ENG-2010 — Page 356

298 The Environment

ago, the region was inundated by a shallow sea, where limestones (now marble) and siltstones, found in western and central New Territories were deposited. From 170 to 140 million years ago, violent volcanic eruptions occurred with deposition of thick ash and lava layers and the development of several calderas (giant craters). At deeper levels, molten magma was intruded and slowly crystallised to form granite. Layered rocks now seen on the island of Ping Chau are younger sediments, laid down in a lake on the edge of a desert, about 55 million years ago.

During the last 2.6 million years, major glaciations caused successive lowering of sea level by up to 120 metres. This resulted in the coastline receding by about 100 kilometres south of Hong Kong. During interglacial periods, including the present time, the sea level rose and marine sediments were deposited.

A series of fifteen 1:20 000-scale geological maps and six accompanying geological memoirs have been produced by the Hong Kong Geological Survey. Two summary memoirs and a set of 1:100 000-scale geological and thematic maps that synthesise the geology of Hong Kong as well as a popular account of the geology of Hong Kong, in Chinese and English, have also been published. Geological information can be accessed from the website of the Civil Engineering and Development Department.

Energy

Gas

Town gas and liquefied petroleum gas (LPG) are the main types of fuel gas used in Hong Kong for domestic, commercial and industrial purposes. LPG is also used as a fuel by nearly all taxis and over 60 per cent of public light buses while natural gas is used for electricity generation and production of town gas.

Hong Kong has about 2.3 million gas customers in the domestic, commercial and industrial sectors, of which town gas and LPG respectively accounted for 84.7 and 15.3 per cent of the total fuel gas sold in these sectors.

Town gas is manufactured at plants in Tai Po and Ma Tau Kok which have daily throughput capacities of 9.66 and 2.6 million cubic metres respectively. A pipe network of some 3 400 kilometres supplies town gas to about 1.7 million customers.

LPG is imported into Hong Kong by sea and stored at five terminals on Tsing Yi Island before being distributed to approximately 631 000 customers and 62 LPG filling stations.

Natural gas is imported from the Mainland via submarine pipelines to the Black Point, Castle Peak and Lamma Power Stations for electricity generation and to the Tai Po Plant for production of town gas.

The Gas Safety Ordinance regulates the importation, manufacture, storage, transport, supply and use of fuel gas. All gas supply companies, gas installers and contractors must be registered with the Gas Authority (the Director of Electrical and Mechanical Services).