Tectonic activity in the Caribbean Abstract The main tectonic process in the Caribbean is that of subduction, creating the curved island arc system. This Geocase looks at the geological structure of the Caribbean, the process of subduction, the characteristics associated with island arcs and a case study of the on-going eruption of the Soufriere volcano in Montserrat, and attempts to redevelop the island. Introduction Tectonic activity formed the islands of the Caribbean and continues to affect them. For example, Jamaica has been hit by many earthquakes such as in 1692 when the old city of Port Royal, south of Kingston Harbour, was destroyed and 2000 people killed. Later, in 1907 over 800 people were killed by a powerful earthquake and then by a fire. Volcanic activity is also present. The on-going eruption of Soufriere (Montserrat) and the bubbling of the Soufriere hot springs in St Lucia are witness to this potent force. The main danger areas – a seismic zone where no energy has been released in the twentieth century – are from the Cayman Islands to Haiti and from Grenada to St Lucia. It is likely that some time in the future there will be a large earthquake in one of these regions. Figure 1 The geological structure of the Caribbean Zone 1: The oldest rocks in Jamaica, Hispaniola and Puerto Rico were formed as part of an island arc system about 100 million years ago. This is no longer a subduction zone but is now a transform plate boundary. On the northern edge of the Caribbean plate, the Caribbean plate moves past the southern edge of the North American plate. There is a short divergent plate – the Cayman Island Ridge – to the west of Jamaica. Here, two plates are being pushed apart and molten rock is forced slowly upwards between the two plates and forces the two plates apart. Zone 2: The Bahamas and Cuba are part of the North American plate. The Bahamas are geologically stable. Zone 3: The eastern Caribbean is an island arc which follows the line of the subduction zone along the edge of the Caribbean plate. There is a convergent plate margin on the east coast of the Caribbean plate where the Atlantic plate is being pushed under the Caribbean plate. In addition, the Cocos plate is moving east, and subducting under the Caribbean plate. Zone 4: Mexico and Central America is a very complex area. There is a destructive plate boundary along the Pacific Coast. Earthquakes are very common here and there are many active volcanoes. Zone 5: There are more fold mountains along the southern edge of the Caribbean, in Northern Venezuela and Trinidad. Earthquakes occur at all margins but most volcanic activity occurs at divergent and convergent margins. Subduction zones Subduction zones form where an oceanic lithospheric plate collides with another plate, whether continental or oceanic. The density of the oceanic plate is similar to that of the aesthenosphere hence it can be easily pushed down into the upper mantle. Subducted (lithospheric) oceanic crust remains cooler, and therefore denser than the surrounding mantle, for millions of years, so once initiated subduction carries on, driven, in part, by the weight of the subducting crust. As the earth has not grown significantly in size – not enough to accommodate the new crustal material created at mid Ocean Ridges – the amount of subduction roughly balances the amount of production at the constructive plate margins. The evidence for subduction is varied: The existence of certain landforms such as deep sea trenches and folded sediments – normally arc-shaped and containing volcanoes. The Benioff zone – a narrow zone of earthquakes dipping away from the deep sea trench. The distribution of temperature at depth – the oceanic slab is surrounded by higher temperatures. Figure 2 Ocean-ocean subduction zone Ocean–ocean subduction zones The Lesser Antilles (Eastern Caribbean) Arc shows all the features of a typical island arc. Ocean-ocean subduction zones tend to be simpler than ocean-continental subduction zones. In a typical ocean-ocean subduction zone there are a number of characteristic features. Ahead of the subduction zone there is a low bulge on the sea floor (known as the trench outer rise) caused by the bending of the plate as it subducts. One of the most well known features is the trench that marks the boundary between the two plates. In the Esatern Caribbean, the trench associated with the subduction zone is largely filled with sediment from the Orinoco River. These sediments, more than 20km thick, have been deformed and folded into the Barbados Ridge, which emerges above the sea at Barbados. The outer slope of the trench is generally gentle, but broken by faults as the plate bends. The floor of the trench is often flat and covered by sediment (turbidites) and ash. The trench inner slope is steeper and contains fragments of the subducting plate, scraped off like shavings from a carpenter’s plane. The subduction complex (also known as acrretionary prisms) is the slice of the descending slab and may form significant landforms. For example in the Lesser Antilles, the islands of Trinidad, Tobago and Barbados are actually the top of the subduction complex. Most subduction zones contain an island arc, located parallel to the trench on the overriding plate. Typically they are found some 150-200 km from the trench. Volcanic island arcs, such as those in the Caribbean including the islands from Grenada to St Kitts, are island arcs above sea