Lava Delta Ponta dos Biscoitos near Santa Cruz das Ribeiras

Lava deltas, similar to

river delta A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rare ...

s, form wherever sufficient

sub-aerial In natural science, subaerial (literally "under the air"), has been used since 1833,Subaerial
in the Merriam ...

flows of

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...

enter standing bodies of water. The lava cools and breaks up as it encounters the water, with the resulting fragments filling in the adjacent

seabed topography The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

such that the flow can move further offshore sub-aerially. Lava deltas are generally associated with large-scale, effusive type

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

Occurrence

Lava deltas are found mainly associated with volcanic islands, particularly those formed at

hotspots Hotspot, Hot Spot or Hot spot may refer to: Places * Hot Spot, Kentucky, a community in the United States Arts, entertainment, and media Fictional entities * Hot Spot (comics), a name for the DC Comics character Isaiah Crockett * Hot Spot (Tra ...

as they produce the necessary effusive basaltic flows. The largest lava delta systems known are associated with formation of volcanic type

passive margin A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting cre ...

s. Just prior to break-up along the northern Atlantic in the late

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pala ...

, massive eruptions occurred along the eventual line of break-up. This volcanism, part of the North Atlantic Igneous Province, led to the formation of two extensive lava escarpments, interpreted as deltas, extending from the

Faeroes The Faroe Islands ( ), or simply the Faroes ( fo, Føroyar ; da, Færøerne ), are a North Atlantic island group and an autonomous territory of the Kingdom of Denmark. They are located north-northwest of Scotland, and about halfway betwee ...

onto the More Margin (the Faeroe-Shetland escarpment) and the Vøring escarpment on the Vøring margin, a combined distance of approximately . As these deltas were prograding into water of relatively constant depth, they were able to extend as much as from their original vents.

Formation

When a sub-aerial lava flow reaches the ocean (or other large body of water), contact with the water causes both rapid cooling of the lava and steam explosions that fragment it. The glassy fragments that are formed, known as hyaloclastites, fall down to the seabed forming foresets. As the seabed topography becomes infilled, the subaerial flow is able to build out. The process continues as long as the lava supply is maintained, creating a lava bench. A lava bench is a volcanic landform with a horizontal surface raised above the level of the surrounding area. The

Hawaiian islands The Hawaiian Islands ( haw, Nā Mokupuni o Hawai‘i) are an archipelago of eight major islands, several atolls, and numerous smaller islets in the North Pacific Ocean, extending some from the island of Hawaii in the south to northernmost ...

are an example of land that was formed this way, and the Big Island is currently still expanding due to lava benches. The Kilauea Volcano releases lava that flows down the slope of the volcano and eventually encounters the ocean; this lava flow hardens when it comes into contact with the significantly cooler water of the ocean and forms an unstable lava bench. Eventually, when the material beneath the lava bench stabilizes, it becomes stable land that has been added to the island. Most lava deltas are formed by relatively low

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...

pāhoehoe Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

type flows and the lava reaches the sea via a system of small

lava tube A lava tube, or pyroduct, is a natural conduit formed by flowing lava from a volcanic vent that moves beneath the hardened surface of a lava flow. If lava in the tube empties, it will leave a cave. Formation A lava tube is a type of lava ...

s, their entry into the water being marked by a series of steam plumes.

Hazards

Collapse of the frontal part of lava deltas is common during their formation, representing a hazard for any people that are watching from a solidified part of the delta and such areas are normally marked as dangerous. If a newly formed lava bench rests on

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

s, it may pose hazards due to its extremely unstable structure. Oftentimes, these benches are so unstable that they collapse into the sea, exposing the water to the hot lava on the interior of the bench and releasing acres of land into the ocean. When the hot lava hits the water, violent explosions of steam can shoot large rocks and molten lava up to inland. These collapses are extremely dangerous because they can happen without warning, and anyone or anything on the bench will be caught in the collapse. For safety, people are advised not to walk on lava benches because of their unstable nature, and they must maintain a safe distance from the lava bench.Advertiser, Final
Big Island gets smaller as lava bench breaks off
Honolulu Advertiser, The (HI) 04 Aug. 2006: NewsBank. Web. 26 Oct. 2016. People standing too close to the edge of an active delta are not only at risk from being thrown in the water but from the steam explosions and the accompanying acidic clouds ('' laze'') that result from the collapse as seawater comes into contact with the active lava tube system.

Uses

On steep-sided volcanic islands, lava deltas make attractive sites for building and many villages and towns are located on old lava deltas, such as Garachico on

Tenerife Tenerife (; ; formerly spelled ''Teneriffe'') is the largest and most populous island of the Canary Islands. It is home to 43% of the total population of the Archipelago, archipelago. With a land area of and a population of 978,100 inhabitant ...

References

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External links