Types of Volcanoes and Eruptions

Types of Volcanoes

Volcanoes can be classified by type. The style of eruption, structure and composition determine a volcano’s type.

• Shield volcanoesShield volcanoes, such as Ambrym and Ambae, are characterized by a broad, flat and convex shape with a summit calderas. The vents are mostly localized at the summit and on the rift zones. Eruption frequency is variable, decade to century. The eruptions consist of fluid lava flows, weak fountaining explosions, summit explosions and ash fall.
• Caldera volcanoesCaldera volcanoes, such as Ambrym, Ambae, Gaua and Yasur are characterized by a central or summit location with the predictable eruption location and often have a small volcanoes cone and lakes inside them. The caldera forming are from very large eruptions. Eruptions are rare. Eruption frequency is from 1000’s years.
• Cone volcanoesCone volcanoes, such as Lopevi, Gaua, Ambrym and Yasur are characterized by a central or summit vent with the predictable eruption location, steep-sided cone shape and multiple vents can build small volcano complexes over extended area. Eruption frequency is between 10’s – 100’s years. Eruptions are explosive often powerful and consist of lava domes, lava flows, mud flows, floods and infrequent major collapses.
• Scoria cones and MaarsScoria cones and Maars (mostly along rifts) are characterized by numerous small volcanoes over extended area with random eruption location such as present in Vanuatu along the rifts of  Ambrym and Ambae. There are variety of eruption style, dry creates cones and wet creates maars. Eruption frequency 100’s to 1000’s years.

•  Strombolian and Hawaiian eruptions

• Vulcanian eruptions

• Sub Plinian and Plinian eruptions

• Phreatic eruptions

• Phreatomagmatic eruptions

Volcanic Hazards

Volcanic hazards are defined as any potentially dangerous volcanic process that puts human life, livelihoods and/or infrastructure at risk of harm in a given area within a period of time.

Numerous types of volcanic hazards may result from an eruption. The type and intensity of a given hazard will vary from volcano to volcano.

Hazards may also vary from eruption to eruption as well as across the sequence of a single eruption.

These hazards can have severe impacts on people, infrastructure, and the environment. Below is a list of common volcano hazards:

Ballistic or bomb

Lahars or mudflow

Lahars are “mudflows”, mixtures of volcanic ash, blocks and water, formed on volcanoes. The source of a lahar maybe a crater lake, a dam collapse or heavy rainfall washing from the slope of a volcano. People caught in the path of a lahar have a high risk of death from severe injuries. Lahars are extremely destructive causing destruction of buildings, installations and vegetation caught in their path. People should remain at a safe distance.

Hot ash flow or pyroclastic flow

If a large volume of volcanic debris is erupted quickly from a volcano, the eruption column can collapse. As the eruption column collapse it can transform into an outwardly expanding flood of hot solid ejecta in a fluidizing gas cloud, knowing as a pyroclastic flow, or surge. These flows are a common devastating result of certain explosive eruptions; they normally touch the ground and hurtle downhill, or spread laterally under gravity. People caught in the direct path of a flow or surge are most unlikely to survive. The best protection is to evacuate prior to the event.

Volcanic gases

Volcanic gases predominately consist of steam and followed by other gases (carbon dioxide, Sulphur dioxide and chlorine compounds). Hazardous concentrations of gases are present only very close to the crater (within 1 -2 km). Away from the vent gases pose no more than an irritant or nuisance.

Ash fall

When a volcano erupts, it will eject a wide variety of material into the air. The fine material (millimeter-sized ash), which is derived from the glass, rock and crystal particles, can be  carried by currents in the eruption column above the volcano and pass the downwind plume to rain out forming ash deposits. The impacts of ash fall on people, structures and equipment’s depend largely on ash thickness. For more information regarding the ” Volcanic Ash Impacts & Mitigation “, click here.

Lava

Lava flows are streams of molten rock that will travel down valleys on the slopes of volcanoes. The distance they travel depends on the viscosity of the lava, output rates, volume erupted, steepness of the slope, topography and obstructions in the flow path. Lava flows will seldom threaten human life because of their slow rate of movement. However, the steep fronts of flows may became unstable and can collapse, causing small pyroclastic flows. Lava flows will causes total destruction of buildings and other infrastructure in their path.

Tsunami

Tsunamis are seismic sea waves of long period caused by disturbances on the sea floor. Volcanic tsunami may be produced by landslides or debris avalanches flowing into the sea around an island volcano such as Lopevi.

Many of these phenomena will only affect an area very close to the volcano. However, volcanic ash fall can be deposited hundred to thousands of kilometers from its source, making it the product most likely to affect the largest area and the greatest number of people.

Monitoring Methods

Vanuatu Geohazards Division monitor a range of Vanuatu volcanoes. Various agencies are notified when the Volcanic Alert Level changes for Vanuatu’s active volcanoes.

Activity is monitored using the technique of Volcanic Surveillance which is based on the assumption that movement of molten rock or magma beneath a volcano will occur before any eruption can start and this movement of magma is detectable using various methods. Volcanologist use many techniques to monitor an active volcano. Monitoring data collected from different technologies are  continuously analyzed and interpreted to help give an understanding of the behavior of the volcanoes and an insight to future eruptions. Some of the main techniques use by the Vanuatu Geohazards Division are :

1. Seismic Monitoring – Almost all volcanoes in the world have some kind of seismic monitoring system and it is usually the first technique applied when scientist begin to monitor a volcano. Image below shows the typical seismic shed in Vanuatu, seismometer (seismograph) are usually buried and attached to bedrock and the waveforms data produced by the seismometer.
   
2. Visual and cameras – The safest and cheapest way of monitoring a volcano is just to look at it. Images below shows the webcam used in Vanuatu and its corresponding images of the volcano.
   
   
3. Satellite images – MOUNTS satellite image gives daily flux of Sulphur dioxide (S02) and MIROVA gives a near-real-time thermal monitoring of global volcanism using MODIS instrument. In addition, true color imagery is also used to monitor significant observable changes in and around the volcano using CORPENICUS BROWSER.
   
4. Geochemistry – This technique is in its early stages and it requires field measurements of gas emission, sample collection and lab analyses to understand the behavior of a volcano and it complements the seismic and other monitoring techniques.

Volcanic Alert Level System

In Vanuatu, we use a system of Volcanic Alert Level to define the current status of each volcano. The alert levels range from 0 to 5. The alert levels are used to guide any appropriate response.