The Geological Time Scale

Order these units of the geological time scale from longest to shortest duration: epoch, eon, period, and era. Give a real example of each.

Name the four eons of Earth's history, order them from oldest to most recent, and indicate in which one we live and since when, approximately.

The Phanerozoic eon began $541$ million years ago, and Earth formed about $4\,600$ million years ago. Calculate what percentage of Earth's history the Phanerozoic occupies and explain why the previous eons (the "Precambrian") are much less well understood.

Name the three eras of the Phanerozoic eon, list them in order from oldest to newest, and cite a characteristic group of living organisms for each.

Place each of these events into the corresponding era and, if possible, period: the extinction of non-avian dinosaurs, the appearance of the genus Homo, and the "Cambrian explosion" of animals.

What event marks the beginning of the Phanerozoic eon and of the Cambrian period? Explain what the "Cambrian explosion" was and what change it represents in the fossil record.

Place the Cretaceous period in context by answering the following questions:

1. a) To which geological era does it belong?
2. b) Between what approximate ages (in millions of years) does it span?
3. c) What event brought it to an end?

The diagram summarizes the major intervals of geological time (eons and eras) with their boundary ages. Name the interval marked with "?", state between which ages it extends, and cite an important biological event that occurred during it.

The Mesozoic extends from $252$ million years ago to $66$ million years ago. Calculate its duration and what percentage of the Phanerozoic ($541$ million years) it represents, rounding the result.

Explain why the boundaries between eras on the geological time scale coincide with mass extinctions, and use the Mesozoic-Cenozoic boundary as an example, stating what caused it.

The end-Permian extinction was the greatest in history: the trilobites and the vast majority of marine species disappeared. Explain which era it brought to a close, which groups benefited from the ecological void that followed, and what relationship this crisis has with the beginning of the age of the dinosaurs.

The official boundaries of the scale are fixed at specific profiles called stratotypes or "golden spikes" (GSSP). Explain what a golden spike is, why a physical place must be agreed upon for each boundary, and what characteristics that profile must have.

Some scientists propose recognizing a new epoch: the Anthropocene. Explain what geological signals would mark it (radionuclides from bombs, plastics, coal ash, changes in faunas) and why other geologists resist formalizing it.

A geology team analyzes two continuous stratigraphic columns with a thickness of 150 meters: Column A corresponds to the Jurassic period ($201$-$145$ Ma) and Column B belongs to an Archean sedimentary basin ($3\,200$-$3\,000$ Ma).

1. a) Justify why in Column A it is feasible to establish a high-resolution chronology by subdividing strata into biozones spanning barely hundreds of thousands of years, whereas in Column B this method is completely inapplicable. What biological and taphonomic conditions exclusive to the Phanerozoic explain this?
2. b) Design a geochronological strategy to date and correlate the strata of Column B. Specify which isotopic or geochemical methods you would use and what obstacles arising from the rock cycle (metamorphism, diagenesis, tectonics) you must anticipate due to the antiquity of the materials.
3. c) Refute the following conclusion drawn by a student: "Since the Archean lasted more than three times longer than the entire Phanerozoic, its stratigraphic record should be much broader and easier to decipher than that of the Jurassic."

The International Commission on Stratigraphy (ICS) defines the boundaries between units of the Geologic Time Scale using Global Boundary Stratotype Sections and Points (GSSPs or "golden spikes").

Imagine you are part of the committee that must decide between two proposals to mark the basal boundary of a new geological age:

• Proposal A: A record in a stalagmite from a karst cave showing a $\delta^{18}\text{O}$ signal synchronous on a planetary scale (an abrupt global mega-drought) dated with very high radiometric precision using uranium-series dating ($^{230}\text{Th}/^{234}\text{U}$) at $4\,200$ years before present.

• Proposal B: A marine sedimentary section containing the local disappearance of a benthic foraminifer, with an approximate biostratigraphic correlation dating of $\pm 1.5$ Ma.

