Geologic Activity

sedimentary rocks at Exit Glacier
Take a walk in Exit Glacier Valley and you are surrounded by evidence of geologic change. The mountains around you formed at the bottom of the sea. Layers of dark-colored mud, sand, and gravel eroded from an ancient shore and were deposited by underwater landslides into the sea. These sediments transformed slowly, under heat and pressure into sandstone, mudstone, and unsorted rock assemblages called conglomerate. Here, in Kenai Fjords National Park, these sediments were uplifted as mountains when the North Pacific Oceanic plate and the North American continental plate collided.

Light-colored granodiorite is common along much of the park’s coast and formed when a piece of the Pacific plate subducted underneath the North
American plate. During the subduction, some rock melted into magma, which intruded and cooled to form plutons inside the local bedrock. In many
areas, the softer sedimentary rocks that once surrounded the granodiorite plutons have eroded, leaving the harder rock standing as sea stacks and capes.

NPS Photo by Paul Ollig

The stark beauty of a land shrouded in ice, bordering the ocean, and teeming with wildlife covers a plot taking place underneath it all. Beneath the splendor, larger forces are at work. The movement of tectonic plates and the delivery and formation of various rock types prescribe where birds nest, where Steller sea lions breed, and where glaciers flow. The varying rock of the fjords underwrites the spectacle seen above— in one place eroding into graceful arches, in another withstanding the ocean’s constant blows.

Crustal Collision
Plate tectonics theory describes how the Earth is put together, and how it changes over time. The center of the Earth is a tremendously hot core of iron and nickel. Surrounding this is a hot mantle of liquid rock. On the outer edge of the Earth, where it is at last cool enough for the magma to harden into solid rock, the material forms a crust. Convection currents are at work within the Earth. Boiling magma rises from the mantle, cooling near the crust and sinking back again to be re-warmed. This movement disrupts the crust, breaking it into pieces and moving them around.

Continental plates are pieces of crust visible as the continents of the Earth. Oceanic plates are heavier pieces of crust, sinking lower into the mantle and are covered by oceans. The plates collide at their edges, causing earthquakes. The collision of plates also builds mountains.

Mysteries of Movement
The rocks that make up Kenai Fjords National Park have sometimes been carried great distances. Some rock was once coral reef close to the equator: It was carried along as the Pacific plate rotated counterclockwise, traveling north, transforming en route to stone. In the far western end of the park, a mixture of chert and basalt scraped from the ocean floor is jumbled with spectacular white blocks of limestone that carries fossils matching those found in China and Afghanistan. These segments of rock moved from their original home are called terranes. The entire coastline of Alaska is made of a mixture of terranes and local igneous material.

In the park, as the North Pacific oceanic plate subducts beneath us, it is now dragging the edge of the continent down with it. The Kenai Mountains are very gradually sinking below the sea. Evidence for this is found in the lovely half-moon coves of Aialik Bay. In the heyday of the Ice Age, which began about 1.8 million years ago, ice built up in the curve of every peak, gouging out cirques; high valleys that nourished glaciers. A warming climate raised sea level 10,000 years ago and snow falling at lower elevations melted in the summers, no longer transforming to glacial ice. The cirques are now drowned in ocean water and the peaks that edged above Pleistocene ice are now islands surrounded by sea.

Last updated: April 14, 2015

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