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.
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
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.
Capes and Cliffs
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 hardens to 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 Fjord. Granite 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. Birds seem to have an affinity for granite-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.
NPS Photo by Paul Ollig
Seams So Real
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.
Did You Know?
Did you know that the Harding Icefield is the largest icefield solely contained within the United States. More...