Geology question, re: Rock Cycle?

Question

I teach science, 6th gr. We have a new science curriculum that discusses in length the "Rock Cycle", see link:
http://www.minsocam.org/MSA/K12/rkcycle/rkcycleindex.html
My problem is this - Is it accurate to describe this as a cycle? It is obvious the Earth's crust has been shaped by erosion, but I don't think the ENTIRE surface has been through the "Rock Cycle" for even one cycle. It might be applicable to less than 1% of the mass of the crust, or more probably much less than even that. So, how can we describe this as a cycle if very little mass has actually been through it?! Is the terminology just for convience sake?

thx Luka, will look into the maps more, nice resource, will use in my room. Super answer!

Answer 1

Actually, virtually the entire mass of the earth's surface has been through at least one cycle.

Most scientists believe that the earth formed about 4.5 billion years ago. This is based on the dating of meteorites thought to have formed during the creation of solar system.

The oldest earth minerals that have been dated gives dates of about 4.4 billion years old--these are recycled zircons in some rocks from Australia. The zircons are fragments of igneous rocks that were eroded and redeposited in sediments, which were cemented to form sedimentary rocks, (estimate 3.6-3.8 billion years ago) and then metamorphosed. (1) The Acasta Gneiss Complex near Great Slave Lake, Canada has been dated as about 4.030 billion years.(2) Gneiss is a metamorphic rock (therefore--at least two/3 parts of the rock cycle represented here alone.

In fact, rocks as old as the earth do not cover the earth--they have been recycled:
A. The ocean basins cover half the earth, and the basement rocks of the ocean basins are "relatively" young, not older than 180 million years old--formed as igneous rocks along spreading ridges--from recycled material.
B. Continents cover the other half of the earth. If you look at geologic maps of the continents, you will see that only the cores of the continents are made of old Precambrian rocks. For example: here's a map that shows the location of Precambrian rocks in the conterminous United States. They make up only a small part of the States. All the rest of the rocks are younger. The Precambrian includes all the rocks older than about 544 million years--which includes mostly rocks way younger than the inferred 4.5 billion year age of the Earth.
The following website shows a map of all the rocks-by-age at the surface of the conterminous United States: http://tapestry.usgs.gov/two/color.html. And the next url gives the legend for the map. http://tapestry.usgs.gov/ages/ages.html.

Answer 2

it looks pretty good