.

  • Rock layers situated below a key bed are older than the key bed.

• Constructing the Geologic History:

  • Geologists combine multiple principles and evidence to establish the relative chronological order of Earth history:

  • Relative-age dating principles (e.g., superposition).

  • Fossil record and index fossil distribution.

  • Preserved geological evidence of major events and key beds.

Structure and Organization of the Geologic Time Scale

• Overview of the Geologic Time Scale:

  • The Geologic Time Scale is a standardized model of Earth's history from its origin approximately $4.6\text{ billion years}$ ($4,600,000,000\text{ years}$) ago to the present day.

  • Purpose: Helps geologists correlate rock units across countries and continents and provides a standard vocabulary for describing deep time.

  • Evolution of the Scale: It is a dynamic work in progress; boundaries and time spans change as scientists make new discoveries.

• Hierarchical Divisions of Geologic Time:

  • Units are organized hierarchically from largest to smallest duration:

  1. Eons: The longest units of geologic time.

  2. Eras: Subdivisions of eons.

  3. Periods: Subdivisions of eras.

  4. Epochs: Subdivisions of periods.

• Unequal Duration of Time Units:

  • Units of time in the geologic time scale are not equal in duration (e.g., the Paleozoic Era is longer than the Mesozoic and Cenozoic Eras combined).

  • Historical reason: Geologists originally chose boundaries between time units based on fossil changes observed in rock layers. Because time spans between major changes in the fossil record were unequal, the resulting geological time units are also unequal in length.

Chronological Divisions of Geologic Time

• Eons of Earth History:

  • Hadean Eon: From $4600\text{ mya}$ to $4000\text{ mya}$.

  • Archean Eon: From $4000\text{ mya}$ to $2500\text{ mya}$.

  • Proterozoic Eon: From $2500\text{ mya}$ to $541.0\text{ mya}$.

  • Phanerozoic Eon: Current eon, beginning $541.0\text{ mya}$ and continuing to the present.

• Eras of the Phanerozoic Eon:

  • Paleozoic Era: Began $541.0\text{ mya}$ and ended $251.9\text{ mya}$.

  • Mesozoic Era: Began $251.9\text{ mya}$ and ended $66.0\text{ mya}$.

  • Cenozoic Era: Current era, beginning $66.0\text{ mya}$ and continuing to the present.

• Geologic Periods:

  • Ediacaran Period: Began $635\text{ mya}$ and ended $541.0\text{ mya}$.

  • Paleozoic Periods:

  • Cambrian Period: $541.0\text{ mya}$ to $485.4\text{ mya}$.

  • Ordovician Period: $485.4\text{ mya}$ to $443.8\text{ mya}$.

  • Silurian Period: $443.8\text{ mya}$ to $419.2\text{ mya}$.

  • Devonian Period: $358.9\text{ mya}$ to $419.2\text{ mya}$.

  • Carboniferous Period: $358.9\text{ mya}$ to $298.9\text{ mya}$.

  • Permian Period: $298.9\text{ mya}$ to $251.9\text{ mya}$.

  • Mesozoic Periods:

  • Triassic Period: $251.9\text{ mya}$ to $201.3\text{ mya}$.

  • Jurassic Period: $201.3\text{ mya}$ to $145.0\text{ mya}$.

  • Cretaceous Period: $145.0\text{ mya}$ to $66.0\text{ mya}$.

  • Cenozoic Periods:

  • Paleogene Period: $66.0\text{ mya}$ to $23.0\text{ mya}$.

  • Neogene Period: $23.0\text{ mya}$ to $2.6\text{ mya}$.

  • Quaternary Period: Current period, beginning $2.6\text{ mya}$ and continuing to the present.

• Epochs of the Cenozoic Era:

  • Paleogene Epochs:

  • Paleocene Epoch: $66.0\text{ mya}$ to $56.0\text{ mya}$.

  • Eocene Epoch: $56.0\text{ mya}$ to $33.9\text{ mya}$.

  • Oligocene Epoch: $33.9\text{ mya}$ to $23.0\text{ mya}$.

  • Neogene Epochs:

  • Miocene Epoch: $23.0\text{ mya}$ to $5.3\text{ mya}$.

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

  • Quaternary Epochs:

  • Pleistocene Epoch: $2.6\text{ mya}$ to $0.01\text{ mya}$ ($11,600\text{ years}$ ago).

  • Holocene Epoch: Current epoch, beginning $0.01\text{ mya}$ ($11,600\text{ years}$ ago) and continuing to the present.

Comprehending Deep Time through Relative Comparisons

• Understanding Earth's Age:

  • The age of Earth ($4,600,000,000\text{ years}$) is an extraordinarily large duration that is difficult to comprehend relative to human life experiences.

  • Geologists use relative-time comparisons to conceptualize deep time:

  • One-Year Calendar Model: Compressing Earth's entire $4.6\text{ billion-year}$ history into a single $365\text{-day}$ calendar year.

  • 24-Hour Clock Model: Scaling Earth's total history into a single $24\text{-hour}$ clock day.

Vocabulary and Terminology Reference

• Academic Vocabulary:

  • Expose (verb): To make visible; uncover.

  • Correlate (verb): To connect things, facts, or ideas where one thing depends on or affects the other.

  • Infer (verb): To conclude from facts.

  • Duration (noun): The length of time something continues.

  • Standard (adjective): Well established; widely recognized as acceptable.

  • Comprehend (verb): To understand the nature, significance, or meaning of.

• Scientific Vocabulary:

  • Tilted (adjective): Not perfectly horizontal or vertical.

  • Evidence (noun): Information that shows.

  • Distinctive (adjective): Different in a way that is easy to notice.

