Geographic History of Hong Kong

Geographic overview A region in the southernmost corner of China, Hong Kong is part of the southeastern Pearl River estuary, neighbouring on Mount Nanling in the north and South China Sea in the south. Local topography is hilly with scattered coastal plains. The Hong Kong SAR comprises Hong Kong Island, Kowloon Peninsula, the New Territories which is connected to Mainland China, Lantau Island and more than 200 outlying islands. Approximate total land area is 1,104 square kilometres. Hong Kong has a myriad of landforms. The long, sinuous coastline is indeed a natural geological gallery, featuring spectacular landforms and rock formations shaped by waves and weathering. Here, one finds wave-cut sea cliffs, sea caves, sea arches, geos, sea stacks, notches and blowholes. Lying alongside are beaches, alluvial plains and mudflats which have come about thanks to millions of years of sedimentation. Inland areas showcase a variety of weathering characteristics. Terrain features arising from physical, chemical or biological weathering effects are prominent across peaks and valleys, ravines and escarpments. For further details, please refer to the publication of CEDD: Hong Kong Geology - A 400-Million Year Journey History

Years ago(Million) Over billions of years, the Earth went through many glacial ages of prolonged low temperature. The last ice age advanced the evolution of Hong Kong's landscape. In the last 2 million years, sea level saw a series of rises and falls. Correspondingly the land surface of Hong Kong was submerged and exposed by turns. There were significant temperature variations during this glacial period. When it was cold, ice glaciers around the world gained size and water was solidified into ice on land, Sea level dropped as the ice melted and land was submerged. During this time seawater of Hong Kong had dropped to 120 metres below the modern day level, while the coastline was 120 kilometres farther to the south. At the end of the last glacial age, rising sea level flooded land areas which were once river valleys. Previous valleys and mid-slopes turned into sinuous coasts and some peaks became islands the jagged coastline of Double Haven (Yan Chau Tong) and Sai Kung and the islands scattered offshore. Indeed, geological vicissitudes have created the most stunning masterpieces. The local climate got warmer and more humid in the last 60 million years. This change led to chemical decomposition of surface rocks and a soft loose layer vulnerable to erosion was established. Erosion persisted. Terrain features continued to evolve. As a result Hong Kong is bestowed with enchanting landforms. The local climate became extremely dry bout 100 million years ago. Conditions were similar to some areas of the Middle East today. Storms activated great and sudden floods. Sharp rocks that came off the peaks settled below the precipitous slopes, forming fan-shaped lithic shards between which intermittent streams coursed down to the lowlands. Along meandering river channels that flowed only in the wet season, pebbles settled while in other places wind carved out striking red sand dunes. Aided by river sediments these alluvial plains gave rise to the magenta sedimentary rocks seen today in Hung Shek Mun and Port Island(Chek Chau) of the Northeastern New Territories. Slowly, arid land moistened between 80 million and 50 million years ago. Rainfall was merely sufficient to form salt lakes nonetheless. In the rainy season when streams flowed, they washed mud, sand and clay into these water bodies. At other times the lakes were so dry the bottom mud cracked open. With the moisture evaporated, salt crystals appeared. In humid times, plants could survive and in turn supported the growth of insects. Remains of dead insects became fossils in the mud. In low-lying areas around Tung Ping Chau in the Northeastern New Territories, huge lakes once existed. As seasons and environments changed, mud and sand settled orderly in layers of different grain sizes. They formed range upon range of sedimentary rocks. These strata, extremely rich in fossils of plants, spore powder, archaeostraca and insects, are the youngest rocks in Hong Kong. Between 165 million and 140 million years ago, volcanic activities rocked the entire region known as the southeastern China coast today. With the magma chamber lying merely 1 to 2 kilometres below surface, eruptions were violent and produced huge clouds of volcanic ash. Falling ash and lava spread across the land, covering small bays and lake beds. These events changed the original landscape. When pyroclastic substances cooled down, they became volcanic rocks that we now see in Sai Kung, Lantau Island and Tai Mo Shan. In some instances where eruption-derived substances were so great in volume that the volcanoes collapsed. Huge round cavities appeared calderas. When volcanoes were less active, the calderas may get waterlogged. According to data for determining the absolute age of rocks, volcanic eruptions and magma intrusion activities took a cyclical pulse pattern. There were great magma intrusions during large eruptions in every cycle. The intrusive rocks so formed are the exposed granite distributed across the territory today, such as the granite strata of Hong Kong Island and the Kowloon Hills. Rainfall sometimes led to cyclical saturation of ground ash cover. For this reason rainfall had been an important factor in the formation of Hong Kong's ancient landscape. Water-saturated substances travelled down the slopes to reach lowlands. These volcanic mudflows were dense mixtures of water and ash. 140 million years ago, volcanic eruptions came to an end. There are no traces of volcanic cones in modern day Hong Kong because subsequently the mountains were seriously eroded. In a later age, sea level dropped considerably. Land around the edges was exposed and coastal marshes emerged. Later when these were submerged yet again their places were taken over by river deltas. During this time the sea level continued to rise, until 280 million years ago the whole territory became the bottom of a deep sea with rich deposits of silt and mud. Following a subsequent period of rising sea level and erosion, the ocean settled and became a medium depth environment between 210 million and 190 million years ago. A cover of silt and mud reappeared and ammonites throve in these waters. Their remains were buried in the mud for long ages and formed the fossils now found within mudstone. In the Early Carboniferous some 360 million years ago, a transgression induced profound changes to Hong Kong's landscape. The sea level rose progressively. Later, between 360 million and 320 million years ago, coastal floodplains were submerged by the tropical ocean. As there were only a handful of major rivers going out to sea, freshwater inflow was lucid and free of sediments. Warm and relatively silt-free ocean water made favourable habitats for diverse marine organisms, in particular those with skeleton and calcium carbonate shells. These species flourished in abundance. Their debris and shells settled on the seabed and formed a thick layer limestone. Covered by sediments, limestone was exposed to increasing pressure and heat. In time it was transformed into subterraneous grained white marble found under some areas of Hong Kong today. Given the many changes over the ages, fossils in these rocks are indistinguishable now. A good guess is that corals and algae, amongst others, lived and bred here. The oldest rocks in Hong Kong, formed by sediments, date back 400 million to 360 million years. Outcropped stratum can only be seen on the north coast of Tolo Channel in Northeastern Hong Kong. In the remote antiquity, stream originating in the southeastern uplands flowed down the hills, bringing sediments to settle at the estuaries. As multiple stream networks washed down sand and pebbles, local sand dunes were erosion. Meanwhile the uplands were reduced in elevation as a result of erosion. Their remaining masses produced finer sand and silt. The estuary floodplains covered by these substances were intruded intermittently by seawater.