Geology of The Bay Islands
Where We’re From and Where We’re Going

May 1st, 2010
by Katie Dickey

[private] v8-5-feature-Seismic-MapMillions of years ago, giant tectonic plates scraped against one another in a truggle for space and power that gave birth to the underwater mountain range called the Bonacca ridge that embodies the Bay Islands. The upwelling of magma and the crumbling of basalt rock (the core of the islands) built, over millennia, a mountain range that manages to peak out of the ocean. Over time these lands gave birth to vegetation, animals and eventually civilization.

From the rolling hills and valleys of the mainland to our treasured reefs, this type of tropical paradise certainly does not form over night. In most cases, it seems that life has sprung up out of the sea and it is no different for the formation of the Bay Islands.

Differing local opinions suggest several sources of the formation of the Bay Islands from volcanic eruption to sections of the mainland that have just floated away. Although technically these theories are incorrect, in a way both of them do lend themselves to the actual formation of our island chain. Roatan and the Bay Islands, although not volcanic do sit atop mountain peaks. We are part of a large underwater mountain range called the Bonacca Ridge. The Bonacca ridge, a large submarine mountain range, lies south and parallel to the Motagua Swan Islands Fault, a meeting place of tectonic plates. This ridge extends underwater from mainland Honduras. Thus, the theory that we are in fact a piece of the mainland is in part, true. The geological structure and bedrock of the Bay Islands, as far as rock composition, is strikingly similar to that of the Honduran and Guatemalan mainland. Thus, we are part of the same continental crust and come from the same roots as mainland Honduras and Guatemala.

That very formation began millions and millions of years ago when the meeting of giant plates at fault boundaries gave birth to the Bonacca Mountain range. Geologists estimate the time of our emergence from the sea to have been primarily in the Eocene and Oligocene time periods, meaning between 56-34 and 34-23 million years ago. Keep in mind, it is a gradual process. The mountains were formed and lifted out of the sea by an upwelling of magma via the separation and parallel movement of the Motagua and Swan Islands fault. The Bonacca ridge is a Horst which is a type of geological formation of up thrown blocks bounded on either side by normal faulting. And, notably we continue to grow. The faults continue to move and change the geological structure of the islands at a continuous rate, called a slip rate. Some of the faults are held together by friction causing a build up of tension that then results in earthquakes. However, earthquakes are not only a destructive force but a productive one as well. Evidence of earthquakes that have caused the sudden uplifting of the Bay Islands can be seen through geological samplings but also through simple exploration of the islands. There are uplifted and warped landforms on the western part of Roatan that come from prehistoric earthquakes of magnitude 7 or higher. This uplifting that causes Roatan’s unique geological structure comes from a subsidiary fault to the Swan Islands fault called the Flowers Bay Fault, the name of which should sound familiar to some.

The ground shook unexpectedly at 2:24 AM on May 28th 2009 and so did our senses of security. But, since that unsettling moment about a year ago, our little group of islands has sat peacefully in the Caribbean Sea. Now in the wake of the disastrous earthquake that struck Port-au-Prince, Haiti on Tuesday January 12th, as well as those more recent quakes in California, Chili, Indonesia and China we find ourselves thinking more about where we came from and where we might be going; in both an unearthly and earthly sense. The magnitude 7.3 earthquake that occurred off the coast of Roatan in May of last year leaves us with many questions. Why did this happen? What are the affects of earthquakes on the islands? Why did we feel only slight tremors when the mainland saw such destruction? What can we do to make ourselves more prepared for the future? Is it likely that we see another earthquake in the near future? These types of natural phenomena serve to give us a sense of primeval wonder that the very tectonic plates we sit on may be the sources of our destruction as they were the sources of our creation millions of years ago. In order to understand the answers to these questions we must first return to the roots of our geological knowledge. Thus, we return to the basic processes of the creation and deformation of our islands and the reasoning behind such natural phenomena as the formation of land masses, earthquakes and tsunamis. They all come from just a few basic principles.

Basic Plate tectonics

The earth’s crust is made up of a series of solid blocks called plates, the movement of which gives us explanations for the world as we know it today. These solid blocks of the earth’s surface or lithosphere move independently of one another and sit atop a more plastic or malleable part of the earth called the asthenosphere. The asthenosphere causes the movement of these tectonic plates by a process called convection. The molten rock of the asthenosphere is heated by more active elements towards the center of the earth creating this circular motion of heated molten earth rising closer to the surface and parts of the lithosphere being driven down by means of moving plates. This convection processes is the reason for the shifting of these giant tectonic plates.

