US Forest Service

Mount St. Helens

Mount St. Helens National Monument

Mount St. Helens
Pacific Northwest Research Station
1220 SW 3rd Ave.
Portland, OR 97204

(503) 808-2100

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Abundance: The total number of individuals of a taxon or of taxa in an area, volume, population, or community.

Adaptation: The process of adjustment of an individual organism to environmental stress.

Aerobic: Growing or occurring only in the presence of molecular oxygen.

Age class: A category comprising individuals of a given age group within a population; cohort.

Aggradation: The filling of a channel by sediment deposition.

Airfall (Ashfall): Volcanic ash and coarser material that has fallen through the air from an eruption cloud.

A deposit so formed is commonly well sorted and layered.

Anadromous: Migrating from salt to freshwater, as in the case of a fish moving from the sea into a river to spawn.

Anaerobic: Growing or occurring in the absence of molecular oxygen.

Andesite: A volcanic rock containing 53 to 63 percent silica (SiO2); has moderate viscosity in molten state.

Annual: Pertaining to a year; living for one year.

Arboreal: Living in trees; adapted for life in trees.

Ash: Fine (less than 0.1 inch diameter) particles of pulverized rock blown from an explosion vent. Measuring less than 0.1 inch in diameter, ash particles may be either solid or molten when first erupted. A common variety is vitric ash, glassy particles formed by gas bubbles bursting through liquid magma.

Ashfall: Volcanic ash and coarser material that has fallen through the air from an eruption cloud. A deposit so formed is commonly well sorted and layered.

Ashflow (see also pyroclastic flow): A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure. Ashflows are usually of high temperature and move rapidly over the land surface.

Assemblage: A group of species occurring together in the same geographic area.



Basalt: A volcanic rock containing less than 53 percent silica (SiO2); has low viscosity in the molten state.

Biodiversity: The variety of organisms considered at all levels, from genetic variants of a single species through arrays of species to arrays of genera, families, and still higher taxonomic levels; the totality of biological diversity.

Biomass: Any quantitative estimate of the total mass of organisms comprising all or part of a population or any other specified unit or within a given area at a given time; measured as volume, mass, or energy; standing crop; standing stock.

Biome: A biogeographical regional ecological community characterized by distinctive life forms and principal plant or animal species.

Biota: The total flora and fauna of a given area.

Biotic: Pertaining to life or living organisms; caused by, produced by, or comprising living organisms.

Blast, lateral: Rapid, lateral movement of a cloud of hot ash and coarser fragments across the landscape.

Blast area: The area subjected to the effects of a lateral blast, including the tree removal zone, the blowdown zone, and the scorch zone of standing, dead vegetation, where the hot blast cloud lacked the force necessary to topple trees, but hot gases scorched the foliage.

Blowdown: Toppling of forest vegetation by strong winds, in this case the shock wave and winds from explosive volcanic activity.

Blowdown zone: The area where forest was toppled by the lateral blast of Mount St. Helens on May 18, 1980.

Boulder: Sediment particle more than 10 inches in diameter.

Breccia: A coarse-grained rock composed of angular, broken rock fragments cemented in a fine-grained matrix.

Bryophyte: Mosses and liverworts.



Caldera: A large depression at the summit of a volcano created after a loss of structural support, most often after significant volumes of magma are evacuated from a volcano’s underground reservoir.

Climax: A more or less stable biotic community that is in equilibrium with existing environmental conditions and is the final stage of an ecological succession as long as the environment is unchanged.

Cobble: Sediment particle about 2.5 to 10 inches in diameter.

Colonization: The successful invasion of a new habitat by a species.

Community: Any number of organisms belonging to a number of different species that occur in the same habitat or area and interact in that area.

Competition: The simultaneous demand by two or more organisms or species for an essential common resource that is or potentially is in limited supply (exploitation competition); the detrimental interaction between two or more organisms or species seeking a common resource that is not limiting (interference competition).

Competitive exclusion: The exclusion of one species by another when they compete for a common resource that is in limited supply.

Connectivity: The interrelationships between different components of an ecosystem.