level. Figure 3 Lesser Antilles island arc Figure 4 Cross-section of the Caribbean The islands of the Eastern Caribbean form a double arc. The inner islands are volcanic in origin. Most of the volcanic activity has long ceased, although Soufriere on Montserrat is an exception. In the last century volcanic activity has been on Mt. Pelée (1902) in Martinique and Soufriere (1979) in St Vincent. The outer islands are sedimentary – uplifted coral limestone built upon a base of solid rock. Trinidad and Tobago are different having broken off from the South American plates. Its southern part is formed of deposits from the Orinoco River, while its northern Range in an extension of the Andes. The fore-arc basin is located between the subduction complex (if developed) and the volcanic arc. This may contain sediments from both the volcanic arc and the subduction complex. Between the Barbados Ridge and the island arc is the forearc basin, known as the Tobago Trough. The marginal basin or back arc basin is a backwards-spreading centre caused by tensional forces (i.e. pulling apart) on the over-riding plate caused by the subduction process. Marginal basins are therefore similar to midocean ridges and may produce new ocean crust. However, owing to water present in the subducted plates, the new oceanic crusts are chemically different to those formed at mid-ocean ridges. Famous marginal basins include the Japan Sea. The Grenada trough is a back-arc basin is located between the inner side of the island arc, and the Aves Ridge, a remnant arc. Subduction zones generally dip at between 30o and 70o, although some may be steeper. The dip is related to The speed of convergence at the trench Time since subduction Age – the older the crust, the steeper it dips. Oceanic volcanic arcs are surrounded by large quantities of volcanic debris – often many kilometres thick. Although the base of the arc is dominated by lava, the deposits on top are a mix of pyroclastic flows, ash, soot and pulverised rock. Sediments around island arcs show a number of characteristics. Their distribution is asymmetric due to prevailing wind patterns, ocean currents, and slope angles. As the island arc develops, older volcanic deposits may be blanketed by redeposited turbidites. The position of a volcano along a trench can change over time (as subduction proceeds). As a volcano grows into shallow water and emerges, eruptions become more explosive, and ash is dispersed greater distance from the volcano. Sedimentary processes sort volcanic deposits into proximal (closest) course-grained sediments, medical (middle) debris flow deposits, and distal (further away) turbidites fall out ash. The rates of arc volcanism and back arc spreading vary with periods of continuous volcanism and reduced spreading favouring sedimentation. Tectonic activity in St Lucia Figure 5 Volcanic activity in St. Lucia 1. Most of the rocks in this region were laid down about 10 million years before present (MYBP). The original volcanic landscape has been completely altered by subsequent erosion. There are many ridges of high ground formed by dykes. 2. Mount Gimie is the largest of a group of volcanic cones which was formed about 1.7 MYBP. 3. Several hundred thousand years ago, there was a very large volcano, centred near to the town of Soufriere. 4. About 40,000 years ago there was a major ignimbrite eruption from this volcano. 5. After the eruption, the centre of the volcano collapsed, leaving a large caldera. The western side of the caldera is missing. 6. Inside the caldera are fifteen intruded viscous lava plugs. The largest of them are the two pitons. 7. This headland is formed of layers of ash and lava laid down around 10 MYBP. The steep cliffs have been produced by subsequent wave erosion. 8. Many of the headlands of this stretch of coastline are formed by old lava flows. In a number of areas, molten rock remains close to the surface. For example St Lucia has hot springs – water seeps through the surface rocks, is heated by the molten rock, and then erupts back up to the surface. Many hot springs contain high levels of sulphur. Fumaroles are formed where steam enriched with sulphur issue from the surface. The St Lucia Soufriere fumarole is located in a volcanic crater. Nevertheless, there has been no major eruption in St Lucia for 40,000 years. Montserrat update Figure 7 Sketch map of Montserrat Key facts Population 8,437 Population growth rate 8.43% (2002 est.) Birth rate 17.54 per thousand Death rate 7.47 per thousand Infant mortality rate 7.98 per 1000 live births Life expectancy 78.2 years (80.4 years for females, 76.1 years for males) Montserrat is one of the Leeward Islands chain in the Eastern Caribbean. Montserrat is part of the Lesser Antilles, which is a chain of volcanic islands formed along the junction of the Caribbean and North American crustal plates. There are at least 15 potentially active volcanoes spread along the length of the chain, of which the Soufrière Hills volcano of southern Montserrat is one and the one currently most active. Almost all of Montserrat is composed of volcanic rocks, which give rise to its black sand, and rugged mountains covered in lush tropical vegetation. The current eruption of the Soufrière Hills volcano began in 1995. The first phreatic eruption of the Soufrière Hills volcano occurred on 18th July 1995. Earthquakes started in 1992, some three years before the 1995 eruption. In November 1995 lava was seen