Denali National Park and Preserve - Geology

Geology

Denali National Park and Preserve is located in the central area of the Alaska Range, a mountain chain extending 600 miles across Alaska. The most well-known geologic feature is Mount McKinley, also known by its Athabascan name of Denali. Its elevation of 20,230 feet makes it the highest mountain in North America, and its vertical relief (distance from base to peak) of 18,000 feet is the highest of any mountain in the world. The mountain is still gaining about 1 mm in height each year due to the continued convergence of the North American and Pacific Plates. The mountain is primarily made of granite, a hard rock that does not erode easily; this is why it has retained such a great height rather than being eroded.

There are three major rock provinces that run in east-west bands through the park. The oldest is in the north, and the younger ones in the south. The area is characterized by collision tectonics: over the past millions of years, exotic terranes in the Pacific Ocean have been moving toward the North American landmass and accreting, or attaching, to the area that now makes up Alaska.

The oldest rocks in the park are part of the Yukon-Tanana terrane. They originated from ocean sediments deposited between 400 million and 1 billion years ago. The original rocks have been affected by the processes of regional metamorphism, folding, and faulting to form rocks such as schist, quartzite, phyllite, slate, marble, and limestone.

The next oldest group of rocks is the Farewell terrane. It is composed of rocks from the Paleozoic era (250-500 million years old). The sediments that make up these rocks were deposited in a variety of marine environments, ranging from deep ocean basins to continental shelf areas. The abundant marine fossils are evidence that around 380 million years ago, this area had a warm, tropical climate.

The Pingston, McKinley, and Chulitna terranes are the next oldest; they were deposited in the Mesozoic era. The rock types include marble, chert, limestone, shale, and sandstone. There are intrusions of igneous rocks, such as gabbro, diabase, and diorite. Special features include pillow basalts, which are formed when molten lava flows into water and a hard outer crust forms, making a puffy, pillow shaped feature; as well as an ophiolite sequence, which is a distinct sequence of rocks indicating that a section of oceanic crust has been uplifted and thrust onto a continental area.

Some of the youngest rocks in the park include the Kahlitna terrane, which is a flysch sequence (a sedimentary rock sequence deposited in a marine environment during the early stages of mountain building) formed about 100 million years ago, during late Cretaceous time. Another rock sequence is the McKinley Intrusive Sequence, which includes Mount McKinley. The Cantwell Volcanics include basalt and rhyolite flows, as well as ash deposits. An example can be seen at Polychrome Pass in the park.

Denali National Park and Preserve is located in an area of intense tectonic activity: the Pacific Plate is subducting under the North American plate, creating the Denali fault system, which is a right-lateral strike-slip fault over 720 miles long. This is a part of the larger fault system which includes the famous San Andreas Fault of California. Over 600 earthquakes occur in the park each year, helping seismologists to understand this fault system. Most of these earthquakes are too small to be felt, although two large earthquakes did occur in 2002. On October 23, 2002 a magnitude 6.7 earthquake occurred in the park, and on November 3, 2002, a magnitude 7.9 earthquake occurred. These earthquakes did not cause a significant loss of life or property, since the area is very sparsely populated, but they did trigger thousands of landslides.