Volcanoes

According to the CSEC Syllabus, the specific objectives related to Volcanoes are as follows:

Explain the formation of intrusive and extrusive volcanic features.

As such, we will follow these objectives for this unit.

Volcanoes are a prominent geographical feature in several Caribbean Islands. In fact, our Archipelago was formed on a Convergent Plate Boundary. We reside on what can be described as volcanic islands, formed from the cooling of magma after volcanic eruptions. However, that doesn't tell the full story so let's get into it.

What are volcanoes?

A volcano is an opening in the Earth's crust where magma and gases trapped under the surface erupt, often forming a hill or mountain.

Simple definition right? Good. Keep that in mind. For you see, Volcanoes can be found in three separate classifications. Each classification is used to describe how dangerous a volcano is. These are the following classifications:

Active volcanoes: These are volcanoes that have a recent history of eruptions; they are likely to erupt again in the future. Examples include: La Soufriere in St. Vincent and the Grenadines and Soufrière Volcanic Centre in St. Lucia.
Dormant volcanoes: These are volcanoes that have not erupted for a very long time but may erupt at a future time. Dormant volcanoes are constantly storing energy and are monitored for potential threats. Example: La Grande Soufrière, located in Basse-Terre, Haiti, has been dormant since the 1970s which was its last recording of volcanic activity.
Extinct volcanoes: Now these are the volcanoes that are not expected to erupt in the future. These volcanoes have either eroded away or have outright ceased any kind of activity. Examples include: Mount Saint Catherine and Grand Etang Lake, both of which are located in Grenada.

This video depicts the volcanoes located in the Caribbean.

This video recaps plate boundaries and depicts the way in which volcanoes are formed in the Caribbean.

Now that we understand the three categories of Volcanoes, let's get into the various types of volcanoes. There are four types of Volcanoes:

Stratovolcanoes (Composite Volcano)
Shield Volcanoes
Cinder Cones
Lava Domes

For consistency, the CSEC syllabus currently only asks that we investigate the two main types: Stratovolcanoes (Composite Volcano) and Shield Volcanoes.

Image taken from CSEC Geography Study Guide

Now let's take a deeper look at both of these types volcanoes.

Composite Volcanoes

Composite Volcanoes are the steeper of both volcanoes. These volcanoes form themselves into a conical shape that grow overtime with each eruption as magma cools on the surface. These volcanoes are associated with Convergent Plate Boundaries (also known as Destructive Plate Boundary).

Earlier we stated that the Caribbean is a chain of volcanic islands formed on a Convergent Plate Boundary. If you paid attention to the earlier video it would explain the formation of our island archipelago and how each island has continued to grow overtime. So what does this mean? Well, its simple, every volcano in the Caribbean is some form of Composite Volcano.

Diagram of the Cross Section of a Composite Volcano.

The sides of the Composite Volcano are created by the continuous layering of ash and magma. Now, to the very interesting stuff. Composite Volcanoes are known for their violent eruptions. This is because the energy stored in the magma chamber suddenly bursts through the crater and have been known to cause landslides and send seismic waves throughout the surrounding area. The following explosion shoots heavy ash clouds into the sky blackening the sky. Just look at the video below for an example.

Volcanic Eruption of Soufrière Hills, Montserrat (3 Aug 1996). Viewer Discretion Advised for Language.

SHIELD VOLCANOES

Unlike Composite Volcanoes, Shield Volcanoes are often formed at Divergent Plate Boundaries (also known as constructive plate boundaries).

At Divergent Plate Boundaries, magma is constantly rising slowly through the opening of the earth's crust via gentle eruptions. Upon reaching the surface, this magma cools and spreads. Overtime, this spreading creates layers which form the gentle slopes that help to create the dome of these volcanoes.

Diagram of the cross section of a Shield Volcano.

Some of the most notable examples globally of famous Shield Volcanoes would be Mauna Loa, located in Hawaii, USA (fun fact: it is the largest active volcano globally, standing at 13,681 feet/4170m above sea level) and Fernandina located on the Galapagos Islands.

It should be noted that Mauna Loa is also not formed at a Divergent Plate Boundary but at a Hotspot. Hotspots parts of the Earth's crust that is particularly weak, allowing the underlying magma to break through along cracks and fractures.

Watch the video above for another explanation of shield volcanoes.

Types of magma (Lava)

There are two types of magma (lava) and they both have differing characteristics. These two types of magma (lava) are acidic magma and basic magma. Here's a breakdown of both:

acidic magma (Lava)

Acid magma (lava) is widely associated with destructive plate boundaries which means they belong with Composite Volcanoes and violent eruptions. So here are some characteristics:

Volcano: Composite Volcanoes
Color: Acidic magma is lighter in color than basic magma.
Viscosity: Extremely viscous; resistant to flow (thick, less fluid).
Temperature: Very low temperatures (800-1,000°C)
Eruptions: Violent eruptions that are usually explosive, often producing ash and pumice. This is due to the magma's high gas content and viscosity.
Silica content: Acidic magma has a high silica content, usually between 75–80%.
Lava flows: Acidic magma forms thick, blocky lava flows or steep-sided lava domes. This is because it does not flow far on the surface before it solidifies.

