Istheusia [ɪs'θu:ʒə] (Arithide Istheussein [is'θu:si:n]; Dethric Isthausan [is'θo:sa:n]) is the easternmost continent of Ilethes, as well as the biggest, longest, and most varied in terms of its climate, physical features and cultural landscape. The continent constitutes 35% of the total land area of the planet, but, owing to large areas of desert and advanced demographic trends, is responsible for only 13.4% of world population and falling. The western "Head" of the continent is the site of one of the three cradles of civilisation of the world, and home to the Karyāba, the oldest extant civilisation known.
The English name Istheusia derives originally from the Dethric, but subsequently heavily influenced by the Arithide, owing to the greater wealth of information available about the continent in that language, due to centuries of contact. The Arithide name (which gave rise to the Dethric one) comes ultimately from the an-Kiryeb Istyusi [istjo'si], "homeland". Hindsight reveals the Areth choice of source name to have been uncannily accurate: mitochondrial DNA traces the origin of every race back to west-central Istheusia.
The continent sits on an eponymous tectonic plate, the largest known plate to date, atop a rather fast-moving convection current that propels it more than 5cm a year against the Marcasian and Maellorian plates, from which results the crustal upheaval that has eventuated in the Taza-a Lami in the continental Head, and in theKulyub Dur range on its southwestern coast. To the south the Arophanian plate pulls away from it, while the Sadon plate and the Naria plate subduct on its east to form the Abayan Trench.
Istheusian rock, and consequently the magma on which it floats, is predominantly felsic: i.e. of high silica and aluminium content, which renders the lava of its volcanos viscous and quick to solidify. While this geological coincidence has created some of the tallest and most majestic volcanic peaks in the world, the occasional violent eruption is much more devastating (see 1644 Eruption of Mount Kinaya).