Download Tectonic activity in the Caribbean Abstract The main tectonic

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.
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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.
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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.