Student 1: Low Excellence
Rock type 1: Formation of Greywacke sandstone
Our youngest Greywacke which makes up the bedrock of the Southern Alps and the North Island,
originates from the erosion and deposition of sediment on deep ocean floors are derived from ancient
Gondwana rocks (now Queensland) some 250mya. Our oldest greywacke formed by similar processes
about 490-443mya and are found today on the West Coast.
Greywacke is a sedimentary rock because it is made from particles of sand and mud (sediment) that
has been eroded from land and then transported by glaciers and rivers to form sea floor fans in
deeper offshore water. The mud fraction is transported further than the sands because they are much
smaller in size and takes less energy to move the particles.
This process of deposition, burial and squeezing occurs on a vast geographic scale. Movement of the
Pacific plate by sea floor spreading as it pushes against continental crust produces folding, faulting
and uplift on a large scale to form mountains. Our local area is only a tiny part of this. Weathering,
erosion, transport and deposition of sediment are key processes in the rock cycle. This is the way in
which materials are recycled.
Rock type 2: Formation of Limestone
Limestone is formed by the deposition of calcium/magnesium carbonate by chemical precipitation out
of sea water and/or the continual settling out of billions of carbonate shelled micro-organisms and
macro shells onto a low energy shallow (but not always warm water) sea floor. Younger limestones
are 23-35 million years old.
An easily observed landform
Limestone is a very common rock in NZ and is responsible for much of our easily observed
landscapes. It is generally easily eroded. These landscapes are formed by the chemical weathering of
calcium carbonate and surface and underground water erosion by the rivers bedload. Rivers carry
large rock particles which erode river banks and river beds. Limestone is often jointed and fractured
and acidic water easily weathers out the joints. This can lead to internal collapse and sinkholes.
Acidified water continuously decomposes the lime. Limestone is a chemical sedimentary rock made
mostly of calcite from shells with various amounts of mud and sand. It is chemical because the lime
comes from once living organisms that secrete a hard shell.