Geological History and Hong Kong Rocks
Detailed studies of the rocks enable the geological history and the evolution of the tectonic setting to be deciphered. The large variety of rock types present in Hong Kong reflects the complexity of the geology of the region. |
- Geological History of Hong Kong
- Tectonic Framework of Southeastern China
- Palaeozoic to Mesozoic Pre-volcanic Period
- Mesozoic Volcanism and Plutonism
- Mesozoic Post-volcanic and Cenozoic Period
- Quaternary Period
Geological History of Hong KongThe geological history of Hong Kong has been strongly controlled by changes in the plate tectonic setting over the past 400 million years. Significantly, there are major gaps in the geological record in Hong Kong, which probably reflect major changes in the tectonic regime of the region. These have led to shifts in depositional environments ranging from rivers and deltas, to a warm shallow sea, to a deep continental sea, to a volcanically active continental margin, to an arid, block-faulted, continental terrestrial setting (Figure 1). The oldest rocks in Hong Kong are of late Palaeozoic age and comprise non-marine and shallow marine sedimentary rocks. These Devonian, Carboniferous and Permian rocks crop out mainly in the northeast and northwest of Hong Kong. The Mesozoic pre-volcanic sedimentary rocks comprise Early and Middle Jurassic sandstones, siltstones, and mudstones that were deposited in an alluvial, shallow marine, and sub-tidal environments. Mesozoic volcanic and plutonic rocks are the dominant rock types in Hong Kong. They comprise granite plutons, rhyolitic dykes and thick rhyolitic tuffs and lavas. Most of the igneous rocks are of Late Jurassic to Early Cretaceous age. Mesozoic sedimentary rocks, which are younger than the main volcanic episode, and Tertiary sedimentary rocks, are exposed mainly in northeastern Hong Kong. They comprise non-marine red bed and evaporitic sediments that were deposited in fault-controlled basins. Weathering and erosion have shaped the present landscape of Hong Kong and led to the accumulation of locally thick Quaternary superficial deposits. |
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Figure 1: Hong Kong Geological Time Scale (Click to enlarge). |
Tectonic Framework of Southeastern ChinaThe unexposed (buried) continental basement of Hong Kong is thought to be older than 550 million years. However, in neighbouring Guangdong Province, crystalline rocks as old as 2.5 billion years have been discovered. Southeastern China is believed to comprise a series of three old crustal blocks. These blocks are known as the North China Block, the Yangtze Block and the Cathaysia Block (Figure 2). The crustal blocks have been joined together (sutured) by continent - continent collision events over the past 1,000 million years. The present-day tectonic setting of southeastern China is what is termed a passive continental margin (i.e. there is no plate boundary along the edge of the continental platform). Southeastern China forms part of the Eurasian Plate, which is in continent-continent collision with the Indo-Australian Plate along its southern border (forming the Himalayan Mountain Chain). In the east, the Philippine Plate is moving northwestwards forming a complex oceanic subduction zone system along the margin of the Eurasian Plate. North of Taiwan, the Philippine Plate is being subducted beneath the Eurasian Plate. South of Taiwan, both the Philippine and Eurasian Plates appear to be subducting beneath the islands of the Philippines, leading to a complex history of volcanism and earthquakes. |
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Figure 2: Crustal blocks and the present-day tectonic setting of southeastern China. |
Palaeozoic to Mesozoic Pre-volcanic PeriodAbout 1,000 million years ago, the continental basement of Hong Kong (the Cathaysia Block) collided with the Yangtze Block in a convergent margin tectonic setting. By about 600 million years ago, a large part of southeastern China was submerged beneath a shallow continental sea. By 400 million years ago, sediments from rivers and their deltas were being deposited in the Hong Kong region. These sediments now form the oldest rocks in Hong Kong, which are sedimentary rocks of the Devonian Bluff Head Formation. Tectonically, the area was relatively stable. |
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Over the following 50 to 70 million years, the area was submerged beneath a warm shallow sea in which calcareous muds were deposited. These deposits are preserved in Hong Kong as marble beneath Yuen Long. By about 300 million years ago, the region had become a deep continental sea. |
