Although this principle is generally applied to relative dating it is also the basis for evolution. Fragments of rock that are enclosed within another rock are older than the enclosing rock. Principles of Relative 4. This page was last edited on 24 Januaryat For example, if a valley is formed inside an impact craterthe valley must be younger than the crater. If you date the igneous rock, you know the youngest possible absolute age for the sedimentary rocks. The principle of intrusive relationships concerns crosscutting intrusions.
Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault.
I. Principles of Relative Dating
Essentially, this law states that clasts in a rock are older than the rock itself. Because sedimentary particles settle under the influence of gravity, sedimentary layers of rock are deposited horizontally. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known. Weathering Rinds Rind of chemical weathering formed as water penetrates into rock and minerals are altered to more stable minerals Rind gets thicker with time. The original top and bottom of a sedimentary unit often can be determined from sedimentary structures, such as mud cracks, cross beds, and ripple marks. The formation of melt inclusions appears to be a normal part of the crystallization of minerals within magmas, and they can be found in both volcanic and plutonic rocks. Using microscopic observations and a range of chemical microanalysis techniques geochemists and igneous petrologists can obtain a range of useful information from melt inclusions.