What are the three primary mechanisms that produce magma within the Earth?

Magma forms through three main processes: addition of heat, addition of volatiles, and decompression. Each mechanism generates specific magma types with distinct eruptive styles and volcanic structures. These different melting processes directly influence whether a volcano produces explosive or quiescent eruptions.

How does magma viscosity and volatile content affect eruption type?

Highly viscous magmas with high volatile contents produce the most explosive eruptions, while low viscosity and low volatile content magmas generate quiescent eruptions. Viscous, felsic magmas form from continental crust melting and create tall stratovolcanoes. Less viscous, mafic magmas from oceanic lithosphere produce shield volcanoes with free-flowing lava.

What is pyroclastic material and when does it form?

Pyroclastic material consists of magma, rock, and gas collectively ejected from a volcano during explosive eruptions. It forms when highly viscous magmas with significant volatile content undergo rapid decompression. This material travels at high velocity above the volcanic edifice, contrasting with slower lava flows from quiescent eruptions.

How do successive lava flows create volcanic layering?

Successive lava depositions build volcanic layers over time, demonstrating the principle of superposition where older layers lie beneath newer ones. Layers thin with distance from the magma source, and subsequent hot eruptions can partially melt underlying layers. This layering provides a geological record of past volcanic activity and regional history.

What is the difference between stratovolcanoes and shield volcanoes?

Stratovolcanoes form from highly viscous lava, creating tall, steep volcanic edifices with alternating layers of lava and pyroclastic material. Shield volcanoes develop from free-flowing, low-viscosity lava that travels farther before solidifying, resulting in short, low-profile structures. Magma composition and viscosity directly determine which volcano type forms.

How can volcanic rock composition inform geologists about eruption hazards?

Recognizing volcanic rock types in the field allows geologists to link them to specific eruptive styles, revealing threats to nearby communities. This information supports eruption emergency planning and targeted safety construction. Volcanic layering also provides insights into past eruption severity, helping with land use planning and assessing agricultural potential.

What does volcanic layering reveal about geological history?

Volcanic layers contain information about past climate, environment, and life, serving as easy-to-date time markers in geologic investigations. The principle of superposition shows older layers at the bottom with newer deposits stratified above. This layering provides a window into regional geological history and helps reconstruct past environmental conditions.

Igneous Volcanic Rock

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1. CO₂ Volcano

Fill a plastic container with a thin neck (a 16-oz. soda bottle for instance) about half-full with warm water.
Bury the bottle beneath modeling clay or dough, leaving just the neck (opening) of the bottle exposed, simulating the structure of a volcano.
Add a few drops of dishwashing liquid (in order to make the liquid frothy and likely to produce bubbles).
Using a folded piece of paper as a funnel, add 4 teaspoons (approximately 15-20 mL) of baking soda.
Gradually add red vinegar to the plastic container. If using a 16-oz. soda bottle, add 8-10 oz. of vinegar. Add the vinegar to the container until it begins to effervesce.
If desired, cork the container for a “violent eruption” or leave it uncorked for a more quiescent eruption.

2. Lava Layering

Warm paraffin on a hot plate so it becomes a viscous fluid.
Take a thin section of cardboard, and bend in to form bends and troughs of various shapes. Pour the liquid paraffin onto the inclined cardboard surface. As the paraffin flows over the uneven surface it will form a layer of varying thickness, as would be seen in a real lava flow.
After the paraffin has cooled and solidified, repeat the process two or three times, in order to simulate successive lava flows.

Volcanic rock is a specific type of igneous rock that is formed when magma breaches the surface and solidifies in the subaerial environment. Its study provides insights into past, and possibly future, volcanic activity.

Magma is molten rock, which is produced within the Earth and reaches temperatures from 800 to 1,200 °C. There are three primary mechanisms of magma production: addition of heat, addition of volatiles, or decompression. Each of these different types of melting produces specific types of magma, and therefore generate volcanoes with different eruptive styles and structure. This video will illustrate the differences between types of lava deposition on a small scale using paraffin wax, and different eruption types using a CO₂ based demonstration.

Highly viscous magmas with high volatile contents tend to produce the most explosive eruptions, compared to low viscosity and low volatile content magmas, which generally produce the most quiescent eruptions.

