The ever-changing character of Alaska is evident by the various geologic forces that we are reminded of everyday. Earthquake, volcanoes, mountains, and glaciers are ever-present reminders of Alaska’s dynamic and complex geologic composition. When one considers the formation of the land that we know of as Alaska, its creation has occurred relatively recently within the context of geologic time. Geologic time began 4.5 billion years ago with the formation of the earth and continues to present day.

Alaska has been pieced together in complex ways over millions of years. At this time, geologists divide Alaska into two parts of somewhat different origin. These two parts are North American Alaska and Accreted Alaska.

North American Alaska largely consists of those parts formed in North America, although they may be somewhat displaced from their original locations.

Accreted Alaska contains many parts that could be exotic to Alaska. Basically speaking, the North Slope, Brooks Range, and Yukon-Tanana Upland are North American Alaska; southern Alaska is Accreted Alaska. In some areas, the boundary between these two general types is controversial.

As many as fifty pieces (the precise number/s a controversial debate among geologists), or terranes, have been added onto each other in waves. In fact, the entire Pacific Coast of North America, from Baja California to the Alaska Peninsula, are terranes that have been grafted together. As the North America continental plate and the Pacific Plate push and slide against one another, Alaska has been on the receiving end of the terranes that have been pushed northward. These terrains were created by a variety of geologic processes, and have been pieced together during the last 220 million years.

Geologists are still working on trying to understand Alaska’s complex geology. The entire sequence of events that created Alaska cannot be documented. Some rocks of southwestern Alaska are as old as two billion years, but researchers are not sure if these rocks were formed in North America. In the interior of Alaska, there are rocks 500 million to one billion years old that were formed in ancestral North America. In the 1960s, researchers at the Unive rsity of Alaska-Fairbanks found evidence that the Alaska Peninsula and regions in southwestern Alaska were formed much farther south from their present location. By using paleomagnetic signatures (when rocks form, magnetic particles in the rock align themselves with the earth’s magnetic field and their alignment can be measured to determine the latitude that the rock was formed at), geologists have determined that some terranes in Alaska were formed near the equator. The Alaska Peninsula and the Wrangell Mountains were formed in equatorial regions. In some other terranes, the complex nature of their formation prevents the use of paleomagnetic signatures, and thus determining the latitude of their origin.

Plate Tectonics

The earth’s crust is made up of a dozen or so large fragments called “plates”. Most of these plates are more than a thousand miles across and more than 40 miles thick. The plates move steadily, but slowly, past each other at rates currently up to 4 inches per year. Continental plates are less dense and more buoyant than the oceanic plates. There are three ways that plates can move relative to each other: 

1. The plates move towards each other and one (the denser one) dives underneath the other, which is called subduction. This is the situation in southern Alaska and along the Aleutian Islands where the Pacific plate dives beneath the North American plate.

2. The plates slide by each other. This is the geologic setting offshore of southeastern Alaska, where the North American plate and the Pacific plate slide past each other on the Fairweather-Queen Charlotte fault. Fault lines occur between plates as well as within a plate (e.g., the Denali fault in Denali NP).

3. The plates move away from each other. This is called a spreading zone and it occurs mostly in deep oceans. 

The collision of tectonic plates also results in the folding, faulting, and thickening of rocks. Rocks of the continental plate are pushed upwards to form mountains. Throughout Alaska, the evidence of plate collisions is abundant by the presence of mountains. Seismic activity within mountain ranges may indicate ongoing collisions or post-collisional adjustments. On some occasions, mountains are also built a part of a subducted oceanic plate scrapes off and is grafted onto the continental plate. The Chugach Mountains are an example of a terrain that was grafted onto the existing continental plate.

Earthquakes

Earthquakes occur either due to subduction of one plate underneath another or due to two plates sliding by each other. The immense plates move at a steady rate, but at their edges the sliding motion is neither smooth nor constant. Plates tend to get stuck in places and when they are not slipping by each other they are “locked” together. Eventually, enough strain will build up, the locked section will break the two plates will slide past each other, and we experience an earthquake. The whole process is much like pulling a concrete block with a bungee cord. At first you pull on the bungee cord and it stretches out and the block does not move. Eventually, the pull on the cord is strong enough to get the block moving. It slides forward with a jerk and then stops. If you keep pulling the cycle repeats itself, just like the earthquake cycle. 

