The younger Free-Ride offspring shares a diagram from school.
As promised, much of the science so far this school year has been earthy, and part of that is learning where rocks come from. (No, it’s not the stork.)
Different kinds of rocks, of course, have different origins. They also present different spelling challenges. (Yes, I’m looking at you, “metamorfic”.)
I kind of want to ask if there was a discussion about the size cutoff between a sedimentary rock and a sediment. (What’s the biggest you could get and still be a sediment? What’s the smallest clump of compressed sediments that you’d count as a rock?) But my offspring inform me that this kind of request that they spell out distinctions makes me sound like a philosopher or something.
Molten rock is always the prettiest.
But, I’ll confess, I look at all the forces at the center of this schematic of the rock cycle:
and I feel a lot of empathy for the rocks. I think these are some of the same forces acting upon me this semester.
Brian Romans (who blogs at Clastic Detritus) could probably give a better answer to the question of “sediment” versus “sedimentary rock.” But from my (metamorphic) perspective: sedimentary rock is stuck together (that’s what the cement does), and sediment is crumbly and falls apart (that’s what weathering does).
And it’s possible for a rock to be both: a pebble of sandstone would be both a sedimentary rock and a piece of sediment.
Ah yes, the question of when sediment becomes sedimentary rock. As Kim says above, when a pile of loose sedimentary particles becomes cemented together it can probably be considered a sedimentary rock.
In general (but not always) the further back in time in the geologic record the more cemented sedimentary rocks are. Preserved piles of sediment in the hundreds of thousands of years to a bit over a million years old usually fall into an ‘in-betweener’ state of compacted and packed sediment that may not be very well cemented.
In terms of size cutoff, there isn’t one. Sediments or sedimentary rocks can be made of particles from clay particles (<4 microns) up to building-sized boulders. However, the latter is very rare. Most sediments/sedimentary rocks are in the clay to cobble-sized range. Google 'Wentworth grain size scale' for more.