The history of life on Earth is chronicled in which type of rock?

Question

BlackTom AI · Accepted Answer

Sedimentary rocks are the primary archive of Earth’s biological history because they form by the gradual deposition and lithification of sediments in environments where organisms live, die, and become buried.

1. Relevant knowledge points and concepts
- Types of rocks: sedimentary (formed from deposited sediments), igneous (cooled from molten magma/lava), metamorphic (altered by heat and pressure).
- Fossil preservation: requires rapid burial and low heat/pressure so organic remains are not destroyed.
- Stratigraphy principles: superposition and original horizontality allow reconstruction of relative time order in layered sediments.
- Lithification: compaction and cementation turn sediments (mud, sand, shells) into shale, sandstone, limestone—common fossil-bearing rocks.

2. Step-by-step reasoning process
- Organisms die in or near depositional environments (rivers, lakes, seas, swamps).
- Their shells, bones, and impressions become buried by additional sediment.
- Over time sediments compact and cement into sedimentary rock, preserving fossils and sedimentary structures.
- Layering (strata) preserves a vertical record; older layers are below younger ones, enabling reconstruction of temporal changes in life.

3. Intermediate conceptual steps
- Identify depositional environment → burial of remains → protection from decay → lithification → preservation of fossil content and assemblages.
- Relative dating of layers (stratigraphy) and occasional absolute dating of interbedded volcanic ash (igneous) allow chronology.

4. Why the correct answer is correct
Sedimentary rocks routinely contain well-preserved fossils and stratified records of past environments and life forms (e.g., limestone with marine fossils, shale with plant material). These characteristics make sedimentary rocks the chroniclers of life’s history.

5. Why other options are incorrect
- Igneous rocks form from molten material at high temperatures that destroy organic remains; they seldom preserve fossils (though ash layers can be used to date sedimentary layers).
- Metamorphic rocks have been subjected to heat and pressure that usually deform or obliterate fossils and original sedimentary structures, so they do not reliably record biological history.

6. Conclusion
Therefore, the history of life on Earth is chronicled in sedimentary rocks. This matches the provided correct answer: sedimentary.

The history of life on Earth is chronicled in which type of rock?单项选择题

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