If you visit the Cook Inlet side of Lake Clark National Park and Preserve in Alaska, you will no doubt notice a couple of lofty, cone-shaped mountains from which steam issues skyward. Designated as National Natural Landmarks and towering above the landscape at a little over 10,000 feet (3,048 m) in elevation each, Mount Redoubt and Mount Iliamna are physical manifestations of the dynamic geological forces at work within the park.
Plate Tectonics
Lake Clark is about 150 miles (241 km) north of Katmai National Park and Preserve, so the geology between the two is similar. Both parks are a part of the Peninsular terrane, which is part of the larger Wrangellia composite terrane. A terrane is a package of rocks with a different geologic history than the other rocks around it. This package of rocks was transported to its present location via plate tectonics. The arc-shaped terranes composing Alaska are sheets of rocks which collided with other sheets of rocks, suturing themselves together at fault zones.
Map of Alaska's geologic terranes, including Lake Clark National Park and Preserve / Silberling et al via NPS
Think of Earth’s outermost layer as a series of irregular slabs, or sheets, riding above mobile, molten material beneath these slabs. This molten mantle material (the asthenosphere) is constantly in motion, ferrying the sheets all over the globe. Plate tectonics are always on the move, albeit slowly. Plate movement ranges from one centimeter to more than 15 centimeters (0.39 – 5.9 inches) per year, which sounds negligible until you start adding those centimeters/inches up over the course of millions of years.
Earth’s outer shell is broken into tectonic plates that move relative to one another / Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers via NPS
Mounts Redoubt and Iliamna are the result of Alaska and Lake Clark’s location along the northern boundary of the Circum-Pacific Belt (aka Pacific Rim of Fire), where 90 percent of the world’s earthquakes and most of the world’s volcanism occurs. At this boundary, plates of mostly oceanic crust are sinking (subducting) beneath a larger tectonic plate. Most of Alaska is a part of the North American Plate. When other tectonic plates subduct beneath the North American Plate, the water in the material subducted heats up due to high temperatures and pressures at depth, melting the neighboring rocks to create molten magma. Magma is lighter than the surrounding rock and rises back up to the surface, forming chains of volcanoes.
A diagram of the southern Alaska subduction zone / NPS modified from Robert J. Lillie "Parks and Plates, The Geology of Our National Parks, Monuments, and Seashores"
Volcanoes
Mount Iliamna, Lake Clark National Park and Preserve / Rebecca Latson
Mounts Redoubt and Iliamna are different from the volcanoes of Hawai’i Volcanoes National Park, not only in shape but also in lava composition and eruption type. Redoubt and Iliamna are stratovolcanoes, also known as composite volcanoes, made up of layers of lava, pumice, ash, and rock fragments. Composed primarily of an igneous rock called andesite, the eruptions from these two volcanoes can be quite explosive, due to the higher gas and silica content than the lava of Hawaiian volcanoes.
While Redoubt volcano has erupted at least four times during recorded history: 1902, 1966–1968, 1989–1990, and 2009, scientists estimate this volcano may have been erupting as far back as 0.88 million years ago, based upon potassium-argon radiometric dating.
Mount Redoubt, Lake Clark National Park and Preserve / AVO-USGS-R.G. McGimsey
Mount Iliamna is not quite as active as Redoubt, having no confirmed eruptions described in recorded history. Age dating this volcano’s rocks, however, indicates eruptions occurring 300 and 140 years ago.
Glaciers
Much like Mount Rainier at Mount Rainier National Park in Washington state, glaciers radiate down and out from the snowy flanks of each of these two snow-and-ice-covered Alaskan volcanoes.
Map of glacier extent between 1957 - 2008, Lake Clark National Park and Preserve /Loso et al via NPS Geologic Inventory Report
Glaciers within this park and preserve have existed and sculpted the park’s landscape for thousands of years. These “rivers of ice” cover approximately 14 percent of the park as of 2008. Found in the central and eastern portions of the park, Lake Clark’s glaciers are concentrated around Redoubt and Iliamna volcanoes as well as along the crests of the Chigmit and Neacola Mountains. As of 2008, the park contained over 1,500 glaciers, 17 of which are formally named. These glaciers range in size from small cirque glaciers covering less than 1 km2 (0.4 mi2 ) to Double Glacier, a mountain glacier spread over 137 km2 (52.9 mi2). The features formed and deposits left by these glaciers in Lake Clark National Park and preserve are textbook examples of the power of ice.
Diagram of common glacial features and deposits, Lake Clark National Park and Preserve / Trista Thornberry-Ehrlich via NPS
For a more detailed look at Lake Clark’s geology, you can read and download the 2021 Lake Clark National Park and Preserve Geologic Resources Inventory Report.