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Between 250 and 200 million years ago, there was a major tectonic event in China, possibly resulting from major plate reorganisation. This was accompanied by the intrusion of granite magmas. The older rocks in Hong Kong were strongly deformed. From about 200 to 80 million years ago, a convergent margin tectonic setting developed along the southeastern coast of China. In the Hong Kong region, active volcanoes erupted ash and lava between about 165 and 140 million years ago. |
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Mesozoic Volcanism and PlutonismEvidence for the oldest volcanoes in Hong Kong occurs in the Tuen Mun area, where a relatively small volume of volcanic ash and lavas rich in calcium, sodium, iron, and magnesium aluminium silicate minerals are exposed. Preliminary radiometric dating of these rocks suggests that they are about 180 million years old. They probably represent the remnants of a chain of andesitic stratovolcanoes that developed along the southeastern coast of China after the onset of a convergent margin tectonic setting in the Early to Middle Jurassic Period (200 to 175 million years ago). |
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As the convergent margin matured, the subduction zone migrated southeastwards, and volcanoes of a slightly different type and composition soon developed. Volcanic ash and lava rich in dominantly silica (quartz), potassium, sodium and iron aluminium silicate minerals were erupted from large caldera-type volcanoes during the major period of volcanic activity that affected Hong Kong between 165 and 140 million years ago. Eruptions occurred in four distinct episodes (Figure 12): 165-160 million years, 148-146 million years, 143-142 million years and 140 millions years, and were accompanied by the intrusion of large granite plutons. |
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Figure 12: Inferred locations of the Mesozoic volcanic centres or calderas in Hong Kong. |
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Major Volcanic Episodes 165-160 million years The first major episode of eruption formed much of the volcanic rock exposed in the central New Territories. These rocks are dominantly crystal-rich ashes containing abundant rock fragments. The volcanic eruptions were very violent, and there is very little evidence of any lava flows. Although the caldera that produced the ash has long since disappeared, its approximate location can be determined from the concentration of volcanic vent-type materials and the associated plutonic rocks. Volcanic vent materials and related plutonic bodies form a discontinuous, northeast-oriented ring-like structure that probably marks the original caldera boundary (Figure 13). Northeast-trending faults, subparallel to the convergent margin, probably controlled the shape and size of the caldera. |
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Figure 13: Schematic representation of caldera development and
related subvolcanic intrusions |
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148-146 million years Further migration of the convergent margin southeastward led to a shift in focus of volcanism towards the southeast and towards the development of stronger northwest-southeast tensional forces. A major new volcanic caldera developed in the area that is now the central part of Lantau Island (Figure 18). This caldera was fed from a deep magma chamber, with the magma injected along dykes. A granite body in the vicinity of Sha Tin represents the remnants of the magma chamber, while numerous east-northeast and northeast-trending dykes on northeast Lantau Island, Ma Wan and Tsing Yi represent the feeder dyke plumbing system of the volcano. Measurement of the width of the dyke complex suggests that approximately 6 km of northwest-southeast crustal extension took place over about 1.5 million years. |
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Figure 18: Schematic representation of caldera development and
related subvolcanic intrusions |
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Compared with the earlier volcanic episode, the volcanic activity appears to have been hotter, less crystal-rich, and more violent. In places, the volcanic ash was so hot that it fused together to form lava. This process, called welding, is thought to be the origin of some of the banded volcanic rocks on Lantau Island. |
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143-142 million years The scale and intensity of Mesozoic volcanic activity continued to increase as the convergent margin migrated southeastwards. The third major episode of volcanic activity in the Hong Kong region is particularly complex, with the formation of at least two calderas that each erupted volcanic materials of slightly different compositions. Evidence suggests that one caldera, centred on Hong Kong Island, erupted volcanic ash with very little crystal content, while the other caldera, centred in the area of Sai Kung and Long Harbour, erupted volcanic ash with abundant crystal content (Figure 23). |