The size of an earthquake is commonly stated in terms of magnitude. There are several ways that earthquake magnitude is expressed. The most famous was devised in 1934 by Charles F. Richter. On the Richter scale, each whole number step represents a thirty-fold increase in the size of seismic waves measured on a seismograph, a machine that measures how much the ground moves in an earthquake.

Alaska is the most seismically active state in the United States, with an average of 1,000 earthquakes per year that are 3.5 or more on the Richter scale. The most seismically active part of the state is the Aleutian Island chain. Seismic related to the Aleutian Islands extends into the Gulf of Alaska and north to near Denali (Mt. McKinley).

Along the edges of the Pacific Plate, tremendous is created and release. This area is called the “Ring of Fire,” due to the seismic and volcanic activity that occurs along the edges of the plate. Seventy-five percent of the world’s earthquakes occur along the Ring of Fire, and eleven percent of the world’s earthquakes occur in Alaska. Alaska experienced three of the Top 10 worldwide earthquakes during the 20th century.

Between 1899 and 1995, ten earthquakes occurred within Alaska that equaled or exceeded magnitude of 8.0 on the Richter scale. More than 75 earthquakes occurred during that time frame that were magnitudes of 7.0 or greater on the Richter scale, with the most recent occurring in the Gulf of Alaska on March 6, 1988, that registered 7.6 on the Richter scale.

1964 Earthquake

On Good Friday, March 27, 1964, at 5:36 PM, the largest recorded earthquake in North America occurred in Alaska. At the time of the quake, it was rated between 8.4 and 8.6 on the Richter scale; however, its equivalent moment magnitude has been revised upward to 9.2 (new ways of measuring and ranking very large earthquakes have been developed by seismologists). The energy released by the Good Friday earthquake equals that of 73,000 Hiroshima-size atomic bombs. The epicenter of the earthquake was located beneath Miners Lake in northern Prince William Sound (about 70 miles east of Anchorage). The vibrational waves – which were felt 700 miles away – shook south-central Alaska for approximately 7 minutes. The 1964 earthquake moved the earth farter, both horizontally and vertically, than any other earthquake ever recorded except the 1960 Chilean earthquake. There were 12,000 after-shocks of 3.5 or greater during the 2.5 months after the original quake.

As a result of the earthquake, land west of Prince William Sound subsided. (22,000 square miles of land dropped as much as 5.4 feet), while land east of the epicenter was uplifted (12,000 square miles rose as much as 7.5 feet). The earthquake and tsunami (seismic ocean waves) killed 131 people, 115 of them Alaskans. Sixteen of the deaths occurred as a result of a tsunami that hit the coast of Oregon and California. The numerous tsunamis accounted for 119 of the 131 deaths that occurred. Many portions of Anchorage were hard-hit by the earthquake, as much of the soil that the city is built upon is subject to liquefaction. During an earthquake, some of the soil types act like a liquid as a result of the vibrations, and the liquefaction-affected soil cannot support structures. The soils are also prone to landslides, and many massive slides occurred in and around Anchorage during the earthquake.

Volcanoes

When an oceanic plate subducts underneath a continental plate, the oceanic plate dives down into the earth’s magma. The deeper it dives, the more it heats up until it melts. The molten rock is lighter than the surrounding magma and will move upwards. When it hits the earth’s crust it either forms a pocket in the hard rock of the crust cooling off slowly (this is called a pluton) or it breaks through the earth’s surface and spills out as a volcanic eruption. A volcano constitutes a vent, a pipe, a crater, and a cone. The vent is an opening at the earth’s surface. The pipe is a passageway in the volcano in which the magma rises through to the surface during an eruption. The crater is a bowl-shaped depression at the top of a volcano where volcanic materials like ashes and lava and other pyroclastic (mixture of hot gas and ash) material are released. Solidified lava and ashes form the cone. Layers of ashes and lava build the steep sided cone higher and higher.

