Chat cam room 05 03 2013 sara - Earth science relative dating activity

However, note that because of the "principle of cross-cutting relationships", careful examination of the contact between the cave infill and the surrounding rock will reveal the true relative age relationships, as will the "principle of inclusion" if fragments of the surrounding rock are found within the infill.

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The simplest situation for a geologist is a "layer cake" succession of sedimentary or extrusive igneous rock units arranged in nearly horizontal layers.

In such a situation, the "principle of superposition" is easily applied, and the strata towards the bottom are older, those towards the top are younger.

Much of the Earth's geology consists of successional layers of different rock types, piled one on top of another.

The most common rocks observed in this form are sedimentary rocks (derived from what were formerly sediments), and extrusive igneous rocks (e.g., lavas, volcanic ash, and other formerly molten rocks extruded onto the Earth's surface).

This document is partly based on a prior posting composed in reply to Ted Holden.

My thanks to both him and other critics for motivating me.

Despite this, the "principle of cross cutting relationships" can be used to determine the sequence of deposition, folds, and faults based on their intersections -- if folds and faults deform or cut across the sedimentary layers and surfaces, then they obviously came after deposition of the sediments.

You can't deform a structure (e.g., bedding) that is not there yet!

This orientation is not an assumption, because in virtually all situations, it is also possible to determine the original "way up" in the stratigraphic succession from "way up indicators".

For example, wave ripples have their pointed crests on the "up" side, and more rounded troughs on the "down" side.

Geochronologists do not claim that radiometric dating is foolproof (no scientific method is), but it does work reliably for most samples.

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