Corridor: A generally linear, more or less continuous connection between separate habitat patches of the same type.

Coverage: That part of a sampled area covered by a particular plant species or individual plant canopy; typically expressed as a percentage.

Crater: A steep-sided, usually circular or bow-shaped depression formed by either explosion or collapse of a volcanic vent.

Cross pollination: Transfer of pollen from one flower to the stigma of a flower on another plant of the same species.



Dacite: A volcanic rock containing 63 to 68 percent silica (SiO2); has high viscosity in the molten state.

Debris avalanche: Rapid flow of large, unsorted masses of rock material, possibly in combination with liquid water or snow and ice, that moves rapidly down slope under the influence of gravity. Collapse of a volcano’s flank can trigger massive debris avalanches.

Debris-avalanche deposit: An accumulation of poorly sorted, rock material emplaced by debris avalanches. These can form extensive areas of irregular terrain with hummocks (irregular mounds) up to tens of feet high and undrained depressions, which become ponds. Deposits may be up to several hundred feet thick.

Dispersal: Outward spreading of organisms from their point of origin; one-way movement of organisms from one home site to another; the outward extension of a species’ range, typically by a chance event.

Disturbance: “Any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment.” (White and Pickett 1985). Disturbances are an integral part of most ecological systems. Rather than being catastrophic agents of destruction, many disturbances are normal, perhaps even integral, parts of long-term system dynamics.

Dome: A steep-sided mass of highly viscous lava extruded from a volcanic vent. Typically circular from an aerial view, with rounded or flat top and a spiny surface.


Ecosystem: The level in the biological hierarchy of organization that includes the biological community and the physical environment in which it occurs.

Ejecta: Material that is thrown out by a volcano, including pyroclastic material and lava bombs.

Endemic: Native to and restricted to a particular geographical region.

Eruption cloud: A cloud of gas, ash, and larger rock fragments blown away from an eruption column by the wind.

Eruption column: The vertical column of gas, ash, and larger rock fragments rising from a crater or other vent during a volcanic eruption.

Eruption: The often-violent processes by which solid, liquid, and gaseous materials are ejected into the atmosphere or onto the Earth’s surface by volcanic activity.

Establishment: Growing and reproducing successfully in a given area.

Extirpation: Local extinction of a species, or the loss of some but not all populations of a species.


Facilitation: Enhancement of the behavior or performance of an organism or system by the presence or actions of other organisms.

Fauna: The entire animal life of a given region, habitat, or geological stratum.

Flora: The plant life of a given region, habitat, or geological stratum.

Flood plain: Land on either or both sides of a stream or river that is flooded when water levels are high.

Freshet: A great rise in, or sudden overflowing of a small stream usually caused by heavy rains or rapid snowmelt.


Gravel: Sediment particles from 0.2 to 1 inch in diameter.

Guild: A group of species having similar ecological resource requirements and foraging strategies and therefore having similar roles in the ecological community.


Habitat: The locality, site, and particular type of local environment occupied by an organism.

Herbivory: Feeding on plants.

Heterogeneity: Variation or diversity in the environment.

Hummock: A rounded, conical, steep-sided mound up to several tens of feet high. At Mount St. Helens, hummocks formed as a result of the avalanche-debris flow.



Indigenous: Native to a particular area.

Inhibition: Any process that acts to restrain reactions or behavior.



Keystone species: Organisms that disproportionately affect the organization of their communities.



Lahar: Indonesian term for volcanic mudflows (see “mudflow”). Lahars can have various types of origin, including hot tephra falling from an eruption column onto snow, melting and mixing with it to form a cool slurry flowing down a volcano’s flank.

Landscape: Area that includes a number of dissimilar land covers, such as forest stands of different types and ages, meadows, streams, and wetlands.

Lateral blast: A sideways explosion of materials from a volcano during an eruption.

Lava: Magma that has been extruded in liquid form onto the surface of the earth. Most commonly refers to streams of liquid rock that flow from a vent. Also refers to cooled, solidified rock of this origin.