Along the Ring of Fire, the zone where the Pacific plate subducts beneath other plates, volcanoes are a common occurrence; in fact, over half of the world’s active volcanoes are located here.

Alaska has more than 10% of the world’s identified volcanoes. Most of them are located in a volcanic belt that extends from Mount Spurr, about 80 miles west of Anchorage, to beyond Buldir Island in the western Aleutians. Alaska’s volcanic belt on the chain of volcanoes contains about 80 major centers with one or more volcanoes that have erupted in geologically recent times, and activity has been recorded at about 40 of these volcanoes since 1700. Pavlov Volcano on the Alaska Peninsula, Alaska’s most active volcano, has erupted about 40 times since 1790.

The largest eruption in the world in the 20th century occurred in 1912 at Novarupta on the Alaska Peninsula. An estimated 15 cubic kilometers of magma was explosively erupted during 60 hours beginning on June 6 – about 30 times the volume erupted by Mount St. Helens in 1980. The expulsion of such a large volume of magma excavated a funnel-shaped vent 1.2 miles wide and triggered the collapse of Mount Katmai volcano 6 miles away to form a summit caldera (depression) 200 feet deep and about 2 miles across. Extrusion of the lava dome, called Novarupta, near the center of the 1912 vent marked the end of the eruption. (If you read up on the climbing history of Denali, you will notice that the Parker/Browne expedition attempting to reach the summit in 1912 was turned away by a storm, which forced them off the mountain. After they got off, a big earthquake hit the region that changed the face of Denali forever and would have killed the climbing party had they still been on the mountain. This earthquake was caused by the Novarupta eruption.) Little was known about the spectacular effects of this great eruption until 1916, when a scientific expedition sponsored by National Geographic Society visited the area. To their amazement, they found a broad valley northwest of Novarupta marked by a flat plane of loose, “sandy” ash material from which thousands of jet streams were hissing. The eruption produced pyroclastic flows that swept about 14 miles down the upper Ukak River valley. The thickness of the resulting pumice and ash deposits is not known but may be as great as 600 feet. In 1916, the deposits were still hot enough to boil water and form countless steaming fumaroles; hence the expedition named this part of the Ukak River the “Valley of Ten Thousand Smokes.” 

Today, scientists are particularly concerned about volcanoes whose eruptions can affect the Cook Inlet region, where about 60% of Alaska’s population lives. Redoubt volcano (on a clear day is easily visible on your drive along Turnagain Arm, or in Anchorage), erupted for the fourth time in the 20th century on December 14, 1989. Following several days of strong explosive activity, a series of lava domes grew in Redoubt’s summit crater during the next four months. Most of the domes were destroyed by explosions or collapsed down the volcano’s north flank. Ash produced by the eruptions affected air traffic en route to Anchorage. Many domestic carriers suspended service to Alaska following major explosive events, and several international carriers temporarily rerouted flights around Alaska. On December 15, a jetliner en route to Japan encountered an ash cloud while descending into Anchorage. The plane quickly lost power in all four engines and dropped 12,000 feet in altitude before the pilots were able to restart the engines. The aircraft landed safely in Anchorage but it sustained more than $80 million in damage.

Lahars (mudflows formed by the mixing of volcanic particles and water) generated during the Redoubt eruption threatened an oil-storage facility located on the banks of the Drift River on the west side of Cook Inlet. Oil is pumped from more than a dozen wells in Cook Inlet to the facility and then loaded onto tankers, which docked just offshore. A lahar on January 2nd flooded part of the facility with neatly 3 feet of water, forcing a shutdown until workers could restore power. This and subsequent lahars prompted Cook Inlet Pipeline Company to temporarily halt oil production from some oil wells and reduce the amount of oil stored at the facility between tanker loadings.

