Earth’s Past: A Geological Journey Through Time
ITEM 13: Getting the Earth and its Past
Readers Rock
In the development of geology as a science, three key ideas have been central:
- The Earth is immensely old. The Earth originated about 4,560 Ma; however, for many centuries, it was thought to be scarcely 6,000 years old.
- The Earth is constantly changing. The Earth’s surface changes permanently due to slow, gradual processes such as mountain erosion or continental movement, but also thanks to sporadic and intense processes, such as volcanic eruptions, earthquakes, or asteroid impacts.
- Rocks are records of the Earth.
Scales in Geology
- Timescale. The unit of time in geology is the Ma (Mega annum). However, some processes require the use of geological time units above and below the Ma. Thus, units used include seconds, days, years, and thousands of years; but to study the Earth’s origin, we must go back thousands of Mas.
- Spatial scale. The diversity of spatial scales used in geology is no less significant than the temporal scales. Spatial scales range from kilometers to centimeters, and millimeters to angstroms.
Past Earth: Reconstruction
Reconstructing a story involves two kinds of activities: investigating the events and ordering these events temporally.
How We Investigate What Has Occurred
- Geological events generate changes. If the event is significant, the changes it causes will be substantial; if the event is minor, the changes will also be minor.
- Changes leave traces. Since almost all events throughout Earth’s history could not be observed by people, they must be inferred from the traces they left. Geological changes are detected through:
- The materials they create. For example, a volcano eruption produces pyroclastic material (ash, lapilli, bombs) or volcanic casts. A flood leaves mud, and a glacier leaves till.
- The resulting structures. Compressive stress creates folds or reverse faults. The presence of normal faults indicates a period of extension.
- The forms they leave. A glacier excavates a U-shaped valley.
The Beginning of the Present
The principle of uniformitarianism argues that the processes occurring at present are the key to interpreting what happened in the past.
Dating Methods
Once it is known that geological events have occurred in a given area, it is necessary to reconstruct the temporal order of the history. There are two ways to sort:
- By relative dating, which establishes what occurred before and after, without giving numerical figures.
- By absolute dating, which indicates the number of years, or M.a., since each event occurred.
Fundamental Principles of Dating (Nicolas Steno)
- Principle of original horizontality of strata: Deposited sediments form horizontal layers. If we find a series of layers arranged non-horizontally, we can conclude that after its formation, some kind of process has changed its original arrangement.
- Principle of lateral continuity of strata: Strata originally extend laterally and thin at the edges. The age is the same throughout the surface of a stratum.
- Principle of superposition of layers: Sediments are deposited on top of each other, so that in a series that is in its original arrangement, the lower stratum is the oldest and the higher, the most recent. We represent the materials in chronological order using the stratigraphic column.
Roof and Wall
The surfaces that bound a layer are called bedding planes. The top and most recent surface of a stratum is called the roof. The base is the wall. The same terms are used to refer to a series of strata. The vertical distance between the roof and the wall of a layer is called the thickness.
Polarity Criteria (where the layers are vertical)
The so-called roof-wall criteria are a set of sedimentary structures that orient strata. These structures are sometimes present on the surface of strata and others inside.
- Desiccation cracks. Formed by drying clay sediments. They are very open on the surface and close in depth. In a cross-section, the cracks will have a V-shape, with the apex pointing toward the wall of the stratum.
- Ripple marks. Those formed by waves or wind have sharper crests toward the roof. Ripple marks most often caused by processes such as drying, disappear; however, sometimes they remain in the rock.
- Cross-bedding. May arise from sand deposits transported by wind. In it, the laminae have a gentle inclination toward the wall.
- Graded bedding. Is formed when materials of different sizes, transported by a stream, are deposited. Thicker material is closer to the roof and thinner toward the wall.
Agreement and Disagreement
Two materials are concordant if the surface separating them is parallel to the bedding planes of both; otherwise, it will be discordant. A mismatch between the deposition of a material and the following one implies that some process has occurred. If there has been erosion, it is called erosional unconformity. If there has been folding, it will be an angular unconformity; and if there has been folding and erosion, it will be an angular and erosional unconformity.