The volcano erupted numerous times in the late Pleistocene[a] and Holocene,[b] forming widespread tephra deposits both in the proximity of Hudson and in the wider region.

During the 1991 eruption, volcanic ash covered a large area in Chile and neighbouring Argentina, causing high mortality in farm animals, aggravating an existing economic crisis, and reaching as far as Antarctica.

[16] Cinder and spatter cones reach heights of 200 to 300 metres (660 to 980 ft) and are sources of lava flows outside of the caldera, especially in the Sorpresa Sur valley.

[42] Older volcanism in the region includes back-arc volcanoes in Patagonia and adakitic rocks in the Taitao Peninsula that were emplaced during the last 4 million years.

[43] Hudson rises from the Patagonian Batholith, a 1,000-kilometre-long (600 mi) formation made up of intrusive rocks (diorite, gabbro, granite, granodiorite and tonalite[23]) that were variously emplaced during the Cretaceous[g]-Miocene.

[45] In the Hudson area, the LOFZ is formed by two branches connected through perpendicular faults[40] and lies 30 kilometres (19 mi) west of the volcano.

[49][50][51] Rocks contain only a few phenocrysts,[26] including andesine, apatite, clinopyroxene, ilmenite, oligoclase, olivine, orthopyroxene, plagioclase and titanomagnetite.

[56] The three major Holocene eruptions produced uniform magmas with temperatures of 943 to 972 °C (1,729 to 1,782 °F), a few percent water by weight and a trachyandesitic to trachydacitic composition.

[11] The northeastern sector of the volcano is older than the southeastern, which has yielded ages of 120,000–100,000 years,[68] but the incomplete stratigraphy of the edifice, which is largely covered with ice, precludes establishing a proper history of its growth.

[73] The deglaciation that began 17,900 years ago[4] may have enhanced volcanic activity;[74] the largest eruptions of Hudson, Llaima and Villarrica took place at that time.

[26] Pre-caldera outcrops are rare and consist of breccias formed by hyaloclastite, lahars[j], mafic lavas and pyroclastic rocks; they occur mostly on the northeastern and southern sides of the caldera.

[100] Tephra layers from 1035 AD[101] and 9,216 BC in the Siple Dome of Antarctica have been attributed to Hudson, but for the older eruption there is no evidence in South America of an appropriately sized event.

[20] It is the largest known eruption of Hudson, yielding more than 20 cubic kilometres (4.8 cu mi)[m] of tephra, and may have initiated the growth of the caldera.

[77] It produced about 18 cubic kilometres (4.3 cu mi) of trachydacitic or trachyandesitic rocks,[26][115][55] thus reaching a volcanic explosivity index of 6.

[113] It has been recovered from wind-transported sediments,[122] lakes like Lago Cardiel and Laguna Potrok Aike, peat bogs including at Puerto del Hambre and Punta Arenas, and archaeological sites.

[77] The wide dispersal of the ash was either due to the eruption column exceeding 55 kilometres (34 mi) height or to strong winds.

[132] More controversially,[133] the eruption may have caused a cessation of the southern Patagonian obsidian trade,[134][135] and a shift towards the use of coastal resources by people in Patagonia.

[142] After the eruption, activities at the Túnel 1 archaeological site changed from a terrestrial lifestyle to one that relied on coastal food sources[143] which were less vulnerable to volcanic impacts.

[157] Initially, basaltic magma rose in the edifice and entered a trachyandesitic reservoir at 2 to 3 kilometres (1.2 to 1.9 mi) depth, until the stresses opened up another pathway along local-scale fractures.

[168] More than 4 cubic kilometres (1 cu mi) of tephra fell along two axes: A narrow northern one and a much wider and longer east-southeast trending axis from the volcano in southern Patagonia and the South Atlantic Ocean.

[169] Ash fell over an area of about 150,000 square kilometres (58,000 sq mi) in Chile and Argentina,[25] reaching as far as the Falkland Islands and South Georgia.

[170] The ash fall buried vegetation and roads, caused house roofs to collapse and contaminated water supplies.

[171][168] Ailments[p] caused by the ash and preceding harsh winter killed about half of all grazing animals in the directly affected areas such as Argentina's Santa Cruz Province,[173] where damage exceeded 10,000,000 dollars.

[175] Winds transported the plume towards Antarctica and in the westerlies surrounding the polar vortex, circling the continent in a month[176] and reaching Chile again after a week.

[177] However, the Hudson cloud led to substantial ozone loss over Antarctica and had comparable effects in the southern hemisphere to the Pinatubo eruption.

[10] An eruption column rose 5 to 12 kilometres (3 to 7 mi) above the volcano and deposited tephra to the east into the South Atlantic Ocean.

[183] The lahar dragged blocks of ice along,[188] swept the valley clear of trees and produced a pumice raft in the sea off the mouth of the Huemules River.

[156] Presently, shallow seismicity takes place under Hudson and south of it, between 0 to 10 kilometres (0 to 6 mi) underground, and is probably related to volcanic activity.

[9] Despite the low population density in the regions of Argentina downwind of Hudson, ash fall could cause serious impacts on farming and animal husbandry.

Mudflows caused by melting of ice or erosion of tephra and pyroclastic deposits have occurred in the Huemules and Ibáñez valleys.