Glaciers

In Alaska, the effects of glaciation, both from present valley glaciers and from the recent Ice Age, dominate the landscape. There are an estimated 100,000 glaciers in Alaska covering about 5% of the state, or 29,000 square miles! About three-quarters of all fresh water in Alaska is stored as glacial ice.

Most of Alaska’s glaciers do not occur in the north where temperatures are colder and the air is drier, but along the coast where snowfall is greatest. The Chugach and St. Elias Mountains rise abruptly from the Gulf of Alaska and contain the highest concentration of glaciers in the state. Mt. Marcus Baker, the highest mountain in the Chugach Range, reaches a height of 13,176 feet just a few miles from the sea. These high mountains intercept moisture-laden air from the gulf, resulting in snowfalls of over 100 feet annually. That’s not 100 inches, but 100 feet, of snowfall each winter – the height of a 10-story building! (Mt. Marcus Baker is located north of Prince William Sound along College Fjord.)

The largest glacier within Alaska is considered to be the Bering Glacier complex, which is about 2,250 square miles in size, and includes the Bagley Icefield. The Malaspina Glacier is the largest piedmont lob glacier in Alaska, and it is located along the coast of Wrangell-St. Elias National Park. At 850 square miles, the Malaspina Glacier is larger than the state of Rhode Island.

What is a Glacier?

To be considered a glacier, there must be enough snow so that is has been compacted into ice and the ice must be flowing downhill. Under enough pressure, ice is no longer the brittle solid we are familiar with, but softens (imagine silly putty or very thick molasses), yields to gravity, and flows downhill. Usually the ice needs to be 100 feet thick or more to be under enough pressure to soften and flow. Typical glacier movement is a few inches per day, but some glaciers move much slower than others much faster. The area on a glacier where snow accumulates and turns into ice is called the accumulation area. It is in the ablation area that snow and ice melt. The line between those two areas is the annual snow line.

Depending on the balance between how much snow and ice accumulates, and how much melts at the bottom of the glacier, a glacier may get longer (advance; there is more ice accumulating than melting) or shorter (retreat; more ice melts than accumulates at the top). However, whether advancing or retreating, the ice within a glacier is always flowing downhill, much like a river of ice. It’s this movement of ice downhill that truly defines a glacier.

Types of Glaciers

Valley Glacier

A glacier that occupies a valley; this type of glacier is very common throughout Alaska. Examples include Skilak Glacier, Exit Glacier, Ruth Glacier (seen from the Denali State Park overlook) and the Muldrow Glacier (in Denali National Park, just beyond Eielson Visitor Center.)

Cirque Glaciers

Cirque glaciers form near mountain crests in circular basins or amphitheaters. Although there are numerous examples in Alaska, cirque glaciers are relatively small and rarely named.

Hanging Glacier

If a valley or cirque glacier ends abruptly at the top of a cliff it is called a hanging glacier. Two good examples include Explorer and Middle glaciers in Portage valley (located on the right side of the road as you drive to the Visitor Center). They were formed when a bigger glacier with a lower base filled the valley they are now abruptly ending in. After the last ice age, many of those bigger glaciers have melted and left are the little “feeder glacier” or hanging glaciers.

Tidewater Glacier

A valley glacier that flows into the ocean. Examples of tidewater glaciers include Holgate and Northwestern Glaciers in Kenai Fjords National Park, and Columbia Glacier in Prince William Sound.

Piedmont Glacier

A large valley glacier that flows out from a confined valley into an open area and forms a broad lobe-shaped mass. Examples: Malaspina and Bering Glaciers.

Icefield

A large mass of ice where many valley glaciers flow out on all sides of the icefield. Example: Harding Icefield above Seward feed Skilak, Exit, Holgate, Northwestern and many other glaciers.

Ice Cap

A dome-shaped mass of glacier ice that spreads out in all directions. Larger than an icefield but smaller than 12 million acres. Fairly uncommon, a few ice caps exist in Canada.

Ice Sheet (or Continental Glacier)

A vast ice mass that completely covers a large land mass and is greater than 12 million acres. Examples: During the last Ice Age, ice sheets covered southern Alaska, most of Canada, and parts of the lower 48 states. Today, only the Antarctic Ice Sheet and Greenland Ice Sheet remain.

