A lateral eruption or lateral blast is a

volcanic eruption Several types of volcanic eruptions—during which lava, tephra ( ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are of ...

which is directed laterally from a

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

rather than upwards from the

summit A summit is a point on a surface that is higher in elevation than all points immediately adjacent to it. The topographic terms acme, apex, peak (mountain peak), and zenith are synonymous. The term (mountain top) is generally used only for a m ...

. Lateral eruptions are caused by the outward expansion of flanks due to rising

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural s ...

. Breaking occurs at the flanks of volcanoes making it easier for magma to flow outward. As magma is pushed upward towards the volcano it diverges towards the flanks before it has a chance to erupt from the

crater Crater may refer to: Landforms *Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet *Explosion crater, a hole formed in the ground produced by an explosion near or below the surfac ...

. When the expanding flank finally gives it releases a flow of magma. More

explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An expl ...

lateral eruptions are referred to as lateral blasts. Some of the most notable examples of a lateral eruption include

Mount St. Helens Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United ...

Mount Pelée Mount Pelée or Mont Pelée ( ; french: Montagne Pelée, ; gcf, label=Antillean Creole, Montann Pèlé, meaning "bald mountain" or "peeled mountain") is an active volcano at the northern end of Martinique, an island and French overseas departme ...

, and

Mount Etna Mount Etna, or simply Etna ( it, Etna or ; scn, Muncibbeḍḍu or ; la, Aetna; grc, Αἴτνα and ), is an active stratovolcano on the east coast of Sicily, Italy, in the Metropolitan City of Catania, between the cities of Messina ...

St Helens map showing 1980 eruption deposits

Creation of a lateral eruption

Most eruptions are caused by the immediate decompression of a

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it upw ...

near the flanks of the volcano. During an eruption, the rapid decompression of magma may cause subsidence of the

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher th ...

, and if a flank collapses, a lateral eruption occurs. The directed nature of the blast may cause damage at much greater distances from the peak than a summit eruption would have.

Pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...

s and

lahar A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley. Lahars are extrem ...

s can affect areas originating from the volcano roughly in the shape of a cone that can span hundreds of square kilometers.

Examples

is a stratovolcano located in Washington,

USA The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territo ...

. Volcanic activity beginning in March 1980 saw magma accumulating underneath the mountain's north flank. On May 18, 1980, an earthquake triggered the collapse of the flank and a lateral eruption which killed 57 people. It was the deadliest volcanic event in US history.

Bezymianny Bezymianny (russian: Безымянный ''Bezymyannyyi'', meaning ''unnamed'') is an active stratovolcano in Kamchatka, Russia. Bezymianny volcano had been considered extinct until 1955. Activity started in 1955, culminating in a dramatic erup ...

is a stratovolcano located on the

Kamchatka peninsula The Kamchatka Peninsula (russian: полуостров Камчатка, Poluostrov Kamchatka, ) is a peninsula in the Russian Far East, with an area of about . The Pacific Ocean and the Sea of Okhotsk make up the peninsula's eastern and west ...

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eight ...

. On March 30, 1956, it erupted laterally after a flank collapse similar to that experienced by Mount St. Helens. No fatalities resulted from this eruption due to the remote location of the volcano. Subsequent lava dome growth has since filled the 1956

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber i ...

with a new cone.

is a volcano located on

Martinique Martinique ( , ; gcf, label=Martinican Creole, Matinik or ; Kalinago language, Kalinago: or ) is an island and an Overseas department and region, overseas department/region and single territorial collectivity of France. An integral part of ...

, in the

Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean S ...

. It underwent a lateral eruption on May 8, 1902, killing 28,000 people in the deadliest volcanic event of the 20th century.

References

{{Types of volcanic eruptions Volcanic eruption types