Lava flow: A stream of liquid rock that flows from a vent. Also refers to cooled, solidified rock of this origin.

Legacy: Components of a system that persist through a disturbance. Legacies include living and dead organisms, and physical structures.

Legacy, biological: Organisms, propagules, and organically derived structures and patterns that remain after disturbances. Biological legacies include surviving plants, animals, seeds, spores of micro-organisms, and plant or animal organic matter, such as large woody debris and above- and belowground litter (such as roots) that affect soil properties.

Life form: Categories of plants that have similar positions of their dormant buds relative to the surface (examples are trees, shrubs, etc.).



Magma: Naturally occurring hot, mobile rock material, generated within the earth and capable of intrusion within the earth or extrusion onto the Earth’s surface.

Metamorphosis: A marked structural transformation during the development of an organism, often representing a change from larval stage to adult; an example is caterpillar to butterfly.

Metapopulation: A group of partially isolated populations of one species that are distributed across a landscape in a way that can influence the long-term persistence of the species. The number of populations present at any given time is governed by the spatial structure of suitable and unsuitable habitat, as well as a balance between local extirpation and colonization.

Microsite: Locations where physical conditions are suitable for the establishment of organisms. These are usually small areas that occur in a matrix of relatively inhospitable habitat.

Mudflow: Flow of water-saturated earth commonly moving rapidly and with a high degree of fluidity (see also “lahar”). Lahar is a more specific term for mudflows of volcanic origin.

Mudflow deposits: Accumulations of sediment transported by mudflows. Mudflow deposits are generally thin (up to 3 feet thick), covering flood-plain areas and vegetation.

Mycorrhizae: An association between the fine roots of plants and specialized soil fungi in which both organisms benefit. The fungi provide the plant with certain nutrients, such as phosphorous and water, and in return receive simple carbohydrates from the plant.



Neotropical: Designating or of the biogeographic realm that includes South America, the Indies, Central America, and tropical Mexico.

Nitrogen fixation: The conversion of nitrogen in the atmosphere to ammonia or other forms easily used by other organisms; the nitrogen fixation is carried out by micro-organisms.



Pebble: Sediment particle about 0.2 to 2.5 inches in diameter.

Perennial: Used in describing plants that persist for several years with growth each year.

Plankton: Passively floating organisms in a body of water.

Population: The level in the biological hierarchy that consists of groups of individuals of one species sharing a common gene pool and living in a geographical area.
Predation: The consumption of one animal (the prey) by another animal (the predator); also used to include the consumption of plants by animals, and the partial consumption of a large prey organism by a smaller predator.

Primary succession: An ecological succession starting on a substrate that does not support any organism or contain any biological remnants from previous life.

Propagule: Any part of a plant that, when separated from the parent plant, can give rise to a new individual. Seeds are the best-known propagules; in some plant species, root or shoot fragments can develop into new plants.

Pumice: Light-colored, low-density, frothy, glassy volcanic rock formed by expansion of gas in erupting lava. Fragments range from ash to large blocks.
Pyroclastic: Pertaining to fragmented (clastic) rock formed by volcanic explosion or ejection from a volcanic vent.

Pyroclastic flow: Rapid movement of extremely hot (often more than 1,200 °F), turbulent gases and fragmented rock across a land surface from a volcanic vent. The denser, lower part of a pyroclastic flow hugs the ground and follows topography, moving with great force and speed (up to 125 miles/hour). A lower density ash cloud extends well above the ground surface and moves more slowly.

Pyroclastic-flow deposits: Accumulations of sediment transported by pyroclastic flows, which commonly include a base of gravelly deposits topped by finer-grained, layered deposits from the ash cloud.



Reassembly, biological: The pattern and processes that occur in species accumulation and community reorganization after an ecological disturbance.

Redds: Sites where fish deposit eggs in sediment on a streambed.

Residuals: Surviving individual organisms, vegetative tissue that can regenerate, seeds, and components of the microbial and fungal soil community.

Rhyolite: A volcanic rock containing greater than 68 percent silica (SiO2); has very high viscosity in molten state.

