Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: dlwalke on 18/10/2017 02:19:29
-
Just got back from a long trip through the southwest (from Atlanta). Wow!! Passed through Great Lakes on way back (Apostle Islands, Pictured Rocks Natl Lakeshore). Wow again! Anyway, I have a few questions and found this forum. I have no training in geology other than what I learned way back when in high school so these are very much novice questions. Here's one. I presume there is a great deal of variation from place to place but am wondering what the range might be for how many years are reflected in let's say an inch of sedimentary layers. In other words, if you're looking at the Grand Canyon for example, how many years passed to lay down 1 inch of sediment?
Also, how much overhead material is required to produce enough pressure to turn sand into sandstone, or to create schist or gneiss? For example, if you have a sand layer 200 ft under the surface, will that be enough pressure to create sandstone, or is 1/4 mile or many miles of material necessary?
And for metamorphic rock, does heat generated by the pressure of however many thousands of tons of earth above play a role?
Thanks
Dave
-
I think the thickness of layers is going to vary far too much to make any general statements about correlations between thickness and age. A volcano can lay down meters of ash in a matter of minutes whereas another area might accumulate a few millimeters of dust over a year. Some things, like ice cores, can contain annual layers but those layers no doubt vary in thickness over large spans of time. Wikipedia has an image of an ice core 19 centimeters long with 11 annual layers, so you could roughly approximate an accumulation of 1.73 centimeters per year (which is probably naive). You're better off looking at the number of layers rather than their thickness.
-
how many years are reflected in let's say an inch of sedimentary layers.
Some lakes lay down orderly, thin layers of sediment every year.
In a river estuary, it is a continual tradeoff between deposition and erosion, with material eroded in one place being deposited somewhere else. It is only in a region of geological downwarping (eg the Mississippi basin) that there is a regular buildup of material that exceeds the erosion rate.
In ocean trenches adjacent to continents, years go by with minimal deposition, while sediment builds up on the edge of the trench. This is followed by an undersea landslide which dumps many meters of sediment in one day - a turbidity current. I understand that a number of turbidites (a result of a turbidity currents) are visible in parts of the Grand Canyon. Their boundaries can be identified by big rocks at the bottom (which fall out first) followed by smaller rocks, then silt at the top (which may take a few hours to settle out).
So it is fairly rare to find sediments that deposit at a regular rate.
See: https://en.wikipedia.org/wiki/Turbidity_current