We review the most important types of volcanic hazards that have occurred in Nicaragua during the past ∼40,000 yr and that are expected to occur in the future. Population density within the potential hazard area is clearly essential in defining and understanding volcanic hazard and risk. There are three main groups of volcanic events that pose major hazards: Group 1 comprises several types of explosive volcanic eruptions that impact society (people and infrastructure) directly. The most hazardous types are pyroclastic surges, particularly those generated by water-magma interaction, pyroclastic fallout, and pyroclastic flows, as well as tsunamis generated by volcanic eruptions within and close to Nicaragua's large lakes. Group 2 includes nonexplosive volcanic activity such as lava flows and the permanent or episodic emission of volcanic gases from open vents. Group 3 comprises chiefly lahars generated by mixing of volcanic debris with water and volcano flank collapses (landslides) sometimes unrelated to synchronous volcanic eruptions but being conditioned chiefly by the stability of a volcanic edifice. We discuss the present database on the age and type of the most recent eruptions emphasizing those that potentially pose major hazards to the populated areas. These include volcanogenic tsunamis in Lake Managua and Lake Nicaragua, scoria cone and maar formation chiefly in the western part of Managua, and major explosive eruptions of Chiltepe and Masaya volcanoes, a large eruption from Masaya volcano having devastated the entire area of present Managua only ∼2000 yr ago. We discuss the most important techniques for monitoring volcanoes to detect unrest and predict the time and magnitude of upcoming eruptions, emphasizing techniques presently employed in Nicaragua. Finally, we address the subjects of risk assessment, including hazard and risk maps, and the importance of long-term development plans to reduce vulnerability.

Abstract The mid-American channel connecting the primeval Atlantic and Pacific oceans was subjected to forces in Upper Jurassic time that folded the sea bed into a series of parallel ridges striking SE to NW. The westward and most tenuous of the ridges was ruptured by extrusive material that grew from the channel floor and emerged to form a chain of volcanic islands—the western archipelago. Erosion of the islands and deposition to the northeast provided the sediments of the Nicoya complex now exposed along the west coast of Costa Rica and Panamá. Volcanic eruption and continuing erosion throughout the Cretaceous supplied sediment to the shallowing channel area. Deposition during this period was mainly from the archipelago, although some material was derived from the northern Nuclear Central American mass. By the end of Cretaceous most of the denuded islands had foundered and the western archipelago had disappeared. Diastrophism accompanying the Laramide revolution rejuvenated and further upfolded one of the interior ridges—the “Guanarivas island.” Eastern Panama, belonging to the Choco borderland, which had been emergent throughout the Cretaceous, began to founder in lower Eocene and was submerged by the beginning of the middle Eocene. Guanarivas island and the volcanic islands had disappeared by lower Oligocene time, which was an epoch of comparative quiescence. Renewed activity in the middle Oligocene resulted in the growth of the Talamanca ridge and the appearance of islands in southern Costa Rica and northeastern Panama. Continued growth through early Miocene culminated in development of the West Talamanca fault and total emergence of the ridge by the end of the middle Miocene. The faulted upthrust block was tilted eastward, creating compressive forces that fractured the eastern front of the high area and initiated folding on the Atlantic foreland of southern Costa Rica and northeastern Panama. The Miocene diastrophism was accompanied by the growth of volcanoes on the ridge in Panamá. Total emergence of a narrow strip of land bordered by the Pacific Ocean and the Nicaraguan depression opening to the Caribbean, resulted in the first uninterrupted connection of South America with Nuclear Central America in Pliocene time. By Quaternary time the Talamanca ridge had become stabilized and adjusted, the Nicaraguan depression was filled, leaving only Lakes Nicaragua and Managua and the San Juan River to mark its former course, and the orogen had assumed the shape we know today.