MadSci Network: Earth Sciences

Re: Why is it that some of the highest land in NZ is made of seashell rock?

Date: Sun Dec 3 12:54:56 2000
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 974962959.Es

Dear Georgina,

Your question places you in excellent company.  Scholars in Classical 
Greece and Rome observed the petrified remains of seashells in rocks far 
from the sea.  In Italy in 1484 Leonardo da Vinci was engineer on a 
project to build a canal for the Duke of Milan, and the diggings uncovered 
many fossil shells.  Leonardo reasoned that they were the remains of sea 
creatures that had once been living.  We know now, of course, that this is 
correct.  All fossils, both of sea creatures and of land animals (like 
dinosaurs) and plants, are the remains of once-living animals and plants.

The seashellrock you mention will be what geologists call a sedimentary 
rock, that is, a rock formed from sand or mud deposited often in the sea, 
along shorelines or in lakes.  If itís made of sand it will be a 
sandstone, and if its made of limey mud it will be a limestone.  The 
shells will be fossils, and geologists would call the rock a fossiliferous 
sandstone or fossiliferous limestone.

Two things must happen to put  sandstones or limestones containing fossil 
shells in highland areas.  The first thing that must happen is that shells 
must be buried in sand and mud that later becomes rock.  This can happen 
in the ocean, and also in freshwater lakes and lagoons.  Just as the 
seashells you find on the shore today were formed by creatures living on 
the seabed or along the beach, so fossil shells were originally made by 
creatures that lived in the sea or in lakes.  When they died their shells 
became buried in the sand or mud on the bottom, and over immense periods 
of time the sand and mud were buried more and more deeply and compressed 
until they became rock.  Thus the shells became fossils.

The second thing that has to happen is that the rocks containing the 
fossil shells must be lifted up to form mountains.  To understand how this 
happens you need to understand how the crust of the Earth behaves.  The 
outer part of the Earth is called the crust, and it is mostly between 
about 8 and 50 kilometers thick.  This is very thin compared to the size 
of the Earth, and you can compare the Earthís crust to an eggshell.  What 
is more, the crust is broken up into fragments (called plates) which 
slowly move around and jostle against each other.  The energy that makes 
them move is heat from deep within the Earth.  It is hard to imagine how 
rocks can flow, but the Earth is so hot inside and the pressure is so 
great that over many millions of years the rock beneath the crust actually 
flows like hot toffee, although in very, very slow motion!  As the 
interior of the Earth flows it moves the crustal plates sitting on top.  
When two crustal plates smash together the rock between them becomes 
squeezed and lifted up to form mountains.

It can take several million years for mountains to form in this way.  But 
you might be able to experience the uplift of rocks if you are lucky (or 
unlucky, depending on your point of view), because rocks are lifted up 
during earthquakes.  When crustal plates smash together the pressure 
between them is released as earthquakes, when the pressure squeezing on 
the rocks of the crust becomes so great that  the rocks break and move 
suddenly against each other.  A single earthquake can uplift a piece of 
the crust by several meters, and it only takes time and many earthquakes 
to uplift rocks by thousands of meters to form mountains.  Of course, if 
the rocks being uplifted contain fossil shells, then eventually you will 
get fossils high up in mountains.

I am interested to learn from you that rock full of fossil shells forms 
the highest land in New Zealand.  I live in England, and there are many 
examples here also of hills and mountains made of rocks full of fossils. 

I hope this answers your question.

Best wishes,

David Scarboro

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