Hilima Eruption
The Hilima Eruption, also referred to as the Great Median Eruption, was a catastrophic volcanic eruption estimated to have occurred in 855 BCE on Hilima, an island in the southern Medius Sea, the remnants of which form the Gateway Islands group, currently overseas possessions partitioned between Tvåriken and Vernland.
The eruption of the stratovolcano Kalmisana (the name translates from the Antissan as “thunderbolt”) was the largest volcanic event in Arden’s recorded history, with a Volcanic Explosivity Index (VEI) of 7 and dense-rock equivalent (DRE) of 100+ cubic kilometres. The eruption devastated the island, including the settlements on it, with effects spreading to the communities and agricultural areas on the northern coast of Lestria, the southern coast of Messenia and the eastern coast of Ascesia.
The event
Geological evidence shows that the Kalmisana volcano erupted several times over a period of some 300,000 years before the Hilima eruption. In a repeating process, the volcano would violently erupt, then eventually collapse into a roughly circular caldera filled with sea water, with numerous small islands forming the circle. The caldera would slowly refill with magma, building a new volcano, which erupted and then collapsed in an ongoing cyclical process. Before the eruption, the volcano is estimated to have risen between 2500 and 4000 metres above sea level.
On Megney, today the main island of the Gateways, there is a layer of white tephra some 60 metres (200 feet) deep that overlies the soil, clearly delineating the ground level prior to the eruption. This layer has three distinct bands, indicating the different phases of the eruption. The thinness of the first ash layer, along with the lack of noticeable erosion of that layer by seasonal rains before the next layer was deposited, indicate that the volcano gave the local population a few months’ warning. It is also suggested that several months before the eruption, Hilima experienced one or more earthquakes, which damaged the local settlements.
Intense magmatic activity of the first major phase of the eruption deposited up to seven metres (23 feet) of pumice and ash, with a minor lithic component, to the east and south-east. Archaeological evidence indicated burial of man-made structures with limited damage. The second and third eruption phases involved pyroclastic flow activity and the possible generation of tidal waves. The third phase was also characterized by the initiation of caldera collapse. The fourth, and last, major phase was marked by varied activity: lithic-rich base surge deposits, lahars, debris flows, and co-ignimbrite ash-fall deposits. This phase was characterized by the completion of the caldera’s collapse, which produced a tidal wave without precedent in known history. The explosion fragmented the island of Hilima into two smaller islands, today known as Megney and Lítiltey.
Aftermath
The eruption resulted in a plume of ash and airborne debris some 30 to 35 kilometres (19-22 miles) in height, extending into the stratosphere. In addition, the magma underlying the volcano came into contact with the shallow marine embayment, resulting in a violent steam eruption. This resulted in the formation of large ash deposits as far afield as the Lestrian coast, 300-400 kilometres further to the south-east. Ash layers in cores drilled from the seabed and from lakes in Neyet show that the heaviest ashfall was eastward and south-eastward from Hilima.
The event also generated a tidal wave between 35 and 150 metres (115-490 feet) in height, which devastated the coasts of three continents and obliterated most settlements in the area, including the major population centre of Antos, in what is today southern Siurskeyti. On the island of Källarey, forty kilometres to the north-east of the eruption, ash layers three metres (ten feet) deep have been found, as well as pumice layers on slopes 250 metres (820 feet) above sea level. Elsewhere in the Median basin there are pumice deposits which could have been caused by the Hilima eruption.
The eruption caused significant climatic changes in the region, although debate continues as to the extent to which the masses of detritus thrown into the upper atmosphere exacerbated, or served as a tipping point for, longer-term climatic trends. As well as recorded historical sources which describe massive floods and failing harvests, radiocarbon testing shows evidence of a significant climatic event in the northern hemisphere. The evidence includes crop failure over an area extending from present-day central Helminthasse in the west to Nation 37 and Nation 38 in the east, as well as from dendrochronology samples. The overall effect on the climate of Arden was unparalleled in nature, and unapproached by anything in subsequent history until the years without summers of the late 1950s and early 1960s.
Effects on history
The devastation wrought on the Messenian coastline caused widespread shifts in population as refugees fled inland, away from the zone of catastrophe. The stresses caused by these internal movements rocked the already-weakening Larhine Empire, setting in train a series of events that would bring about the fall of the empire as a definable entity within the next twenty years.
The climatic shifts referred to previously prompted significant cooling across the globe, with mean temperatures falling to historically low levels across the northern latitudes. While settled agricultural communities sought to weather the downturn as well as possible, the effects on wild animals hunted by the more nomadic peoples of the continent forced those nomads to move south to more hospitable regions. These shifts in population added to those arising in the immediate wake of the Hilima event so as to weaken, and in places topple, long-established communities and cultures across the Messenian and Joriscian landmass. The effects were felt particularly strongly in southern Messenia, where the influx of raiders from the northern plains shattered the settled pastoral states of the Prasinian peninsular region, but the eruption has been associated with social disruptions as far afield as Chotar in southeastern Outer Joriscia, being implicated in the ostensible fall of the Ukmai Empire (early Second Empire).
Although not as immediately, the effects of the Hilima event were also felt within the religious practices of the region. The effects on Palthachism, then the dominant faith of the Larhines and other peoples of southern Messenia, were particularly profound, as the faith began a process of splintering into smaller and more restricted churches, many of which varied from each other only in minor points of doctrine. This process would culminate, some 250 years later, in the birth of Palthachism’s successor, Siriash.