Strata accumulate in order from bottom to top. By reading strata, we can learn about past environments and changes over time.
Basic Knowledge
The principle that strata accumulate horizontally is called the law of superposition. It means that lower strata are older and upper strata are newer (unless folding or overturning has occurred).
Strata that formed at the same time should accumulate continuously, but if erosion or a gap in time occurs between them, this is called an unconformity. Fossils and rock facies change greatly above and below an unconformity surface.
For correlation of strata, distinctive layers called key beds are used. Volcanic ash (tuff) layers and layers containing particular fossils are often used.
A geologic column shows the vertical sequence of strata at a location. Underground structures can be reconstructed by comparing several geologic columns.
Law of superposition（The bottom is older and the top is newer. Proposed by Steno in 1669）
Conformity（A state in which strata were deposited continuously over time）
Unconformity（A state in which deposition stopped and resumed after an erosional surface formed）
Key bed（A stratum distributed over a wide area that can identify an age. Tuff is a typical example）
Geologic column（A diagram that schematically represents the vertical cross section of strata at a location）
Graded bedding（Coarse particles at the bottom and fine particles at the top. Formed by submarine landslides and similar events）
Further Study (Background and Significance)
Reading strata means interpreting the story of Earth that continues from the past to the present. For example, if strata at a location are arranged as bottom: marine strata → middle: unconformity → top: terrestrial strata, we can learn that the area was once the seafloor but rose to become land.
Graded bedding indicates submarine landslides caused by earthquakes (turbidites) and provides clues about ancient earthquake activity. From cross-laminae, in which sand grains are aligned in a particular direction, we can estimate the direction of ancient currents.
The ability to observe strata at an outcrop (a place where strata are exposed at the surface) is one of the most basic skills in field-based Earth science. This also led to the idea that the past is the key to the future (uniformitarianism).
- Law of superposition = lower is older and upper is newer
- Conformity = continuous deposition / unconformity = a gap in time
- Key bed = widely distributed and useful for identifying age (tuff)
- Reconstruct underground structures with geologic columns
- Graded bedding = turbidite (submarine landslide)
- Cross-laminae = clue to ancient current direction
- Outcrop observation is basic geology
Points to Note (Easily Confused)
① Distinguish conformity (continuous deposition) from unconformity (a gap in time). ② The law of superposition is based on the assumption that there is no overturning. In places with intense folding, the order may be reversed. ③ A key bed identifies an age, while an index fossil indicates an environment; do not confuse them. ④ A geologic column is a vertical cross section.
Practice
- Which is older, the bottom or the top of a sequence of strata?
- What is a widely distributed layer used for correlating strata called?
- What is the boundary surface where deposition stopped and then resumed called?