Glacial Features

Moraine

Rock debris (silt, gravel, rocks, boulders) that is either on top of , within, or deposited by a glacier.

Lateral moraines

Moraines on top of but along the sides of a glacier. Debris has been scraped from the valley walls by the glacier or added to by rock fall from above the glacier.

Medial moraines

Moraines down the center of a glacier, formed where lateral moraines of tributary glaciers come together. The lower portions of large valley glaciers are frequently completely covered by moraine obscuring the ice due to many glaciers merging further up. The moraine on the lower portion of the Muldrow Glacier in Denali is so thick, trees are growing on it.

Crevasses

Cracks and crevasses form in the upper 100 – 150 feet of the glacier where the ice is brittle. Similar to where rapids form in a river, crevasses occur mainly in steep or narrow sections of a glacier. They also form where the glacier travels over uneven ground.

Blue Ice

Although glacier ice often appears blue, it is actually clear. (Similar to clear water in a blue lake). Glacier ice appears blue because it absorbs light from most colors of the spectrum and scatters the blue light. The blue color is often more intense on overcast days, because clouds also filter out low energy colors such as red, orange and yellow.

Calving

Calving is the fracturing and breaking off of ice from a tidewater glacier or from a glacier that ends in a lake. If the piece that breaks off extends more than 15 feet above water level it is called an iceberg, if it is between 3-15 feet it is a bergy bit (no lie!), and pieces less than three feet above water level are called growlers.

Glacier flour

Rocks being transported at the base of the glacier and scraping against bedrock erode into smaller pieces. The results are gravels, sands, and a fine powder called glacier flour. Glacier flour is so fine it is easily held in suspension in water and is immediately washed away from the base of the glacier or the moraine. Lakes and streams draining glaciers carry a lot of glacier flour and appear very gray and muddy, such as Portage Lake or the Toklat River (in Denali). If the glacial river runs through a lake, the flour settles out and the water appears turquoise blue, such as in Kenai Lake and the Kenai River. Glacial flour settles more easily in salt water and is what creates the dangerous quicksand of Turnagain Arm.

Glacier scouring

Glacially carved valleys are easily recognized by their characteristic U-shape (as opposed to V-shaped, river-carved valleys). The tools with which glaciers carve are not so much ice, but rather rock. As the glacier moves, it picks up and incorporates rock debris into the ice. It is this rock carried by the glacier that grinds and scrapes the underlying bedrock surface.

Terminal moraine or end moraine

When a glacier is stable, which means it neither advances nor recedes, transported rock and debris that is always left behind at its terminus accumulates there. The result is a new landform. A large ridge of rock debris deposited at the end of a glacier is called a terminal moraine. Terminal moraines dam Portage, Wonder, Kenai and Skilak Lakes. At Exit Glacier you get an excellent view of a terminal moraine formed in 1995. Currently, the glacier is receding and not stable enough to form a visible moraine.

Erratic

A large boulder, deposited by a glacier. On the flats below Polychrome Pass in Denali, several erratics are visible. Erratics often are of a different rock formation than their currently surrounding rocks which is a dead give-away for glacial deposition.

Kettle ponds

Small ponds made by the melting of a remnant piece of ice left by a glacier. Most of the ponds between Eielson Visitor Center and Wonder Lake are kettles.

Surging Glaciers

Some glaciers occasionally move very rapidly in an event a glacial surge. Surging glaciers move up to 100 times their normal speed for about one year. During its 1956-1957 surge, the Muldrow Glacier moved as much as 1,150 feet per day!

A surge is different than an advance. A glacier advances because more ice forms than melts. A surge is a rearrangement of the ice that’s already there. For unknown reasons, the upper reaches of the glacier drops, the ice flows down the glacier in a rapid wave, and is pushed out down the valley. The mechanism for this event is unknown, but the most popular theory is that the surge is related to a high amount of water built up beneath the glacier.

