Question

a. What is the principle of “original horizontality” b. What is the principle of superposition? c. What can pieces of an older rock being included in the layer above it tell you? d. What is the principle of cross cutting relationships? e. What is an unconformity? F. What is a disconformity and how would it form? g. What is a nonconformity and how would it form? h. What is an angular unconformity and how would it form?

Answer 1

The Principle of Original Horizontality states that layers of sediment are originally deposited horizontally under the action of gravity. It is a relative dating technique. The principle is important to the analysis of folded and tilted strata.

From these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science of plate tectonics; that movement and collisions of large plates of the Earth's crust is the cause of folded strata.

As one of Steno's Laws, the Principle of Original Horizontality served well in the nascent days of geological science. However, it is now known that not all sedimentary layers are deposited purely horizontally. For instance, coarser grained sediments such as sand may be deposited at angles of up to 15 degrees, held up by the internal friction between grains which prevents them slumping to a lower angle without additional reworking or effort. This is known as the angle of repose, and a prime example is the surface of sand dunes.

Similarly, sediments may drape over a pre-existing inclined surface: these sediments are usually deposited conformably to the pre-existing surface. Also sedimentary beds may pinch out along strike, implying that slight angles existed during their deposition. Thus the Principle of Original Horizontality is widely, but not universally, applicable in the study of sedimentology, stratigraphy and structural geology.

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The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with geological stratigraphy. It is a form of relative dating. In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will be at the bottom of the sequence. This is important to stratigraphic dating, which assumes that the law of superposition holds true and that an object cannot be older than the materials of which it is composed.

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Cross-cutting relationships is a principle of geology that states that the geologic feature which cuts another is the younger of the two features. It is a relative dating technique in geology. It was first developed by Danish geological pioneer Nicholas Steno in Dissertationis prodromus (1669) and later formulated by James Hutton in Theory of the Earth (1795) and embellished upon by Charles Lyell in Principles of Geology (1830).

Types

There are several basic types of cross cutting relationships:

• Structural relationships may be faults or fractures cutting through an older rock.
• Intrusional relationships occur when an igneous pluton or dike is intruded into pre-existing rocks.
• Stratigraphic relationships may be an erosional surface (or unconformity) cuts across older rock layers, geological structures, or other geological features.
• Sedimentological relationships occur where currents have eroded or scoured older sediment in a local area to produce, for example, a channel filled with sand.
• Paleontological relationships occur where animal activity or plant growth produces truncation. This happens, for example, where animal burrows penetrate into pre-existing sedimentary deposits.
• Geomorphological relationships may occur where a surficial feature, such as a river, flows through a gap in a ridge of rock. In a similar example, an impact crater excavates into a subsurface layer of rock

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An unconformity is the contact between sedimentary rocks that are significantly different in age or between sedimentary rocks and older, eroded igneous or metamorphic rocks. Unconformities represent gaps in the geologic record, periods of time that are not represented by any rocks.

Unconformities happen for two reasons: sediment deposition stopped for a considerable time and/or existing rocks were eroded prior to being covered by younger sediment. There is no single time span represented by an unconformity. It depends on how long erosion occurred or for how long deposition ceased.

Some unconformities are easier to identify than others. For example, the contact between a very old granite and a younger sandstone is pretty obvious. On the other hand, figuring out whether two limestone beds are significantly different in age might require more investigation.

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