Tectonic plates come together in several ways, but in the most basic sense, there are three types of formations. The plates come together in convergent, divergent and transform/transcurrent or strike slip fault boundaries. That is, plates that move toward one another, away from one another or slide horizontally past one another. Each different type of movement has a differing geological phenomenon that follows. Divergent boundaries, or the spreading apart of the earth’s crust result in the creation of a new sea floor via the upwelling of magma or molten rock from below the surface of the earth. Convergent plate boundaries, as they are the reverse, are the colliding of two plates that can result in one plate moving over another creating a subduction zone where the lower plate is then driven underneath and heated, continuing the convection process. Transcurrent plates move past each other and connect a network of convergent and divergent plates. The plate boundary that lies adjacent to the Bay Islands is a transcurrent boundary much the same as the volatile San Andreas fault in California. Not only do we share the same type of plate boundary as San Andreas but also the fault that caused the Earthquake in Haiti that’s horror attracted the attention of the whole world.

The fault that caused the earthquake in Haiti is a strike slip and forms the boundary between a smaller plate which is wedged between two larger plates. The smaller plate is called the Gonave microplate and it is situated between the Caribbean and North American plates. Detailed GPS analysis of these faults show that there was an accumulated deficit since the last large earthquake generated by the fault in the 18th century (deficit= stress or the amount the plate should have moved) by about 2 meters. That kind of deficit stress place on these faults can cause an earthquake of exactly the magnitude that struck Port au prince. Thankfully, it is unlikely that the earthquake in Haiti or the disturbances along those fault lines will have an effect on the bay islands as we lie adjacent to a different fault boundary. Yet, this does not negate the volatile nature of that fault and the potential for another earthquake.

Where are we located?

The geographic location of the bay islands seems common knowledge to those of us who live here but it is not often that we give a thought to our geological location or our geological origins. As we do simply lie about forty miles east of the mainland of Honduras there is much more detail of our geological location and geological history.

Concerning specifically the tectonic plates and faults of our area, Roatan and the Bay Islands lie about 15 miles south of a major fault called the Swan Islands fault. We are located on the northern portion of the Caribbean plate which lies directly adjacent to the North American Plate, a transcurrent fault boundary. This is the type of fault, or plate boundary where the faults are moving horizontally, next to one another. We share the same plate major plate boundary as Haiti, the thought of which is quite unnerving. However, Haiti lies across a different fault line called the Eriquillo fault line and it is unlikely that the disturbance of this fault line would make a difference in the movement of tension of the Swan Islands fault line. In short, we share major plate boundary lines but it is the smaller, closer, faults that may be cause for concern.

The Bay islands, from west to east are Utila, Roatan, Barbareta and Guanaja, atop the Bonacca Ridge that runs east/northeast. In combination with these 4 islands there are 4 other islands and sixty five cays, they amount to a total landmass of about ninety two square miles. Roatan and Guanaja sit parallel to the Bartlett trough (boundary between the Caribbean and North American Plate, deepest point in Caribbean Sea) and are cut by a series of subsidiary fault lines. South of the Bonacca Ridge is lies an extremely large and deep marine basin called the Tela Basin, which is on the northern continental margin of Honduras. The Ridge is separated from the Basin by a normal fault system. The Southwestern tip of Roatan is comprised of a prominent east/ northeast ridge. It can be seen at West End Point. This ridge is in between two subsidiary faults that run directly across Roatan. The northwestern end of the one of these faults is marked by a topographic break which forms a bluff at a high angle on the coast of the west end and extends far below sea level to the south west. Thus the reef at West end point is truncated by this fault instead of wrapping around it.

The southeaster fault otherwise known as the flowers bay fault runs along the coast line of Roatan from Coxon Hole to West End point. Evidence of prehistoric earthquakes along these fault lines can be seen in Roatan’s geological structure. Warped landforms on the western coast of Roatan show record of at least two Holocene ( … million years ago) earthquakes at an estimated magnitude of M7 along the flowers bay fault. Another earthquake in the Bay islands, circa 900 AD has been recorded to have produced huge amount of uplift. We are being uplifted out of the water at a steady rate of 3 mm per year, however when we see serious earthquakes such as out most recent instance, our entire structure is changed and although it seems only a small amount, it is equivalent to years and years of gradual slip. Direct evidence of the changing height of the islands suggests that we are in a constant state of change. Karl Stanly creator and operator of the only tourism oriented submarine on the island, is an eye witness to the geological changes that have occurred on Roatan. Over millions of years the sea level of the island has risen and fallen as a result of the movement of tectonic plates, to give us our current paradise. As the mountains rise the sea level falls and thus we can see the evidence of ancient reefs and a past life under the surface of the sea. However, we also see evidence of changing sea level in the opposite direction. As coral can only grow 300 ft below sea level it is natural to see its growth to that point. However, when deep diving in his submarine, Karl Stanley not only sees coral formation at 300ft but up to 700 ft below the surface. This indicates that the sea level of Roatan was previously about 400ft lower than it is today. This change in sea level is a result of the ice ages. Stanley also sees large boulders of limestone (formerly coral) that have fallen to the depths as a result of prehistoric and more recent earthquakes. The underwater world gives us huge insight into our geological history and the movement of the plates we rest on.

