Laki (Icelandic pronunciation: [lac]) or Lakaggar [lakaciar] (Craters of Laki) is a volcanic fissure in the western part of Vatnajkull National Park, Iceland, not far from the volcanic fissure of Eldgj and the small village of Kirkjubjarklaustur. The fissure is properly referred to as Lakaggar, while Laki is a mountain that the fissure bisects. Lakaggar is part of a volcanic system centered on the volcano Grmsvtn and including the volcano rarhyrna. It lies between the glaciers of Mrdalsjkull and Vatnajkull, in an area of fissures that run in a southwest to northeast direction.

The system erupted violently over an eight-month period between June 1783 and February 1784 from the Laki fissure and the adjoining volcano Grmsvtn, pouring out an estimated 42 billion tonnes or 14 km3 (1810^9 cu yd) of basalt lava and clouds of poisonous hydrofluoric acid and sulfur dioxide compounds that contaminated the soil, leading to the death of over 50% of Iceland's livestock population, and the destruction of the vast majority of all crops. This led to a famine which then killed approximately a quarter of the island's human population.The Laki eruption and its aftermath caused a drop in global temperatures, as 120 million tonnes of sulfur dioxide was spewed into the Northern Hemisphere. This caused crop failures in Europe and may have caused droughts in North Africa and India.



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 often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series.

There are three different types of eruptions:

Magmatic eruptions are the most well-observed type of eruption. They involve the decompression of gas within magma that propels it forward.

Phreatic eruptions are driven by the superheating of steam via contact with magma. This type often exhibits no magmatic release, instead causing the granulation of existing rock.

Phreatomagmatic eruptions are driven by the compression of gas within magma, the direct opposite of the process powering magmatic activity.Within these wide-defining eruptive types are several subtypes. The weakest are Hawaiian and submarine, then Strombolian, followed by Vulcanian and Surtseyan. The stronger eruptive types are Pelean eruptions, followed by Plinian eruptions; the strongest eruptions are called Ultra-Plinian. Subglacial and phreatic eruptions are defined by their eruptive mechanism, and vary in strength. An important measure of eruptive strength is the Volcanic Explosivity Index (VEI), an order-of-magnitude scale, ranging from 0 to 8, that often correlates to eruptive types.