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Figure 23: Schematic representation of caldera development and
related subvolcanic intrusions |
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The two volcanoes probably erupted simultaneously for a period. Very little lava appears to have been erupted from either volcano. Over time, the volcanic eruptions became hotter and increasingly more violent. Both volcanoes probably culminated in caldera collapse following a catastrophic eruption. Today, the caldera margins are marked by discontinuous dyke-like intrusions of plutonic rock. |
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140 million years The fourth and final episode of volcanic activity in Hong Kong is marked by the development of a large caldera volcano centred in the Rocky Harbour area, which was fed from northeast-trending fissure dykes along its northern and southern margins (Figure 27). Eruptions were dominated by large volumes of crystal-poor volcanic ash that accumulated as thick layers, which are, in places, strongly fused together. Fusing indicates that the eruptions were extremely hot. |
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Figure 27: Schematic representation of caldera development and
related subvolcanic intrusions |
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The tectonic setting of Hong Kong appears to have been strongly back-arc extensional as the convergent margin continued to shift farther southeastwards. The final volcanic episode culminated with one extremely explosive eruption associated with caldera collapse. Following the caldera collapse, an enormous volume of ash, with a minimum thickness of 400 metres, accumulated in the volcanic depression. The ash slowly cooled to form the spectacular six-sided columns of rock seen at the East Dam of High Island Reservoir. This cataclysmic eruption marked the end of Mesozoic volcanism as recorded in the Hong Kong region. |
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Mesozoic Post-volcanic and Cenozoic PeriodBetween about 140 and 50 million years ago, the Hong Kong region was part of a landmass. Tectonically, the region was relatively stable, except for the development of several block-faulted basins. The climate was hot and dry. |
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From 50 million years to about 2 million years ago, there is no record of any sediments being deposited in the Hong Kong region. However, there is strong evidence that this period was one of continued subtropical weathering. The granitic rocks, which had been emplaced about 2 kilometres below the ground surface, had largely been exposed, and were weathered to produce the thick (up to 250 metres) weathering profiles seen today. From about 5 million years ago to the present, the Eurasian Plate has been in collision with the Philippine Plate, resulting in reactivation of northeast-trending faults in southeastern China. |
Quaternary PeriodThe Quaternary Period, which extends from about 2.6 million years ago to the present-day, is characterised by cyclical climatic changes, during which world sea level periodically fell and rose in response to glacial and interglacial episodes. During the cooler glacial periods, when sea level was as much as 120 metres lower than today, the coastline was about 100 kilometres south of Hong Kong and large volumes of alluvium were deposited on the exposed areas of former seabed. Intervening periods of climatic warming led to melting of the ice sheets, which caused the sea level to rise during these interglacial periods. The rising sea flooded across the alluvial sediments, depositing marine mud over much of Hong Kong waters, with coarser, more sandy sediments accumulating in areas of strong tidal currents. Thus, although Hong Kong was never covered by glacial ice, the stratigraphy of the offshore sediments provides indirect evidence of the major fluctuations in global climate over the Quaternary period. Following the end of the last glacial period, about 11,000 years ago, sea level began to rise rapidly, probably reaching its present height in the Hong Kong region about 8,000 years ago. Quaternary superficial deposits in Hong Kong primarily consist of hillslope deposits (colluvium), river deposits (alluvium), and offshore deposits (mud and sand). About 14% of the land surface of Hong Kong is covered by Quaternary deposits greater than 2 m thick. Man-made deposits, such as reclamations, constitute about 6% of the present land area of Hong Kong (1,105 km2). |
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Onshore Superficial Deposits
Offshore Superficial Deposits
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