I looked up the average depth of the Pacific Ocean; I got 4000 m. I tried to find average height of the North American continent, but neither Google, Lycos, nor Yahoo would let me customize my search, and all only gave links to average heights of humans. I found a solution, though.
I noticed that my searches turned up the word "elevation" with regards to the various land masses (countries, islands, mountain ranges, but not continents). That gave me a "standard phrase". I tried modeling what I needed to do, and determined that I wanted to compare a known depth-area combo (Pacific Ocean) with a calculable height-area combo (North America, because that's where the Rockies are found). In the process, I remembered that I could convert volumes into relative heights, and that averages could be "the value of a quantity of number n plus the value of a quantity of number x, then dividing the resulting combined value by the combine quantities to get the average number" (kind of like mode or median). These led to my solution.
So, I looked for the average elevation of countries [
https://en.wikipedia.org/wiki/List_of_countries_by_average_elevation], the area of countries [
https://en.wikipedia.org/wiki/List_of_countries_and_dependencies_by_area], and a list of countries in North America [
https://en.wikipedia.org/wiki/List_of_sovereign_states_and_dependent_territories_in_North_America].
I copied to tables to a spreadsheet (one table per tab), and then extracted the metric text values and converted them to numbers. (To convert "1,885 m (6,184 ft)" to a metric number, I used "=VALUE(LEFT(B2,(FIND("m",B2)-2)))". I tried " m", but Wikipedia is not using a standard space character, so I adjusted the formula.) Then, I copied the the country names (from the elevation table) into a new tab. Next, I flagged the country names (per the countries in North America list) with #s (North America was "1", for convenience). After that, I used a lookup function to match my country names with the elevation table, and another one to do the same with the area table. Then, I added a "/1000" to the elevation lookup formula to converted the elevations (m) to km and multiplied by area for countable "average volumes above sea level". I summed up the North American volumes, and divided by the summed up areas, which gave me an average elevation for North America.
Last, I made a new table to compare the volumes and depths of the Pacific Ocean to the that of North America. If I did the math right, the Pacific is 50 times the volume of the whole North American continent. Applying the same to the Rockies ( (average elevation (km) x area (km^2)) / Pacific volume ) gives roughly .0066, or the Pacific is 151 x the volume of the Rockies. (1/.0066 ~ 151) Since the measures are all in km, we can multiply the average Rockies height by the discovered relation and see that the whole of the Rockies would make a 29m layer over the Pacific basin.
(I've skipped over some additional effort, as you may be able to find the average area of the Pacific trenches and their average depth. That would give a better ratio of Rockies to trench.)
As to how to think of this?
Essentially, the dust/mud filling the Pacific trench is not only from the Rockies (as there is probably not enough stone in the Rockies to fill the trenches, unless the Rockies' elevation was much closer to sea level (and had become the Rocky canyon)). The dust/mud is coming from everywhere that drains into the Pacific. (Because ocean levels have risen and fallen over the millenia, some of that drainage has been from the continental shelves, when they were closer to the surface or fully above sea level.)
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