In quiescent eruptions, lava flows off the side of the volcano or outward from fissures. Lava flows are typically slow moving, and as such may cause property damage, but rarely loss of life. In contrast, more explosive reactions result in magma, rock, and gas, collectively known as "pyroclastic material", to be ejected from the volcano.

The type of mantle being melted, and the degree of melting, can both affect magma composition. The resultant magma formed will then affect the resulting volcano produced, and the eruption type observed.

Generally, viscous magma is more felsic in composition and forms as a result of melting of continental crust or continental lithosphere. In contrast, less viscous magma is typically mafic, and forms during melting of oceanic lithosphere or asthenopheric mantle melting. For more information on felsic and mafic rock, see this collection's other video on Igneous Rock.

Volcanoes are typically generated by successive depositions of lava over time. Highly viscous lava creates tall, steep edifices, known as stratovolcanoes. In contrast, free-flowing lava travels further before solidifying, creating short, low-profile structures known as shield volcanoes.

Now that we are familiar with the concepts behind magma production, deposition, and volcanic eruption, let's take a look at how these can be simulated in the laboratory.

The first procedure demonstrates quiescent and explosive eruptions. To begin, fill a plastic container with a thin neck to about half full with warm water. To simulate the structure of a volcano, bury the bottle beneath modeling clay or dough, leaving just the neck opening of the bottle exposed. Next, add roughly 4 teaspoons of baking soda.

Add vinegar to the bottle until it begins to effervesce. Including dye can aid with visibility. For a quiescent eruption leave the bottle open. If simulation of a violent eruption is desired, cork the bottle.

In the quiescent eruption, some of the material flowed outwards like a lava flow. The frothy nature of the flow is reminiscent of lava that is charged with volatiles.

Most volcanic eruptions are linked to volatile loss. Those that are particularly explosive will have considerable volatile emanations. In the corked container, the initial eruption involves pyroclastic-type material that is ejected into the air above the volcanic edifice. This also indicates what can happen in naturally blocked volcanoes.

The next demonstration relates to is lava layering. To demonstrate this, warm paraffin on a hot plate until it becomes a viscous fluid. Pour the liquid paraffin onto an inclined thin cardboard surface with bends of various shapes. This varied gradient simulates lava flow on the uneven surface of real volcanoes. As the paraffin flows over the uneven surface, it will form a layer of varying thickness, which simulates what would be seen on the surface of a real volcano. Allow the first paraffin layer to cool, then pour a second layer over the first, starting from the same point. Repeat this process several times to simulate successive lava flows.

Note how the layers thin with distance from the magma source. Also observe that subsequent hot layers or eruptions can partially melt underlying layers.

The layering demonstrates the principle of superposition. Older layers are found at the bottom, with deposits from more recent eruptions stratified above.

Additionally, the bent surface of the card simulates the uneven surface seen on most volcanoes. Different thicknesses of magma will collect on the steeper or shallower parts of the volcano surface, changing the landscape of the volcano with each successive eruption.

Understanding volcanic rock composition, formation, and the properties that lead to different eruption phenomena has vast applications for geologists and human populations as a whole.

Recognizing types of volcanic rock in the field and linking them to specific eruptive styles can inform geologists of the type of threats posed to nearby communities. This information can help with implementing eruption emergency plans, or with targeted safety construction or town planning.

Types of volcanic rock can also be studied to evaluate the severity or explosivity of past eruptions. This information can be helpful when planning land use. As volcanic deposition can also positively influence soil and agriculture, such areas may be economically fruitful if the risk of severe eruption is considered low.

Volcanic layering can be a window into the geological history of a region. Layers can contain information about past climate, environment, and life, and are easy to date, providing useful time markers in geologic investigations. Volcanoes can also create scenic landscapes, including the iconic Arthur's Seat, which overlooks the city of Edinburgh in Scotland. This is the largest remaining part of an extinct volcano that dates back to the Carboniferous period, and is designated a Site of Special Scientific Interest.

You've just watched JoVE's introduction to volcanic igneous rocks. You should now understand the different types of magma and their deposition, principles of quiescent and explosive eruptions, and how to simulate these in the laboratory or at home. Thanks for watching! Thanks for watching!