What does this have to do with natural disaster?

The reasoning behind the May 2009 earthquake is as such, according to Chuck Demets, . The earthquake that Rotan felt in May of 2009 was caused by a rupture of the swan islands fault in response to the motion between the Caribbean and North American plates across that fault. The Caribbean and North American plates move steadily past each other, however, the Swan Islands fault remains in place because of friction, thus, the Swan Islands fault stayed in place when it should have been moving with the surrounding plates. Tension began to rise between the Surrounding plates and built up around the swan islands fault line. The earthquake that we experienced in May of last year was a result of that built up stress. Earthquakes occur when the amount of built up pressure from plate movement exceeds the frictional forces that prevent slip on that fault. Professor Demets has analyzed GPS measurements that were made on Roatan during the May 2009 earthquake and has found that the entire island suddenly moved 14 centimeters during the earthquake. This is equivalent to hundreds of years of movement for the island.

The normal slip rate for the Caribbean North American plate, our geological home, is 20 millimeters per year. That means that every century it is necessary that 2 meters of slip happen along this fault for normal movement. However, when certain faults are locked in place that means that there is a high likelihood of an earthquake occurring,, especially if the fault does not move its 2000 millimeters that century. That amount of built up stress is enough to cause an earthquake of the same magnitude that occurred last year, M7-7.2. Thus, we should expect to see an earthquake of similar magnitude once every one hundred years. More frequently we may experience shaking from surrounding or subsidiary faults but it is difficult to predict exactly and there remains a lot of uncertainty.

When asked if the earthquake last year is indicative of another earthquake happening in the near future, professor Demets replied, “The May 2009 earthquake only ruptured about 100 km of the Swan Islands fault which is about 750 m long in total. In addition the Motagua fault of Guatemala which ruptured catastrophically in 1976 continues westward from the western end of the swan islands fault. All of the fault segments that have not ruptured are capable of generating magnitude 7-8 size earthquakes at any time. We cannot predict when they will rupture but we can foresee with certainty that they will rupture eventually.” Thus, we begin to wonder about how susceptible we are to natural disaster in relation to other islands or in relation to the mainland. The lack of damage done to The Bay Island’s infrastructure presents quite the mystery in comparison, when one considers the damage suffered by the mainland. Especially strange considering we lay much closer to the epicenter of the May 2009 earthquake than the mainland. Demets informs us that Roatan and the other bay island are equally susceptible to natural disasters. The Swan Islands fault presents quite a threat. It is, after all, a major plate boundary. However, despite the threat of the swan islands fault, it is not the worst geological threat against The Bay Islands. There are faults that surround the islands which are lesser and rupture much less frequently but are much closer to the islands and pose quite the hazard. However, the damage done to Roatan in last year’s earthquake was small in comparison to the damage done to the mainland of Honduras. Although this seems strange considering Roatan’s proximity to the epicenter of the quake it is in fact understandable because of Roatan’s infrastructure and population. The damage caused by earthquakes not only depend on the distance from the epicenter of the EQ but also the quality/ type of building contraction and the population density. As the majority of the structures built on Roatan are made of wood, they withstand lateral, or side to side, shaking better than brick or concrete. Also, if structures are build on bedrock as opposed to more soft ground they are more likely to do well when confronted with an earthquake. However, this clearly depends on the location of the structure. Also, it is quite clear that the population density on Roatan is much less than in large cities, thus there were fewer people to experience the event on the island and thus fewer injuries.

Considering long term preparation for earthquakes Demets states, “Building construction and location is the key. Buildings that are made from wood, are tied to their foundations (so the whole building can’t slide off its foundation, and are anchored on bedrock will, on average, be less likely to incur significant damage than (for example) brick buildings, which are bad because brick has little ability to remain standing when it undergoes lateral shaking during earthquakes. In general, the initial cost to construct a building with modest to good resistance to earthquakes only adds a few percent to the initial cost of the building and is a good investment in seismic hazard zones. The other key would be to avoid development in areas of water-saturated soils or in areas where previous shallow water bodies have been filled with sand/soil in order to enable construction.”v8-5-Feature-Western-Faults

However, there are smaller things we can do to be prepared for an earthquake that do not involve changing the construction of our homes. The most important things to do are to be prepared and informed no matter what the structural integrity of our buildings imply. FEMA gives us a seven step process for what to do to safely get through an earthquake. Although they are simple steps they can make all the difference in confronting this type of natural disaster.

The past, present and future of the Bay Islands involves seismic and tectonic movement. We are at the mercy of plate movement, be it gradual or abrupt. The movement of the tectonic plates and the reactions of the faults that surround us is a much larger force than we can overcome. We know now that we would not have the Bay Islands as they are today without disruptive plate movement and that we can be adequately prepared for another earthquake, as we sit in a volatile, but beautiful, position in the sea. [/private]

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