Using tuffs from a volcanic eruption
A tuff is the rock resulting from pyroclastic deposition. when a violent eruption occurs , ash is blasted tens of kilometers into the atmosphere and is laid down over enormous areas exactly the same age- geologically instantaneously.
The tuff that forms from the ash is the best of all rocks to use for correlation because:
- You can carry out chemical analysis - determine exact composition
- Its laid down over large areas rapidly
- Radiometric dating K-Ar
- Can be used for both relative and absolute dating
Using varves from glacial lakes
In the summer, glacial ice melts faster and the increased flows carry down silts into the lake to make a thin pale layer. During the winter there is no meltwater and the lake itself may freeze over. The result is very low-energy deposition of clay particles and organic matter that grows under the ice to produce a thin dark layer. One year is recorded as a pair of layers on the lake bed. this means you count layers to find out how many larers are represented in the sequence. The correlation is provided by thicker layers resulting from hotter summers, the pattern of thick and thin bands being the same for all lakes in the same area.