• Science Use vs. Common Use Terminology:

  • Scale (Science Use): A series of marks or points at known intervals.

  • Scale (Common Use): An instrument used for measuring the weight of an object.

Incomplete Rock Records and Unconformities

• Completeness of the Rock Record:

  • Earth's rock layers do not have a complete record of history because natural processes constantly erase rock material.

  • After rocks form, geological forces push them up to the surface.

  • Wind and rain weather and erode exposed rocks, removing layers and creating missing gaps in Earth's history.

• Definition and Nature of Unconformities:

  • Unconformity: A gap or missing break in the rock record where rock was eroded away before new rock formed on top.

  • What it looks like: It is not a physical hole in the ground; it is simply the eroded surface between old rock underneath and newer rock on top.

  • Time gap: An unconformity represents missing time, ranging from a few hundred years to billions of years.

• The Three Major Types of Unconformities:

  • Disconformity: Flat sedimentary rock layers are eroded, and newer flat sedimentary layers are deposited on top.

  • Angular Unconformity: Older sedimentary layers get tilted or bent, eroded, and then flat, newer sedimentary layers are deposited on top.

  • Nonconformity: Newer sedimentary layers form on top of older, eroded igneous or metamorphic rock.

Strategies for Filling Gaps: Correlation and Index Fossils

• Concept of Correlation:

  • Correlation: The process of matching rock layers and fossils from different locations to fill in missing gaps in history.

  • How geologists correlate rocks:

  • Direct observation: Walking along exposed rock formations to find matching features.

  • Matching separated layers: Comparing exposed rocks in different places when soil or erosion covers the areas in between.

• Regional vs. Intercontinental Correlation:

  • Regional Correlation: Matching rocks located relatively close together (like within a few hundred kilometers) by comparing rock types, structures, and fossils.

  • Intercontinental Correlation: Matching rock formations that are far apart or on different continents using fossils.

  • Main idea: If rock layers in different parts of the world contain fossils of the same age, the rock layers must be the same age too.

• Index Fossils:

  • Why general fossils aren't always helpful: Species that lived for hundreds of millions of years cover too much time to give a specific date.

  • Index Fossils: Fossils of species used to identify specific relative ages. A good index fossil must:

  1. Have lived on Earth for a short period of time.

  2. Have been very common (abundant).

  3. Have lived in many locations across the planet.

  • Trilobites: Certain species of trilobites make great index fossils because they lived briefly across many regions.

Evidence of Major Geologic Events and Key Beds

• Clues Left by Major Events:

  • Volcanic Eruptions: Preserved in rock as layers of volcanic ash.

  • Glacial Movement: Left behind scratched and gouged rock surfaces.

  • Mountain Uplift: Seen through uplifted mountain layers that have been weathered down.

  • Ocean Flooding: Proven by rock layers showing ocean water repeatedly covering and retreating from land.

• Key Beds:

  • Key Bed: A unique, easily recognized layer of rock or sediment used as a marker to match rock layers across large areas.

  • Origin: Formed by sudden global or regional events, like giant meteorite impacts or massive volcanic eruptions.

  • Age rule (Superposition):

  • Rock layers above a key bed are younger.

  • Rock layers below a key bed are older.

Structure and Organization of the Geologic Time Scale

• Overview:

  • The Geologic Time Scale is a timeline model of Earth's history from its origin $4.6\text{ billion years}$ ago to today.

  • Purpose: Helps scientists worldwide talk about Earth's history using a standard calendar.

  • Dynamic nature: Boundaries and dates update as scientists make new discoveries.

• Divisions of Geologic Time (Largest to Smallest):

  1. Eons: The longest divisions of time.

  2. Eras: Subdivisions of eons.

  3. Periods: Subdivisions of eras.

  4. Epochs: Subdivisions of periods.

• Unequal Lengths of Time:

  • Units of time on the scale are not equal in length.

  • Reason: Geologists created divisions based on major changes in fossils. Because major changes in living things did not happen at regular intervals, the time units are unequal.

Chronological Divisions of Geologic Time

• Eons:

  • Hadean Eon: $4600\text{ mya}$ to $4000\text{ mya}$.

  • Archean Eon: $4000\text{ mya}$ to $2500\text{ mya}$.

  • Proterozoic Eon: $2500\text{ mya}$ to $541.0\text{ mya}$.

  • Phanerozoic Eon: $541.0\text{ mya}$ to the present day.

• Eras of the Phanerozoic Eon:

  • Paleozoic Era: $541.0\text{ mya}$ to $251.9\text{ mya}$.

  • Mesozoic Era: $251.9\text{ mya}$ to $66.0\text{ mya}$.

  • Cenozoic Era: $66.0\text{ mya}$ to the present day.

• Cenozoic Periods:

  • Paleogene Period: $66.0\text{ mya}$ to $23.0\text{ mya}$.

  • Neogene Period: $23.0\text{ mya}$ to $2.6\text{ mya}$.

  • Quaternary Period: $2.6\text{ mya}$ to the present day.

• Current Epochs:

  • Pleistocene Epoch: $2.6\text{ mya}$ to $0.01\text{ mya}$ ($11,600\text{ years}$ ago).

  • Holocene Epoch: $0.01\text{ mya}$ ($11,600\text{ years}$ ago) to the present day.

Comprehending Deep Time through Relative Comparisons

• Understanding Deep Time:

  • $4,600,000,000\text{ years}$ is extremely hard to wrap your mind around.

  • Geologists use familiar comparisons to help understand it:

  • One-Year Calendar: Shrinking all $4.6\text{ billion years}$ into a single calendar year ($365\text{ days}$).

  • 24-Hour Clock: Squeezing Earth's whole history into a single $24\text{-hour}$ day.