Question

How does a composite cone form?

Short answer

Composite cones form through alternating explosive and effusive eruptions, leading to layered deposits of lava and ash.

Step-by-step solution

Understanding Volcanic Eruptions

A composite cone, also known as a stratovolcano, forms through a series of eruptions over time. These typically consist of alternating layers of lava flows, volcanic ash, and other volcanic debris. Stratovolcanoes are characterized by their steep, conical shapes.

Eruption Sequence

Composite cones form when eruptions alternate between explosive and effusive activity. Explosive eruptions produce ash and pyroclastic material, which settle around the volcano's vent. Effusive eruptions produce lava flows that layer over the ash, building up the structure.

Layer Accumulation

With each eruption, layers of lava, ash, and tephra accumulate. The repeated sequence of eruptions over centuries to millennia leads to the build-up of the characteristic stratified structure of composite cones, hence the name stratovolcano.

Influence of Magma Composition

The magma feeding composite cones is typically more viscous than that of shield volcanoes, often due to higher silica content. This results in more explosive eruptions and promotes the development of the steep-sided cone.

Key concepts

Stratovolcano

Stratovolcanoes, also known as composite cones, are majestic natural structures that dominate many landscapes. They are recognizable by their classic steep, conical shapes which are quite different from the gentle slopes of shield volcanoes.
This form is a result of their unique composition, which is made from alternating layers of hardened lava and volcanic debris like ash and tephra. Over thousands or even millions of years, these layers build up to create the iconic stratified structure of stratovolcanoes.
Prominent examples include Mount Fuji in Japan, Mount St. Helens in the USA, and Mount Vesuvius in Italy. These volcanoes are famous not just for their beauty, but also for the dramatic eruptions they can produce.

Volcanic Eruptions

Volcanic eruptions are powerful geological events that shape our planet’s surface. Eruptions can be categorized broadly into two types: explosive and effusive. Stratovolcanoes typically experience both types, which contributes to their layered structure.
Explosive eruptions happen when thick, viscous magma traps gases until the pressure becomes too high, leading to a violent release of gas and volcanic material, including ash. This can blanket the surrounding area in layers of debris. Effusive eruptions, on the other hand, allow lava to flow out more gently and steadily, covering previous deposits of ash with new layers of rock.

• Explosive eruptions create dramatic impacts on the surrounding environment and have the potential to affect global climate.
• Effusive eruptions add more lava layers which increase the size and height of the volcano.

Both eruption types are essential for the development and maintenance of a composite cone's structure.

Magma Composition

The composition of magma plays a crucial role in the character of volcanic eruptions and the formation of stratovolcanoes. Magma is a mixture of molten rock, minerals, and volatiles such as gases.

Stratovolcanoes are typically fed by magma that is intermediate to high in silica content. High silica content makes the magma more viscous, meaning it is thicker and less fluid. This leads to:

• More explosive eruptions due to the trapped gases within the magma.
• The ability to build up steep-sided cones due to less flowability, allowing it to pile up around the vent.

The viscosity of the magma influences how the eruptions unfold, where higher viscosity might trap gases, leading to more intense explosions.

Lava Flows

Lava flows are molten rock streams that play a vital role in the formation of composite cones. When a stratovolcano erupts, lava flows can spill out of the vent and down the sides of the mountain.
In stratovolcanoes, these flows are often andesitic, meaning they are of a composition that is neither too thick nor too fluid. This aids in building the steep slopes characteristic of these volcanoes.
Lava flows help to solidify and build up the structure, forming new layers over ash and debris from explosive eruptions. Over time, these layering processes can create the towering structures seen in stratovolcanoes across the world.

Pyroclastic Material

Pyroclastic material is an assortment of volcanic debris that erupts explosively out of a volcano during an explosive eruption. This material includes ash, pumice, and volcanic bombs.
Pyroclastic flows, which are a mix of hot gases and volcanic matter, can travel at rapid speeds down the slopes of a stratovolcano, spreading ash and debris across large areas. This process is integral to the formation of stratovolcanoes and is responsible for:

• Creating thick deposits of ash and debris that contribute to the volcano's structure.
• Shaping the surrounding landscape with layers that can harden into rock over time.

The accumulation of pyroclastic materials is crucial in defining the stratified nature of these impressive volcanic formations.