• pond surrounded by green brush, reflecting a distant range of snow-covered mountains that are dominated by one massive mountain

    Denali

    National Park & Preserve Alaska

Mountains

blue sky, snow-covered Mount McKinley, and low, brown mountains
At 20,320' above sea level, Denali (also known as Mount McKinley) dominates the landscape of the Alaska Range.
NPS Photo / Alex VanderStuyf
 

The Birth of a Mountain Range
The spectacular mountains we see today are a result of millions of years of rock formation, uplift, and erosion. Before we discuss the processes that created today's Alaska Range, let's review some basics about rocks and their origins.

Three major types of rocks make up Earth's crust. Igneous rocks are formed when molten rock (as magma underground or lava on the surface) solidifies. Igneous rocks can be plutonic or volcanic. Plutonic rocks, such as granite, form when magma cools slowly in the depths of the earth. Volcanic rocks, such as basalt, rhyolite, and andesite form when lava cools rapidly on the surface.

Sedimentary rocks are derived from sediments--particles of mineral and organic material which have been deposited by water or wind. Sediments are most commonly carried by rivers or streams and deposited in basins, that is, low areas such lakes and seas. These sediments are buried and compressed into rock strata. Typical sedimentary rocks include sandstone, limestone, shale, and chert. Fossils found in sedimentary rocks provide clues to the environment in which the sediment was deposited.

Metamorphic rocks are pre-existing rocks that have been changed due to intense heat and/or pressure deep within the earth without completely melting. The original rocks could have once been sedimentary, igneous or even another metamorphic rock. When rocks are squeezed and baked beneath Earth's surface or by contact with lava at the surface, the minerals may recrystallize and change form. Wavy layers of minerals called foliation may appear. Typical metamorphic rocks around Denali include schist, slate, quartzite, and marble.

 
A wedge-shaped graphic depicting the layers of the earth (solid core at center, liquid core above, many mantle layers atop that, and two solid layers at the outer layer)

Graphic courtesy United States Geologic Survey (USGS)

How Did Denali Get So High?
Geologists have identified several factors that have probably contributed to Denali's great elevation. The theory of plate tectonics provides part of the answer. To briefly tell this story, consider that Earth's crust is broken into great slabs of rock called tectonic plates. These plates float on the layer of Earth known as the mantle. The mantle is mostly solid, but it can move although very slowly.

Heat from the outer core is transferred to the lower regions of the mantle. As the mantle heats, it becomes less dense. This less dense mantle material begins to rise. The cooler mantle under the crust is denser; it sinks. As it sinks, it warms, setting up a cycle called a convection current. Geologists surmise that the convection currents in the mantle cause the tectonic plates on top to move around the surface.

 
a graphic depicting the Pacific Plate subducting under Alaska (the North American Plate and two subplates), forming mountains in the Alaska Range

Graphic courtesy NASA

One such tectonic plate, known as the Pacific Plate, forms the floor of the Pacific Ocean. It is slowly moving northward at about the rate that your fingernails grow. Oceanic plates are denser than continental plates. When an oceanic plate collides with a continental plate, the oceanic plate sinks below the continental plate in a process called subduction. In the diagram at left, the Pacific Plate is diving below the part of the North American Plate that holds Alaska's mainland. As the Pacific Plate moves northward, it carries chunks of land and pieces of other plates, some from thousands of miles away. These chunks and pieces are called terranes.

The force of the Pacific Plate pushing northward creates tension between the two plates. The build-up and sudden release of tension as these plates slip by one another triggers earthquakes. Much the way the hood of a car buckles under the force of a collision, the process of subduction causes the uplift of the Alaska Range, as well as the coastal ranges.

There are two major faults in Denali National Park and Preserve that contribute to the uplift of Denali. They are the Denali Fault and the Hines Creek Fault. Land south of the faults moves to the west relative to the north at a rate of about 1 centimeter per year. here is a large bend in the Denali Fault directly north of Denali (the mountain) that causes rocks to bunch up inside the bend in the fault. Denali happens to be in this bend; this is one of the reasons Denali is so tall. The forces that caused the uplift of Denali continue today. Scientists have determined that Denali rises at a rate of one half of a millimeter per year. That may not seem like much, but at that rate it will rise one kilometer in the next two million years--a brief period in geologic time.

 

Its composition is another reason that Denali has grown to such a great height. It is composed mainly of the igneous rock granite. Denali's granite formed below the Earth's crust as part of a batholith. A batholith is a bubble or mass of magma within Earth's crust. Plutons are parts of batholiths, defined by their chemical composition. The chemical composition of the magma determines the type of rock that will crystallize. Other intrusive igneous rocks (that is rocks that cool within the crust rather than at the surface) include gabbro, diorite, and pegmatite, to name just a few. Granite usually happens to be less dense than much of the rock that surrounds it. Over millions of years, a granitic pluton will float slowly towards Earth's surface, as it has in the case of Denali. Denali sort of "popped" up to the surface, much like a cork held under water will pop up when released. Just remember that "popping up" can take millions of years in geologic time! Erosion of Earth's surface rocks also helped expose the granitic rocks that make up Denali.

Granite is also very resistant to erosion. The forces of erosion, that is water, ice and wind, have a hard time wearing Denali's rock away. The rock pushes up faster than it is eroded, so Denali continues to grow. Subduction, uplift, and the lack of erosion have all contributed to Denali's great height.

Did You Know?

grizzly bear silhouetted against sky

Denali is home to both black bears and grizzly (brown) bears. Black bears inhabit the forested areas of the park, while grizzly bears mainly live on the open tundra. Almost all bears seen by visitors along the Park Road are grizzlies.