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.