Geog Report-sharp Island

8/10/2019 Geog Report-sharp Island

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S.5 Geography Field Trip

Report

{Sharp Island}

By : Simrandeep Kaur 5M (25)


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Contents

I ntroduction

deposition landform

weathering process

Land form features

Volcanic breccia

Rhyolite

Tuff and Tuffite

Quartz monzonite

Reflection


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Introduction to Sharp Island

2,000 metres off Sai Kung Pier

Shape: long,narrow,extending some 2,500 metres long

and about 500 metres wide from east to west

Highest point : 136 metres above sea level

Geological age : Early cretaceous period (about 140

million years old)

Depositional landform : Tombolo

Formation of Tombolo :

Relative motion of sea currents pushes coastal sand or

gravel to the shore, eventually forming a tombolo,which

connects two islands. This landform is usually found in

narrow sea channels. The submerged in water at high

tide, the sand strip is only visible when tides goes out.


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Weathering process


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Land forms

1. Volcanic breccia

Colour : White/ Grey

Minerals : angular volcanic fragments ( silica

content )

Formation : By violent volcanic eruption, formed by

extrusive vulcanivity.Rocks fell near the caldera andcool and consolidated to volcanic breccias.


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2. Rhyolite

Colour : Brick red (traces of lava flow )

Minerals : silica content

Formation : It is formed by silica lava which is

extruded onto the ground surface during a volcanic

eruption. It cools and consolidates to form a rock.]


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3.1 Tuff

Colour : Grey

Minerals : silica content ; volcanic ash

Formation : Volcanic ash and fragment are

deposited in lowland waters and solidify into tuffite.

3.2 Tuffite

Colour : Grey

Minerals : silica content ; Volcanic ash

Formation : Tuff is formed by solidification of

buried volcanic ash while tuffite is formed by water

sedimentation.


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4.Quartz Monzonite

Colour : Salmon-coloured

Minerals : Feldspar and Quartz

Formation : formed by intrusive vulcanicity.As

magma slowly rise from the depths of the crust, a

small amount intrudes along fissures. It cools down

and solidifies to crystallise.


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Reflection

On the 18th of March, our geography class went to Sharp Island

in Sai Kung.Why Sharp Island ? This is actually because it is wellknown as a memory of volcanic lava flows and it has a lot of

different types of rocks. Some examples of such volcanic rocks

are volcanic breccia,rhyolite,tuff,tuffite and also quartz

monzonite.

Most of their formation occurs when violent volcanic eruptions.

This is called extrusive vulcanicity for many of the rock typesfound on the island. Except quartz monzonite which is formed by

intrusive vulcanicity when magma rises from the cracks

underground. Not only can these rock types be distinguished from

their formations but also their colours and the minerals they

contain. Volcanic breccia and tuff are relatively grey while

rhyolite and quartz monzonite are darker in colour. Most of them

have high silica content and quartz monzonite also contain

feldspar and quartz .

Other than the variety of rocks, Sharp Island is another special

coastal feature. It is a tombolo.The formation is known as

sediment deposition. Sea currents and wave cause coastal sand

and gravel to push towards the shore.Then,a tombolo is found in

narrow sea channels. If there is a high tide, the tombolo is easily

submerged under the sea.However,if there is a low tide, then the

tombolo is visible.

This activity has aroused my interest in studying rocks, not only

through learning inside classroom but also outside. Hong Kong

actually has many interesting features but are needed to be

discovered !

Documents

Geog Report-sharp Island