The Volcanology Presentation is available for download for Field Ecology students only (password protected): Download Here (4.8MB PDF).
The volcanoes local to the study area are Arenal and Cerro Chato. Cerro Chato is considered extinct, but Arenal has been erupting continuously since 1968.
© Scripps Institute of Oceanography
Both Costa Rican geology and topography are dominated by plate tectonics and subduction. The Cocos oceanic place (in the west) is subducting below the Caribbean plate (in the east). This process of subduction is responsible for the ocean trench to the west of Costa Rica, and of the many thrust-fault earthquakes experienced in the area. On at least one occasion, a section of ocean crust has been obducted to form the Nicoya Peninsular ophiolite. This is a province of unusual ultramafic igneous rocks representing a slice of the ocean crust.
Water in the subducted rock, and fluids released by metamorphism are released by the subducting slab and move into the overlying mantle (to the east, directly under Costa Rica). These fluids lower the melting point of the mantle, and partial melting occurs.
Rocks, including the mantle, consist of multiple minerals. These melt at different temperatures, hence melting occurs over a range of temperatures. Partial Melting refers to an intermediate temperature where only some of the rock melts. The composition of the resulting melt depends on the degree of melting. Very small amounts of melting can produce very different compositions, whilst 100% melting will result in a melt with the same composition as the host rock. This is one of the primary causes for the wide variety of igneous rock compositions.
The melt has low density, so it will slowly move upward out of the mantle. At first, this flow is through thin films between crystal faces. As it rises, the melt coalesces and moves through conduits and even 'diapirs' - large rounded intrusions.
As the melt moves upwards through the crust, it will be surrounded by lower containing pressures. With lower pressures, dissolved volatiles may come out of solution to form bubbles. These bubbles lower the density even further and accelerate the upward movement.
Eventually, the rising melt will reach a level of mutual buoyancy. It will stop rising, and form a magma chamber. It is possible that the melt will stay there and cool - to form a batholith.
A volcanic eruption starts when the pressure inside the magma chamber exceeds that of the surrounding rock. Magma is intruded into the surrounding rock as a series of sills and dykes. Magma will also form a conduit to the surface, or use an existing conduit if the volcano has erupted before. As the magma rises, reduced pressures cause volatiles to come out of solution and form bubbles. This lowers the density, causing the magma to become more buoyant and rise more quickly. This acelerates the process. Existing bubbles will grow, and new bubbles may form. This can cause a runaway process. If the bubbles cannot escape before the magma is disrupted into fragments, then an explosive ash eruption is caused (see below). If the bubbles can easily escape from the magma, then the lava will erupt more effusively.
Variations in Rock Types
All rocks can be classified according to their texture and their composition. Igneous rocks are no different, and both should be considered when trying to understand an igneous rock.
The primary way to describe an igneous rock's texture is to look at the grain size. Large crystals are formed by slow cooling, and can only be formed underground. Lavas and small intrusions that cool quickly have small grain sizes. It is possible for a lava to cool so quickly that no crystals form. A glass (obsidian) is formed instead.
Some other igneous textures are described below:
Porphries are often used as decorative stones in architecture and sculpture. A porphry has a fine groundmass surrounding coarse crystals (phenocrysts) that have formed from the same magma. These are formed from a magma with crystals ('crystal mush') which then cools quickly. This magma-crystal mix may form in a magma chamber that has started to cool (and form crystals) but has not cooled sufficiently for different mineral types to form.
Pumice has lots of bubbles (vesicles). Often there are enough bubbles that the rock will float in water. Pumice forms when the magma contains lots of volatiles, but the melt is sufficiently viscous that the resulting bubbles cannot escape from the magma.
Tuffs are rocks formed from volcanic ash. They can form in a variety of different ways - eg. ash fall, pyroclastic flows, and lahars. They will often have sedimentary structures, and can include bombs and other volcanic debris.
There are many different components of igneous rocks, resulting in a wide range of different compositions. Many elements behave in similar ways, so compositions can be simplified by classing similar elements together - eg. Sodum and Potassium; or Iron and Magnesium. Despite this, composition analysis can be complex. By far the most important component is silica.
All igneous rocks (with the exception of some obscure carbonatites) are silicates, and contain between 40 and 80% silica. This greatly affects their behaviour and mineraology.
Rocks with silica levels over 52% are termed over saturated these have surplus silica after all the other elements have been 'used'. This silica typically appears as quartz. Rocks with silica levels below 52% are termed under saturdated. These rocks have no surplus silica to make quartz. Other minerals such as olivine start to appear. With very low silica levels, feldspars are replaced by feldspathoids.
Silica lowers the melting point of a rock. It also increases the viscosity of the magma or lava. The fast flowing low viscosity lavas of Hawai'i are both hot and have relatively low silica. In contrast, the lavas at Chaiten (Chile) and Mt. St. Helens (USA) have high silica levels, resulting in high viscosities that remove their ability to flow. These high silica lavas form domes instead.