for the first time and in December a second evacuation of southern Montserrat took place. In June 1997 19 fatalities occurred within the Exclusion Zone due to a pyroclastic flow. A major dome collapse destroyed W H Bramble airport terminal building. Figure 8 Large areas of Montserrat were devastated by mud flows Economic and social development Although in 1995 there was criticism at the UK government for the slow speed of the aid, and the limited amount of aid, the same cannot be said now. While the southern part of the island has been devastated and remains an Exclusion Zone, the northern part of the island is experiencing rapid development. The capital city Plymouth was abandoned following the 1997 eruption. Temporary government buildings have been built at Brades Estate, in the Carr’s Bay/Little Bay area in north-west Montserrat. There has been a huge increase in the provision of housing in St Johns, and there have been new schools, crèches and hospitals built. Much of the transport infrastructure has been improved, and there has been investment in the port facilities. FIFA are building a football pitch - this may rate as one of the most attractive pitches in the world! – while the Montserrat Governments offices have been relocated and rebuilt in St John. New service industries have developed in Salem, and there has been an increase in ecotourism and adventure tourism. Figure 9 What remains of Bramble Airport, Montserrat Figure 10 Abandoned housing near Plymouth Figure 11 FIFA’s new football pitch near St John’s Figure 12 Much of the economic and population growth is stimulated by reconstruction of the island Prospects for the economy depend largely on public sector construction activity. The UK has launched a three-year $122 million aid programme (known as the Country Policy Plan, 2001-2003) to help reconstruct the economy. The southern third of the island is expected to remain uninhabited for at least another decade. Agriculture accounts for 5.4% of the GDP. The lack of suitable land means that it is unlikely that this figure will increase much in future years. By contrast, over 80% of GDP is generated by services and this is likely to increase as the aid programmes continue. There have been a number of recent developments in the local economy and infrastructure. The Lookout Nursery School, built with DFID funding, was officially opened in March 2002, and a sheltered Housing Scheme at lookout is also near completion. In 2003 a new volcano observatory at Flemings was officially opened in 2003 at the Vue Pointe hotel in Old Road bay, just north of Plymouth. It was later covered in ash as a result of the July eruption! Montserrat has also signed two agreements with medical schools, there is interest being shown in constructing a water bottling plant, and an international lottery is being set up too. There is continuing interest in the exploitation of ash and aggregate. The new airport terminal begun helicopter flights in 2004 and there are plans for a postgraduate school of disaster studies! Nevertheless, Montserratians are unhappy that some of the 1997 refugees are still living in shacks, with sub-standard facilities. Development money (from the UK Government, World Bank and EU) has generally gone into new investments rather than upgrading the facilities hastily constructed in 1997. Recent volcanic activity The Soufrière Hills volcano continues to be active. Areas south of a line extending from the summit of Garibaldi Hill in the west of Pelican Ghaut in the east have been designated an Exclusion Zone into which entry is prohibited. There is also a small Daytime Entry Zone (DTEZ) into which daytime vehicular entry is permitted when the island authorities consider it safe. Figure 13 Soufrière Volcano continues to spew out ash, soot and cinders – volcanologists are not sure what it will do next Figure 14 The world famous Montserrat Volcano Observatory During 2001 and 2002 dome growth continued, reaching record levels in October 2002. Since September 2001, the dome has grown at an average rate of about 2 cubic metres per second (or 400,000 tonnes per day). By October 2002 the lava dome had grown to dangerous levels, reaching its highest point in many years, over 550 metres and nearly 1.5km wide. Scientists from the Montserrat Volcano Observatory feared that the dome could collapse, destroying properties on the volcano’s northern side. In July 2003 the largest dome collapse of the eruption since 1995 occurred. The fall deposit (total of ash, pumice and lithics) reached a maximum thickness of just over 15 cm depth. This is three times the total thickness of deposit that accumulated in the area in the entire period of 1995-1998. The volcano has had a major impact on vegetation and has led to acidification of water sources in the region. pH readings as low as 2.0 have been recorded from lakes at the top of Chance’s Peak. The main hazards remain as pyroclastic flows, explosions, falls of ash and small stones and volcanic mudflows. During a large dome collapse or explosion, heavy ash fall and the fall of small rock fragments can be expected in the populated areas if the wind is in an unfavourable direction. Outside the Exclusion Zone significant falls of rock fragments large enough to cause serious injury are unlikely. Conclusion Tectonic activity continues to have a profound impact in the Caribbean. Its impacts are both beneficial and destructive, depending on time and space.