Basic magma (Lava)

Basic magma (lava) is widely associated with constructive plate boundaries which means they belong with Shield Volcanoes and gentle eruptions. So here are some characteristics:

Volcano: Shield Volcanoes
Color: Basic magma is darker in color than acidic magma.
Viscosity: Lower viscosity; this means that it is more likely to flow as it is less thick and more fluid. (Viscosity similar to ketchup)
Temperature: It is very hot, often reaching temperatures of 1,000-1,200°C
Eruptions: Gentle eruptions due to low gas content and a lower viscosity.
Silica content: Basic magma has a lower silica content, usually between 45–55%.
Lava flows: Basic magma flows and spreads far on the surface before solidifying. This is why they create gentle slopes for their lava domes.

Use the above video for a deeper explanation on both types.

Intrinsic and Extrinsic Features

Now that we know the classification of Volcanoes, the types of volcanoes and the types of magma (lava), we can look at the intrinsic and extrinsic features that help to form the landscape of the volcano.

*NOTE: Intrinsic refers to the features within the Volcano while Extrinsic refers to the features formed on the surface.

Basic Diagram of a Volcano (with descriptions)

Intrinsic Features

Diagram of Intrinsic Volcanic Features (Taken from CSEC Geography Study Guide)

Batholith: Located several kilometers below the earth's surface, the batholith is a large body of igneous rock formed beneath the Earth's surface by the intrusion and solidification of magma.
Dyke: Formed when magma passes through cracks or Joints that cut across beds (layers) of rock underneath the Earth's surface. Dykes often occur in groups called swarms.
Laccolith: This is a mass of igneous rock, typically lens-shaped, that has been intruded/formed between two layers of sedimentary rock causing uplift in the shape of a dome that is visible on the Earth's surface.
Sill: A flat piece of rock formed by the hardening of magma in within the crack of a volcano or when magma forces its way between beds of rock and solidifies.

Extrinsic Features

Diagram showing Extrinsic Volcanic Features

Volcano - The most obvious extrinsic feature.
Craters - Bowl-shaped depressions at the top of a volcano where volcanic materials like ash, lava, and rock fragments are ejected.
Calderas - Large collapse features that can be miles in diameter and form when the magma chamber is partially emptied during a large eruption. (Please Note: If the magma chamber is completely emptied, it can eventually form the origin of a lake.)
Volcanic cones - Form when magma rises from the mantle and erupts through a weakness in the crust. There are two types of cones: Dome Cones (which are associated with Composite Volcanoes and Acidic Lava) and Shield Cones (associated with shield volcanoes and basic lava).
Lava plateau - In places like Iceland on the Mid-Atlantic Ridge, Shield volcanoes will sometimes have areas where fluid (basic) lava spreads out from linear cracks to cover large areas of the ground surface. These eruptions are called fissure eruptions.

Effects of volcanoes

Volcanoes can have both positive and negatives effects. Whilst the positives do not exactly outweigh the negatives, it should be noted that the positive effects assist with some things in regards to the aftermath of volcanic activity.

Positive Effects

Geothermal energy is where heat from within the Earth is used to generate electricity. Geothermal energy can be generated in areas where magma lies close to the surface. This is good for increasing our renewable energy use.
Ash ejected by the volcano acts as a good fertilizer for soils. Hence why some people would explain that volcanic soil is some of the best for growing crops. The ash is rich with nutrients from underneath the Earth's surface.
Volcanoes attract many tourists, who enjoy the dramatic scenery that they produce.
Volcanoes can also build the landscape. As the magma reaches to the sea, it cools and begins to expand the landscape.

Negative Effects

Volcanoes are dangerous. They can kill people and damage property. This is due to their ability to inflict massive damages. Just look at Plymouth, Montserrat for an example.
Economic activity can suffer as it is hard for businesses to operate after an eruption. Farmers who grew crops and raised livestock close to the volcano will also loss their investment as the volcano will burn up all of the arable land.
Habitats and landscapes are damaged by lava flows.
Mass evacuation of people leaving behind their home and disruption to their livelihoods.
Extreme health issues can arise that include:
- - Suffocation
  - Infectious diseases like conjunctivitis
  - Respiratory diseases from breathing in ash, fumes, and gases
  - Burns and traumatic injuries from falling rock
  - Eye and skin irritations from acid rain

Image taken from BBC. Source: https://www.bbc.co.uk/bitesize/guides/z8p9j6f/revision/3

Throughout history, we in the Eastern Caribbean have suffered immensely from Volcanic Eruptions. Whilst uncommon, Volcanic Eruptions still cause hundreds of thousands of dollars in damages to Caribbean Islands. Just look at the table below.

Actual volcanic disasters in the Eastern Caribbean over the past 300 years

The above table was taken from the UWI Seismic Research Centre. Source: https://uwiseismic.com/volcanoes/caribbean-volcanoes/

Pay very close attention to the information in the table. It will become helpful when understanding Case Studies related to Volcano Eruptions.

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