Only a handful of glaciers are known to surge, most of which are located in Alaska. In 1986, the Hubbard Glacier surged, blocking the outlet of Russell Fjord, turning it into a fresh water lake. The event gained national attention when a rescue attempt was made to move seals from the newly formed lake back to the ocean. Eventually, rising waters broke through the ice dam reconnecting Russell Fjord to the sea.

Also in 1986, the Peters Glacier in Denali National Park surged, moving up to 350 feet per day and advancing 3 ½ miles down the valley. This brought Peters Glacier into view from Wonder Lake, although today it is fairly difficult to distinguish

Iceworms

Around one inch long, iceworms are related to earthworms and live between the ice crystals of some glaciers or in permanent patches of snow. They come to the surface to feed on pollen grains and red algae. Only a few glaciers worldwide are known to contain iceworms, but they can be found locally in the snow patch at the base of Byron Glacier in Portage Valley. It’s best to look on a cloudy evening as iceworms burrow deep into the ice to avoid the heat of the day.

The Future of Glaciers

Glaciers are sensitive indicators of climate change. Some glaciers in Alaska are advancing but most are retreating, generally because the climate since the last ice age is naturally getting warmer. Some scientists believe global warming is causing the retreat, but there is much disagreement. Others believe global warming is occurring and will cause the glaciers to advance, not retreat. These scientists believe warmer temperatures will increase snowfall that will create larger glaciers. It is very likely that the earth is in a warmer period between ice advances and the Ice Age is not over. Basically, no one really has a clue about what glaciers in the future will do. (Unlike land-based glaciers, tidewater glaciers are poor indicators of climate change. Their advances and retreats are more influenced by water depth the glacier is in and whether the leading edge is protected from melting influences of water by a terminal moraine).

The Ice Ages in Alaska and Beyond

The most recent 2 million years of the geologic time scale is called the Quaternary period. It is characterized by repeated cool and warm intervals each lasting tens of thousands of years. The Quaternary is divided into two parts. The period up until 10,000 years ago is called the Pleistocene, the last 10,000 years, the current warm interval, have been termed the Holocene. The cool periods, the ice ages, changed the face of Alaska forever. Ice sheets covering Canada and the northern U.S. reached all the way to the Yukon Territory and the southern part of Alaska. Southeast, south central, and southwest Alaska were covered in ice layers thousands of feet thick. There were small glaciers on the southern Seward Peninsula and some more in the Brooks Range. Interestingly enough, the glaciers in the Brooks Range experienced the maximum extent during the middle of the Pleistocene. During the last ice age, the late-Wisconsin (from about 50,000 to 10,000 years ago), Brooks Range glaciers were rather minor in size, even though it is considered the most severe of all known ice ages (compare glacial extent of Brooks Range).

Many visitors to Alaska expect the state to have been covered completely by ice during the ice ages. It is after all, cold up here, isn’t it? This assumption is actually very far from the truth; most of Alaska was ice-free.

Moisture source for the Alaskan glaciers and ice sheets was the Pacific Ocean. Moisture-laden clouds would rise over the ocean and move towards the continent and drop their water in the form of snow onto southern Alaska. When the clouds hit the Alaska Range further to the north, they gained in altitude and therefore dropped the last of their moisture. The air that moved from here further inland was mostly very dry. During earlier ice ages, when temperatures were not at their lowest, there was still enough moisture left to feed some glaciers in the next rise of mountains; the Brooks Range. During the late-Wisconsin, however, the temperatures were at their lowest and the cold air could not hold enough moisture to feed larger glaciers in the Brooks Range. Therefore, Brooks Range glaciers during that period were rather small. This also means that the ice-free part of Alaska, the Interior, the North Slope, and most of western Alaska, was a vast expanse of steppe-like character, covered in thick sheets of wind-blown glacial silt, or loess.

