From what I understand after Mt. St. Erupted, the volcanic ashes and particles did formed sedimentary layers ...
Well, sort of sedimentary. Geologists joke that volcanic ash is "igneous when it goes up and sedimentary when it comes down". (I didn't say it was a
funny joke.)
... But regarding the Grand Canyon they may have been unconsolidated as there are slump at the very bottom, right?
I'm not following you, sorry.
Does Mt. St. Helens show that sedimentary layer can form rapidly, because they're volcanic lava and particles, but not rocks? So rocks takes longer?
It depends on what you mean, and on the rock.
Volcanic ash can form rock almost immediately if it's hot enough when it falls to weld itself together; this gives us the rock known as "welded tuff". Or it can be slowly cemented together by the usual process that also cememts together such things as sandstone, which takes longer.
There are other processes besides volcanism that deposit a relatively thick layer of sediment quickly, such as turbidity currents. That then takes a long time to turn into rock (turbidite).
A layer of basaltic lava can be extruded quite quickly and will be baslat as soon as it cools.
Other things, such as limestone or chert are deposited very slowly and then take a long time to be compacted into rock.
So how long it takes for (a) the original material to be deposited and (b) for the material to be lithified (turned into rock) depends on what sort of rock it is.
If you look at
No webpage found at provided URL: this page and follow links 2-13 where it says "
No webpage found at provided URL: Related articles" you can find out about various sorts of sedimentary rocks and the circumstances under which they're deposited and how we know this.