The geomorphology, stratigraphy, structure, volcanic activity, and geologic history along with related physiographic and geographic features of the present Katmai unit are described below.
The Bruin Bay fault parallels the Alaska Peninsula and divides it into two distinct geologic terrains. East of the fault is a sedimentary province containing Late Jurassic and Cretaceous marine clastic rocks with abundant fossils and few small areas of nonmarine Tertiary clastic rocks. This area also contains all the Quaternary and Recent volcanos. West of the fault there are metamorphic and volcanic rocks of Triassic and Early Jurassic age that are intruded by the Aleutian Range Batholith of Jurassic age and small stocks of Tertiary age. These rocks are locally overlain by nonmarine Tertiary clastic rocks.
In the Bristol Bay lowlands modified moraines extend along parts of the coast and higher lowlands, and slightly modified prominent moraines generally extend over the highest plains and into the upland valleys. Outwash deposits border the older moraines on nearly level slopes. Glacial lake deposits occupy an area west of Naknek Lake. Predominantly nonglacial deposits are associated with the coastlines, rivers, and highlands. Older coastal deposits with some modern beaches, spits, and bars near Bristol Bay extend the coastline. Low terrace deposits occupy sites along the modern flood plains, and higher terrace deposits occupy the valley margins and some areas along the coast. Alluvial fan deposits occupy sites near the base of larger volcanoes.
Volcanism is one of the principal geologic processes at Katmai. The high peaks of the unit were formed by volcanic activity, and many are still active enough to occasionally emit steam, smoke, ash, or lava. For example, Mount Trident discharged steam, ash, or lava in each of the years 1957 through 1965 and in 1968. Mounts martin and Mageik produce steam constantly, and the plumes may often be seen from King Salmon, 60 miles distant. Other peaks in the area have also had periods of volcanic activity. A major eruption may occur at any time.
The Katmai area was largely unknown until 1912, when a geologic event directed worldwide attention to this area. In June 1912, Mount Katmai and Novarupta Volcano erupted with tremendous force and ejected enormous amounts of ash and pumice. Then followed an explosion of hot, glowing ash and pumice from Novarupta and associated fissures. Some of this ash and pumice moved through an adjacent vegetated valley, destroying most living things in its path. Within minutes, more than 40 square miles of this valley were buried by volcanic deposits as much as 300 feet thick. As the ejecta was expelled through Novarupta, the top of Katmai collapsed, forming a large caldera.
Part of the ash was carried by the wind, especially to the east and southeast. This ash fell over a large area downwind from Katmai and Novarupta, accumulating on level ground to depths of 3 to 4 feet at Katmai Bay, 3 feet at Kaflia Bay and 6 to 12 inches at Kodiak on Kodiak Island. Quakes and sounds and the accompanying ashfall caused a complete evacuation of the Native groups at Savonoski, Katmai, Kaflia, and Douglas and a temporary evacuation of Kodiak.
Novarupta quickly became quiescent. Many thousands of fumaroles (steam and gas producing vents) developed as the volcanic materials that inundated the valley settled, cooled and hardened. The vista in 1916 of the coalescing plumes of steam produced by these vents gave the valley its name--Valley of Ten Thousand Smokes. Since its formation, cooling has resulted in diminishing fumarolic activity so that today there are only a few active vents remaining. The semi-consolidated volcanic ejecta is now vegetated, but most of the valley floor consists of multihued rock cut by numerous deep and narrow canyons, some of which are more than 100 feet deep while only 5 to 10 feet wide.
Mount Katmai has additional scientific significance. After the volcanic activity ceased, a glacier formed on the inside wall of the caldera. This glacier, which now extends down to the edge of the caldera lake, is perhaps the only glacier in the world having a known date of origin. Glacial features within Katmai preserve a complete sequence of Wisconsin and Recent (Alaskan) glaciation.
Soils in Katmai vary in composition between the Aleutian Range and the Nushagak-Bristol Bay Lowland physiographic provinces. At high elevations within the Aleutian Range province, the unconsolidated materials are coarse rubble deposits or bare rock. In the mid-to-lower elevations and hilly areas, soils are silty and sandy volcanic ash over gravelly material, stony loam, cinders, or bedrock. Deep depressions in the foothill slopes contain fibrous peat soils with lenses of volcanic ash. Soils in valley bottoms and in depressions in moraine hills along the coast are deep fibrous or partially decomposed peat. There is no permafrost in this province.
Deep, poorly-drained loamy soils with thick overlying peat mat and permafrost occupy lowlands in the Naknek drainage in the Nushagak-Bristol Bay Lowlands province. Poorly-drained, sandy-to-gravelly soils occupy outwash plains and foot slopes from the Naknek Lake area to the Ugashik Lakes. Well-drained, dark, loamy soils from fine ash occupy sites on rolling hills and outwash plains in the Bristol Bay lowlands and the western slopes of the Aleutian Range. Organic peat soils occupy depressions throughout the lowlands of the King Salmon-Naknek areas.
Mineral, petroleum, and geothermal resources within the Katmai area have not been investigated other than through reconnaissance type studies and local explorations. Only a small part Katmai has been subject to detailed geologic study, and most information has been obtained from a few widely spaced traverses of reconnaissance nature. The park includes part of a geologic province that has been involved in at least two episodes of igneous activity.
There are no active mining operations and no records of mineral patents within Katmai National Park and Preserve.