1. a) Justify which of the two proposals meets the formal geological requirements to be approved as a GSSP (resolution, global correlation, isochrony, and preservation), explaining the shortcomings of the discarded proposal.
2. b) In recent decades, the base of the Cambrian Period has been readjusted from about $570$ Ma to be fixed at $538.8 \pm 0.2$ Ma. Explain what advances in radiometric dating techniques (e.g., ID-TIMS on intercalated volcanic zircons) and in ichnostratigraphy (Treptichnus pedum) have allowed this date to be refined without altering the prior relative ordering of the layers.
3. c) Refute the claim of a detractor of geology: "The fact that the age of the Cambrian has changed several times proves that dating methods lack rigor and that the Geologic Time Scale is merely an arbitrary convention."

In March 2024, the International Union of Geological Sciences (IUGS) rejected by a large majority the proposal to formalize the "Anthropocene" as a new epoch of the Quaternary Period with a GSSP located in 1952 (marked by the peak of artificial radioisotopes from thermonuclear tests in the sediments of Crawford Lake, Canada).

1. a) Analyze the major stratigraphic objections that justified this decision: why does restricting the Anthropocene to the last 70 years ignore the deep diachrony of the global human footprint (beginnings of Neolithic agriculture, massive deforestation, industrial revolution) and clash with the stratigraphic principle of isochrony applicable to units of epoch rank?
2. b) Compare the potential preservation in the future lithological record (on a scale of tens of millions of years) of the radioactive pulse of $^{239}\text{Pu}$ and the so-called "technofossils" (plastics, concrete) against classic geological boundary markers such as iridium layers or global biological extinctions.
3. c) Defend why classifying the human impact as a "complex geological event" (analogous to the Great Oxidation Event or oceanic anoxic events) is a methodologically sounder solution than forcing its definition as a formal epoch with a single golden spike.

Unit Challenge. What story do the rocks beneath your feet tell? Become an outreach educator for the United States Geological Survey (USGS) and reconstruct the geological memory of your surroundings.

1. Gathering real data: Access the USGS National Geologic Map Database (NGMDB) viewer. Locate your city (or the nearest National Park) and identify two adjacent rock formations, noting their lithology and chronostratigraphic age (era, period, or epoch).

2. Time scale and calculation: Place both formations on the geological time scale. Calculate what percentage fraction of Earth's history ($4\,560\text{ Ma}$) the oldest rock represents ($t$ in $\text{Ma}$), applying $\% = (t / 4\,560) \times 100$ rounded to at least one decimal place (for example, $12.5\,\%$).

3. Reasoned proposal: Write a brief report arguing which stratigraphic principles of relative dating (superposition, cross-cutting relationships, or index fossils) would allow the local geological events to be ordered, and select a radiometric absolute dating method (such as $^{40}\text{K}/^{40}\text{Ar}$ or $\text{U-Pb}$) justifying its suitability based on the lithology and estimated age.

Assessment criteria:

• Rigor in extracting and interpreting lithological and temporal data from the USGS geologic map viewer.

• Mathematical correctness and precision in contextualizing ages on the time scale.

• Coherent justification of the proposed relative and absolute dating methods according to the geological context.

Order these milestones from oldest to most recent and express their age in millions of years: the formation of the Earth, the appearance of Homo sapiens, the Cambrian explosion, the extinction of the dinosaurs, and the origin of life.

If we assign $1$ year to every $100$ million years, the Earth would be $46$ years old. Calculate at what "age" of this planet-person the dinosaurs appear ($230$ million years ago) and Homo sapiens ($0.3$ million years ago), and comment on what the analogy reveals.

Imagine that the history of the Earth ($4\,600$ Ma) is compressed into a one-calendar-year timeline ending at midnight on December 31.

1. a) Calculate at what approximate date and time you would place three fundamental milestones: the Great Oxidation Event (~$2\,400$ Ma), the appearance of the first macroscopic fossils with shells or skeletons in the Cambrian explosion (~$541$ Ma), and the origin of the genus Homo (~$2.5$ Ma). Show the proportional calculations carried out.
2. b) A popular science documentary states: "The history of the Earth is fundamentally the history of land animals and plants, which have populated the planet since its origins, culminating in the arrival of human beings." Based on the temporal results obtained in the previous section, refute this statement point by point and argue why the geological time scale invalidates any anthropocentric or zoocentric reading of Earth's time.