14.4 Earth's History and Geologic Dating

14.4 Earth's History and Geologic Dating

Quick Answer: Relative dating orders events using superposition, cross-cutting relationships, and fossil succession without giving numeric ages. Absolute (radiometric) dating uses radioactive decay to estimate ages in years. Earth's history records extinctions, volcanism, glaciation, and asteroid impacts. Fossils correlate rock layers across distances—famously matching Grand Canyon wall sequences—building the geologic time scale.

Praxis 5442 items often show a cross-section of rock layers and ask which event is oldest/youngest, or how fossils support correlation. Treat diagrams like logic puzzles.

Stratigraphy: reading the rock record

Stratigraphy studies layered rocks (strata). Three high-yield relative-dating principles:

1. Law of superposition

In undisturbed sedimentary sequences, younger layers overlie older layers. The bottom bed was deposited first.

2. Cross-cutting relationships

A feature that cuts across rocks (igneous intrusion, fault, erosion surface) is younger than the rocks it cuts. A dike intruding layered sediments postdates those sediments.

2b. Intrusive relationships (named ETS tool)

Intrusive relationships are the igneous special case of cross-cutting: magma that intrudes older rock (dikes, sills, plutons) is younger than the host rock. A baked/contact metamorphic zone along an intrusion also shows the intrusion heated preexisting rock—another "intrusion is younger" clue on diagrams.

3. Fossil succession (faunal succession)

Fossil organisms appear in a consistent vertical order worldwide because life evolved through time. Assemblages unique to intervals act as index fossils when they were widespread but short-lived geologically—allowing correlation of layers that are not physically connected.

Additional relative tools (brief): inclusions (rock pieces inside another rock are older); unconformities mark missing time from erosion/non-deposition.

Relative vs absolute dating

Radiometric dating uses radioactive parent isotopes decaying to daughter products at known half-lives. If a mineral starts with parent atoms and retains daughters, the parent/daughter ratio + half-life → age. Different isotope systems (e.g., carbon-14 for recent organic material; uranium-lead, potassium-argon for much older rocks) suit different timescales—middle-school depth is the logic of half-life and closed systems, not memorizing every decay chain.

Exam cue: Sedimentary layers are often dated by dating igneous intrusions/flows that bound them, or by correlating fossils to radiometrically calibrated timelines—not by treating sand grains' mixed origins as a single crystallization age.

Major events that reshape life and landscapes

Earth's history is punctuated by processes that Praxis expects you to recognize as drivers of change:

These are not mutually exclusive: an impact can trigger climate chaos; volcanism can load atmosphere with gases and particles; glaciation redistributes water and erodes landscapes. Teaching scenarios may ask which evidence best supports an impact hypothesis versus a purely volcanic one (multiple lines of evidence preferred).

Fossils correlating Grand Canyon walls

The Grand Canyon exposes a thick stack of Paleozoic (and older basement) rocks—a natural textbook of superposition. Rangers and geologists correlate units along the canyon walls using:

1. Lithology (rock type sequences: sandstone, shale, limestone patterns).
2. Fossil content matching the same ages in separated but equivalent beds.
3. Position in the stack (which unit sits on which).

A limestone with the same distinctive marine fossils appearing at the same relative position on opposite walls supports correlation: those beds were deposited in the same broad time interval, even if later erosion carved the canyon. Unconformities in the canyon sequence show gaps—places where relative dating reveals missing chapters, later constrained by absolute ages elsewhere on the geologic time scale.

Student misconception: "The canyon rocks formed when the river cut them." Clarify: most layered rocks deposited first (often in marine settings); the river eroded the canyon much later—cross-cutting by erosion after deposition.

Building a quick relative-dating solution method

Given a cross-section:

1. Identify sedimentary stacks → apply superposition within each undisturbed block.
2. Find faults/intrusions → apply cross-cutting (cutter is younger).
3. Note baked contacts or inclusions if shown.
4. Use fossils to match ages across gaps or between regions.
5. Only then bring in radiometric numbers if the stem provides isotope data.

Worked micro-example: Layers A (bottom), B, C (top). Dike D cuts A and B but stops below C. Fault F offsets A–C and the dike. Order (oldest→youngest): A, B, dike D, C, fault F—or refine if C was deposited after D and before F as stated. Always re-read what the diagram shows about whether C covers the dike.

Geologic time scale literacy (exam level)

You do not need every period date memorized, but know that:

• Earth is about 4.6 billion years old.
• Life's fossil record becomes rich in the Phanerozoic; major eras (Paleozoic, Mesozoic, Cenozoic) frame big fossil turnovers.
• Absolute ages calibrate the relative fossil order into a numeric timeline.

Teaching-scenario fingerprint

Students dating a sedimentary cliff with carbon-14 on sandstone quartz should be redirected: choose appropriate isotopes, use igneous constraints, or fossil correlation—method must match material and age range.

Bottom line for 14.4: Superposition + cross-cutting/intrusive relationships + fossil succession order events; radiometric dating absolute-ages suitable rocks; extinctions/volcanism/glaciation/impacts punctuate history; fossils correlate sequences such as Grand Canyon strata across space.