Over the past several tutorials, we have
been learning a lot about rocks. We have learned about the types of rocks, those being
igneous, sedimentary, and metamorphic. We have learned about their origins, their mineralogy,
and their properties. With all three types of rocks now understood, we are ready to examine
something called the rock cycle, a construct which magnificently encapsulates a huge breadth
of geological phenomena. All three types of rocks are slowly changed from one type to another in
this process which we refer to as the rock cycle. This involves the formation and destruction of
rock throughout geologic history. For example, in Earth’s early history, all rocks at the surface
were igneous, having formed from the cooling of liquid magma and pushed up to the surface. Over
long periods of time, rain and wind would break these rocks apart and transport their sediments to
the oceans, where they would be buried and later lithified into sedimentary rocks. In parts of
the world where two plates were moving together, these original igneous rocks were metamorphosed by
the heat and pressure of collision, and uplifted to form mountains containing metamorphic rock. And
just as before, these rocks were broken down and their sediments transported to sedimentary
basins. These new sedimentary rocks would later be uplifted and exhumed, where they would
again be attacked by the wind and rain, broken into sediment, and transported to a brand-new
sedimentary basin. In parts of the world that were experiencing subduction, the original rock
in the subducting slab was returned to the mantle, where it would presumably sink to D’’ and later
rise to form a mantle plume, which would melt, create a volcano, and erupt lava to form a
brand-new igneous rock that perhaps shares a few silicon or oxygen atoms in common with
the rock from the previous iteration of the cycle. And then the whole show begins again.
It is by these processes, which are collectively referred to as the rock cycle, that Earth
continuously recycles geologic material, and in some cases, mineral grains from
the primordial Earth have survived over 4 billion years of the rock cycle, and exist
today as parts of ancient sedimentary rocks. A famous formation in Australia, called the Jack
Hills Conglomerate, contains detrital zircon, which is a particularly durable mineral,
that is over 4.4 billion years old. We can look at an image like this to summarize
all of the possibilities within the rock cycle at once. We start with magma, which cools
to form igneous rock. This could melt to form magma again, or it could experience
weathering and erosion to form sediments, which can compact and cement to form sedimentary
rock. Igneous rock can also experience high heat and pressure to form metamorphic rock.
Sedimentary rocks can do the same thing, if they are also subjected to such high heats
and pressures. Then both sedimentary rock and metamorphic rock can experience weathering and
erosion to produce sediments, just like igneous rock, and this sediment will eventually compact to
form still more sedimentary rock. And metamorphic rock can also melt, just like igneous rock, to
produce magma, and return to the source of it all. In describing the rock cycle, and in talking
about the production of sediments in general, we mentioned weathering many times, but we haven’t gone into much detail regarding these
processes. So, let’s move forward and talk about precisely how weathering and
erosion transform rocks into sediment.