The following table summarizes the main silica classifications:
|Group||Silica (%)||Examples / Example Location||Notes|
|Carbonatites||Oldoinyo Lengai, Tanzania||>50% carbonates; WEIRD!|
|Ultramafics||<48%||Mantle||Rare at the surface|
|Mafics||48-52%||Basalt, Gabbro; Ocean crust, Hawai'i||High MgFe, dark; low viscosity lavas|
|Andesites||52-63%||Subduction zone volcanism||Explosive|
|Dacite||63-68%||Mt. St. Helens||Explosive, very viscous lavas|
|Rhyolite||68-77%||Black obsidian, granite; Enchanted Rock, TX||Lava flows are rare (too viscous)|
The forces that bring magma to the Earth's surface have already been described. How this manifests itself depends on a lot of factors. Magma viscosity (which depends on temperature and silica content), magma strength (how easily does the magma fragment?), and volatile content are all important factors. Explosive eruptions can also occur if lava and/or gas cannot escape easily. Viscous magmas might cause this. A volcanic plug that blocks the vent may be another cause. Pressure builds up under the plug until it breaks explosively.
Activity at Arenal
Rather than cataloging the range of eruption types and phenomena, we shall concentrate on activity and phenomena that have been seen at Arenal in the past 40 years.
There is no standard definition of a hot spring other than it is notably warmer than the surrounding rocks. They range from the 'warm' springs of Britain, through to hot springs that are close to boiling. Often (eg. Hot Springs in Arkansas) they are caused by deep water being channelled by the local geology to the surface. Other hot springs are caused by the presence of hotter-than-usual rocks at shallow depths. The Leaves and Lizards property includes a number of hot springs that are formed by this second method.
Fumaroles are similar to hot springs, except they emit more gas than liquid water. The gas is typically dominated by carbon dioxide or water vapour, but it can include many other gases. Sulphur compounds are common. Arenal's "D" crater has only shown fumarole activity during the ongoing eruption.
Named after Stromboli in the Aeolian Islands (Italy), Strombolian eruptions are characterised by the classic "firey spark" time lapse photographs that are often seen in volcano books. The sparks are globs of magma (ie. volcanic bombs). Stromblian activity occurs as a series of bursts separated by minutes or hours. These occur as bubbles coalesce at the throat of the vent. These grow to about a metre in size, and then eventually disrupt the magma. The magma is disrupted into the globs of magma which are thrown out.
Strombolian activity has dominated Arenal's activity since the early 1970s. Both Stromboli and Arenal have shown extended activity over a long period of time, making them ideal destinations for volcano watchers.
These form at the beginning of a new eruption when a cool volcanic plug is broken. Because this plug needs time to form and cool, Vulcanian eruptions tend to be separated by many decades. Vulcanian eruptions are characterised by a large initial explosion that throws out large bombs, and an eruption column that can reach 10-20km high.
Arenal's current activity started with a major Vulcanian eruption on 29th July 1968. It has been calculated that some of the bombs exited Arenal's crater at supersonic speeds, although this calculation has been disputed.
Vulcanian eruptions are not the largest. Plinian eruptions are larger still with eruption columns that reach the stratosphere, and activity that extends over many hours or days. The Vesuvius eruption of 79AD is the most famous example, although Mt St Helens (1980) and Chaiten (Chile, 2008) have both included Plinian activity.
Basalt Lava Flows
Arenal has erupted large numbers of effusive lava flows since the 1970s. Visitors can walk on one from the 1990s in the National Park. Arenal's lava is basaltic but tends to be blocky. This is because the lava is viscous, flowing like toothpaste, rather than as a fluid (ie. at Hawai'i). The slow rate causes the top surface to cool and break, resulting in the blocky appearance. The high viscosity also results in deep flows with significant levees.
Arenal erupts occasional pyroclastic flows. These are ash and rock clouds that resemble avalanches. In reality they are very fast moving (100mph is possible) and very hot. The chance of surviving a pyroclastic flow is very low - the number of pyroclastic flow survivors worldwide in the 20th century can probably be counted on your hands.
Pyroclastic flows have a number of causes, and their sub-classifications continue to be argued amongst volcanologists. Arenal's current pyroclastic activity is mainly in the form of small avalanches, although larger flows have been documented. Arenal has formed a lava lake in the past, and breaches of this lake can form large pyroclastic flows.
Lahars are volcanic mud flows. These are typically formed by rain on fallen ash, or when ash falls on snow. Lahars can occur as flash floods that carry large amounts of rock debris. This rock and ash increases the density of the flow, making them particularly destructive.
Arenal is in a rainforest, so lahars do occur after ash falls and pyroclastic flows. Recent ash activity has been typically restricted in extent, so lahars have tended to be small.