[32] The eruption began with a phreatic (steam explosion powered) stage that deposited 3 centimetres (1.2 in) of ash over 400 square kilometres (150 sq mi) of northwest Lombok.

A magmatic stage followed, and lithic-rich pumice rained down, the fallout reaching a thickness of 8 centimetres (3.1 in) both upwind on East Lombok and on Bali.

[46] Considering the thickness of Samalas Tephra found at Mount Merapi, the total volume may have reached 32–39 cubic kilometres (7.7–9.4 cu mi).

[59] Such large volcanic eruptions can result in catastrophic impacts on humans and widespread loss of life both close to the volcano and at greater distances.

[90] The deposits showed that climate disturbances reported at that time were due to a volcanic event, the global spread indicating a tropical volcano as the cause.

[103] Sulfate deposition from the Samalas eruption has been noted at Svalbard,[104] and the fallout of sulfuric acid from the volcano may have directly affected peatlands in northern Sweden.

[110] Such sulfate aerosols in the case of the Samalas eruption may have remained at high concentrations for about three years according to findings in the Dome C ice core in Antarctica, although a smaller amount may have persisted for an additional time.

[124] The Samalas signal is only inconsistently reported from tree ring climate information,[125][126] and the temperature effects were likewise limited, probably because the large sulfate output altered the average size of particles and thus their radiative forcing.

[127] Climate modelling indicated that the Samalas eruption may have reduced global temperatures by approximately 2 °C (3.6 °F), a value largely not replicated by proxy data.

[146] One hypothesis is that the resulting increase in ultraviolet radiation on the surface of Earth may have led to widespread immunosuppression in human populations, explaining the onset of epidemics in the years following the eruption.

[191] There is little evidence that tree growth was influenced by cold in what is now the Western United States,[192] where the eruption may have interrupted a prolonged drought period.

[63] Very large volcanic eruptions can cause significant human hardship, including famine, away from the volcano due to their effect on climate.

The social effects are often reduced by the resilience of humans; thus there is often uncertainty about causal links between volcano-induced climate variations and societal changes at the same time.

[110] Western and central Indonesia at the time were divided into competing kingdoms that often built temple complexes with inscriptions documenting historical events.

During seven days, big earthquakes shook the Earth, stranded in Leneng, dragged by the boulder flows, People escaped and some of them climbed the hills.The city of Pamatan, capital of a kingdom on Lombok, was destroyed, and both disappeared from the historical record.

[207] The Babad Suwung reports destruction on the western coast of Sumbawa,[208] which was depopulated and remains so to this day; presumably the local populace viewed the area devastated by the eruption as "forbidden" and this memory persisted until recent times.

[177] Climate change triggered by the Samalas eruption and the beginning of the Little Ice Age may have led to people in Polynesia migrating southwestward in the 13th century.

[211] Reports from 1258 in France and England indicate a dry fog, giving the impression of a persistent cloud cover to contemporary observers.

[214][115] In Europe and the Middle East, changes in atmospheric colours, storms, cold, and severe weather were reported in 1258–1259,[134][215] with agricultural problems extending to North Africa.

[224] Iceland and Scandinavia were plagued by cold, bad shipping conditions at sea and lack of food in 1258-1261, prompting Haakon IV of Norway to take legal and political measures to maintain tax revenue and prevent the flight of peasants from rural areas,[225] and the union between Norway and Iceland that took place during that time may have been influenced by the crisis situation.

[94] Matthew Paris of St Albans described how until mid-August 1258, the weather alternated between cold and strong rain, causing high mortality.

[232] In Italy, bad weather including intense rains in 1258 caused crop failures throughout the peninsula, as documented by numerous chronicles,[233] although impacts varied between regions.

[235] The cities of Bologna and Siena in Italy attempted to manage the food crisis by buying and subsidizing grain, banning its export and limiting its price.

[238] Parma ordered the sale of grain and tasked officials with monitoring markets, including closing them on Saturdays,[239] and banned food exports.

[240] It is likely that the overthrow of the podestá (lord) of Parma Giberto da Gente [it] in 1259 was facilitated by the crisis, which induced his supporters to remain passive.

[252] The 1257 Samalas eruption took place during the Pueblo III Period in southwestern North America, during which the Mesa Verde region on the San Juan River was the site of the so-called cliff dwellings.

[219] Along with the Mongol invasions of Korea, hardship caused by the Samalas eruption may have precipitated the downfall of the Goryeo military regime and of its last Choe dictator, Ch'oe Ui.

[260] Monsoon anomalies triggered by the Samalas eruption may have also impacted Angkor Wat in present-day Cambodia, which suffered a population decline at that time.

[177] It may have altered the outcome of the Toluid Civil War[269] and shifted its centre of power towards the Chinese part dominated by Kublai Khan which was more adapted to cold winter conditions.

This warming may have provided the environmental conditions for the spread of Yersinia pestis, the causative agent of the plague,[272] although its diversification in the Tian Shan had begun earlier in the 13th century.