Which statement about rock strata is true?

Which statement about rock strata is true?

• A. They are always horizontal
• B. They represent different periods of history
• C. They are formed only by volcanic activity
• D. They are always metamorphic

Answer: (B)

Rock strata are a record of Earth's past. Each layer formed over a span of time and preserves clues about the environment, life, and events at that moment. Because layers accumulate in a time-ordered sequence, they represent different periods of history, with the oldest layers at the bottom and younger ones above. Fossils and sediment clues within each layer help dating and correlating the sequence across regions, letting us read a chronological story of environments and life. The idea that strata are always horizontal isn’t accurate—deposited layers start roughly horizontal, but tectonic forces can tilt, fold, or fault them, so their present orientation may be anything from upright to steeply inclined. They aren’t formed solely by volcanic activity, either; most strata are sedimentary, built from weathered particles deposited by water, wind, or ice, with volcanic material sometimes adding ash layers but not defining the whole sequence. And they’re not always metamorphic; many strata remain sedimentary rocks, though some can be metamorphosed under heat and pressure.

Rock strata are a record of Earth's past. Each layer formed over a span of time and preserves clues about the environment, life, and events at that moment. Because layers accumulate in a time-ordered sequence, they represent different periods of history, with the oldest layers at the bottom and younger ones above. Fossils and sediment clues within each layer help dating and correlating the sequence across regions, letting us read a chronological story of environments and life.

The idea that strata are always horizontal isn’t accurate—deposited layers start roughly horizontal, but tectonic forces can tilt, fold, or fault them, so their present orientation may be anything from upright to steeply inclined. They aren’t formed solely by volcanic activity, either; most strata are sedimentary, built from weathered particles deposited by water, wind, or ice, with volcanic material sometimes adding ash layers but not defining the whole sequence. And they’re not always metamorphic; many strata remain sedimentary rocks, though some can be metamorphosed under heat and pressure.