Question #2: The Rock Cycle

Ancient rocks from precambrian times are more rare and difficult to find than a more recently formed rock.  This is clearly displayed through the rock cycle.  The rock cycle is simply the constant reformation of igneous rocks into sedimentary rocks into metamorphic rocks and so on.  For example. If sediment from the precambrian eon is compressed and cemented together that sediment forms a sedimentary rock.  Once that sedimentary rock is exposed to extreme heat or pressure, but does not melt, that sedimentary rock will become a metamorphic rock.  In this new metamorphic rock, old crystals have grown and new minerals have formed.  If this metamorphic rock is exposed to even stronger heat or pressure, it will begin to melt into magma or lava.  After a period of time this lava may cool down and begin to solidify and crystallize.  This cooling lava will form either an intrusive or extrusive igneous rock depending on the location of the solidification (inside the earth or near the surface) and the time it took to solidify (rapid cooling=smaller crystals).  After this igneous rock has been weathered and starts to erode, the rock will break down and small particles of sediment will scatter and deposit into the ground.  Now we are back where we started, with sediment.  This cycle could take anywhere from thousands to millions of years depending on how long each rock stays in each step of the cycle.  Now that seems like basic recycling and it would only seem obvious that rocks from 4 billion years ago would still be in this cycle, but there is a catch.  Every time a rock makes a transformation throughout the cycle, old minerals are broken down and new minerals are formed, changing the composition of the rock.  After millions, even billions, of years of this cycle minerals have come and gone billions of times.  Because these rocks have changed their composition so many times, rocks that contain remnants from the precambrian eon are rarely found on the surface of the earth.  If they exist at all, rocks from the precambrian eon would be found beneath the earth's surface where the conditions they exist in stay fairly constant and the pressures and temperatures are not changing enough for those rocks to change their composition and recycle into new rocks.  Because rocks rarely remain the exact same for billions of years, rocks from precambrian times are rarely found on the earth's surface.  Therefore, the rock cycle is completely responsible for recycling old rocks into new ones and renewing minerals.