Riparian zone: Pertaining to, living on, or situated on the banks of streams and lakes.



Safe sites: Places in an environmentally unfavorable landscape with physical conditions that favor the survival and establishment of organisms.

Sand: Sediment particles ranging from .00001 to 0.1 inch diameter.

Scorch zone: The area of standing, dead trees around the perimeter of the blast zone, where hot gases and rock fragments in the blast cloud killed foliage and caused other injuries to the trees, leading to tree death.

Secondary succession: The gradual changes that occur over time in biological and physical conditions after a site has been disturbed, where some legacies of earlier ecosystems remain.

Sere: The sequence of communities that replace each other over time during succession (as an adjective, “seral”).

Silt: Sediment particle less than .00001 inch in diameter.

Sink habitat: A habitat in which there is net immigration such that the population at the site can remain constant or perhaps even grow at a rate that may differ from the rate of immigration (see “source habitat”). Sink habitats are associated with source areas from which the organisms migrate.

Snowmelt: Runoff produced by the melting of snow.

Source habitat: A habitat that supports a net increase in a population (see “sink habitat”).

Species: The level in formal biological classification that consists of organisms capable of interbreeding.

Succession: The process of gradual replacement of one species population by another over time, and the concurrent change in ecosystem properties after a site has been disturbed. The concept can be extended to the replacement of one kind of community by another, the progressive changes in vegetation and animal life that may culminate in dominance by a community that is stable until the next disturbance.

Succession refers to changes that occur over 1 to 500 years and not to seasonal changes in populations and communities.



Taxon (plural taxa): Any of the formal categories in scientific classification of organisms.

Tephra: Fragmental rock material ejected from a volcano during an eruption and deposited by airfall. It is typically composed of ash (less than 0.2 inch in diameter), lapilli (0.2- to 1.3-inch particles), and blocks (angular stones greater than 1.3 inches).

Tolerance: The ability of an organism to endure extreme conditions. The range of environmental factors within which an organism or population can survive. Also, the situation where organisms best able to tolerate prevailing conditions are favored.

Trophic levels: The sequence of stages in a food chain or food pyramid, from producer to primary, secondary, or tertiary consumer.

Tuff: A compacted pyroclastic deposit of volcanic ash.


Vent: Opening in the earth from which volcanic material is emitted.


Wildlife: Animals and vegetation (but especially animals) living in a natural, undomesticated state.



Some definitions are derived from the sources listed above.

Dale, V.H.; Swanson, F.J.; Crisafulli, C.M., eds. 2005. Ecological response of Mount St. Helens after the 1980 eruption. New York: Springer-Verlag.

Munn, T.; Munn, R.E., eds. 2002. Encyclopedia of global environmental change. New York: John Wiley & Sons, Inc. 5 vol.

Foxworthy, B.L.; Hill, M. 1982. Volcanic eruption of 1980 at Mount St. Helens: the first 100 days. [Reston, VA]: U.S. Department of the Interior, Geological Survey. 125 p.

Hanson, H.C. 1962. Dictionary of ecology. New York: Philosophical Library. 382 p.

Korosec, M.A. 1980. The 1980 eruption of Mount St. Helens, Washington. Information circular. Olympia, WA: Washington Department of Natural Resources.

Lincoln, R.J.; Boxshall, G.; Clark, P. 1998. A dictionary of ecology, evolution, and systematics. Cambridge; New York: Cambridge University Press. 361 p.

Lipman, P.W.; Mullineaux, D.R., eds. 1981. The 1980 eruptions of Mount St. Helens, Washington. Professional Paper 1250. Washington, DC: U.S. Government Printing Office; U.S. Department of the Interior, Geological Survey. 844 p.

Macdonald, G.A. 1972. Volcanoes. Englewood Cliffs, NJ: Prentice-Hall. 510 p.

White, P.S.; Pickett, S.T.A. 1985. The ecology of natural disturbance and patch dynamics. New York: Academic Press.



US Forest Service - Pacific Northwest Research Station
Last Modified: Thursday,14May2015 at12:42:26CDT

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