Geologic Formations

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 collision of plates builds mountains at their edges, and the build-up and release of stress at the boundaries causes earthquakes.


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 blocks of limestone that contain 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, pressue builds up and pushes the edge of the continent upward. Since the 1990's, GPS measurements record this upward movement along the coast of the park at an average rate of 10mm/yr. When the pressure along the subduction boundary releases in the form of a large eaerthquake, the ground that was being pushed upward rapidly drops. For example, during the 1964 Great Alaska Earthquake, the coast of Kenai Fjords dropped between 3 and 8 feet. On a longer timescale, sea level has risen and drowned the coast of the park. In the heyday of the last Ice Age, which began about 25,000 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.


Capes and Cliffs
Many visitors to Kenai Fjords National Park take a boat trip out of Resurrection Bay to see the wildlife, glaciers, and unique scenery. On the east side of the bay, Cape Resurrection presents a massive, sheer cliff to the pounding of the waves. The rock here is pillow basalt, a type of igneous rock that formed when lava flowed out underneath the water and cooled rapidly. It gets its name from the bulbous, pillow-like shapes which form as a result of this rapid cooling. The bubbled texture of the cliff provides an ideal habitat for nesting birds like black-legged kittiwakes and horned puffins.

On the opposite side of the bay, the shore is eroded into spires, cliffs, and coves. Exposed areas reveal buckled layers of ancient sediment. Mud, transformed by heat and pressure, becomes shale, a fine-grained, dark-colored stone with many thin layers. With more time and more pressure, the shale becomes slate. If there is sand mixed with the original mud, it may become greywacke instead. These softer layers crumble into the sea more rapidly than basalt. Arches and spires form from the erosive action of waves.

As you round the corner to the west, moving out of Resurrection Bay, Cape Aialik juts into the tumultuous water. The rock here is granodiorite, part of a massive pluton that extends down the shore, cropping up again at the entrance to Northwestern Fiord. Granodiorite is of a lighter color than the ocean sediment rocks. It is also more resistant to weathering, such as by the action of glaciers upon its surface. Where glaciers have carved the bedrock into impossibly steep cliffs, the slate crumbles once the ice melts away - but the granodiorite stands, its sheer surfaces draped with waterfalls.

Cliffs and islands of greywacke, like Nuka Island, have virtually no seabirds nesting on them because their softer surface is too easily eroded to make a safe home. Granitic islands, on the other hand, are packed with birds. The Chiswell Islands, pinnacles of granodiorite stretching up from the sea floor, are home to tens of thousands of puffins, murres, and auklets. The affinity birds seem to have for granite is one of many ways that bedrock influences the life found on its surface. Sea lions, too, congregate on smooth granitic slabs washed by ocean swells to mate, give birth, and rest.

quartz seams in the bedrock at Kenai Fjords National Park
Quartz seams are evident in fractures of greywacke.

NPS / Paul Ollig

Seams So Real
The bedrock of Kenai Fjords National Park was carried north by the action of the tectonic plates. It was heated and pressed into a new form. In certain places, it was also broken. The fractures in the rock filled swiftly with jets of superheated water from deep in the Earth. The water carried dissolved minerals such as iron, silica, arsenic, and gold. These precipitated out, forming white and brown streaks across the darker bedrock.

As park glaciers retreat, more bedrock is exposed. Signs of past events are visible in the white seams of quartz shooting through the dark greywacke and in layers of upended slate which give silent testimony to the slow, inexorable compression of plates. Ice has left its scouring marks across every surface. Once the ice is gone, these signs of the past are quickly engulfed by a verdant wave of life which flows across the landscape, rushing forth to fill every conceivable niche and to hide the larger forces at work.

The Kenai Fjords National Park Geologic Resources Inventory has more information about the geologic formations here in Kenai Fjords National Park

Last updated: September 6, 2020

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