During ice ages, the moisture that evaporates over the oceans and drops into land does not go back into the ocean in the form of run-off but rather stays locked on land in the form of glaciers and ice sheets. As the continental ice sheets grew, the oceans became more and more depleted of water. Sea levels worldwide dropped about 300 feet during the most severe ice age, the late-Wisconsin. It is easy to imagine that the continents looked quite different during those times; shallow oceans dried up and more land was exposed. The Bering and Chukchi Seas between Alaska and Siberia are very shallow (less than 300 feet deep, except for the western part of the Bering Sea which is very deep). The land that was exposed between the Asian and the North American continents is called the Bering Land Bridge. This land bridge was in place intermittently throughout the Pleistocene period and had its maximum extent during the late-Wisconsin, spanning about 1,000 miles from north to south! It was used for migration by many plant and animal species, including humans.

The area between the Mackenzie River in Canada and the Kolyma River in Siberia is called Beringia. During the ice ages Beringia was a mostly ice-free sub-continent, in fact you could walk from Alaska all the way to Europe without being hindered by ice. During the Pleistocene epoch, the most amazing critters roamed the Beringian landscape. The herbivores included mammoths, bison, steppe horses, antelopes, camels, ground sloths, rhinoceros, sheep, caribou, moose, and a giant (5 foot tall!) beaver. The predators included saber-tooth tigers, lions, short-faced bears (9 feet tall!), brown and black bears, and wolves.

How can an ice age landscape support such a high species diversity and why do we have so few species left today? The first question is a million-dollar question and has been hotly debated among scientists. My own (Claudi Hoefle’s) and many other’s research has found that the vegetation of ice age Beringia was significantly different form today’s. As mentioned above, the climate was very, very dry. Today’s plants in Alaska are quite cold tolerant but they cannot handle arid conditions. The vegetation was most likely a mosaic of different plant communities depending on local conditions. But vast stretches of Beringia were probably covered with a steppe-like plant community mostly comprised of grasses. There were some shrubs but no trees in Alaska at that time. The herbivores listed above are predominantly grazers, which need large quantities of grasses to live. There is evidence that the Pleistocene vegetation provided sufficient supplies for this so-called mega-fauna. Most of the fauna species roaming the Beringia landscape died out between 10,000 and 15,000 years ago. There is some speculation as to what role human hunters played in the extinctions but most likely were caused by a radical climate change. During that time the global climate warmed rapidly, ice sheets were melting, and vegetation was changing to the tundra vegetation that is typical of Alaska today. Most Pleistocene herbivores could not tolerate tundra vegetation and ceased to exist. Alaska today is a very low-species environment (for fauna and flora).

With the warming of the climate, sea levels rose again and the Bering Land Bridge was flooded about 10,000 years ago. Today, Alaska and Siberia, which are very similar in their geology and ecology, are called the “divided twins,” waiting to be united once again during the next ice age.

Permafrost

Permafrost is defined as ground that remains frozen for two or more years. It is one of the most important aspects of the high latitude environment but it also occurs at high altitude all across the globe. Approximately 20% of the earth’s surface is underlain by permafrost. About 50% of Canada, 20% of China, much of Siberia, all of Greenland, and 85% of Alaska lie in continuous, discontinuous or sporadic permafrost zones.

Generally, the cold penetrating the ground in the winter works against the warmth penetrating in the summer. Further to the north, the cool summers cannot melt out the ground cooled off in the winter. Here you find continuous permafrost, which means permafrost is everywhere. A little more to the south, warm summer temperatures succeed to melt out the ground completely in some areas but not in others. Here you find discontinuous permafrost. The Interior is primarily a zone of discontinuous permafrost. In general, permafrost here can be found on north-facing slopes (away from the warmth of the sunshine) and in valley bottoms (again, not enough warmth to melt out the seasonal frost). South-facing hills are free of permafrost and are favorite house building sites (you can drill a well for water here, which you cannot do on frozen ground). Sporadic permafrost can be found further, south, mostly in low-lying swamps where a thick vegetation mat insulates the ground, making it impossible for warm air to penetrate.

The maximum thickness of permafrost is over 5000 feet in Siberia and as much as 200 feet in Northern Alaska around Barrow and Prudhoe Bay. The thickness of the permafrost layer generally decreases toward the south; permafrost thickness around Fairbanks is about 90 feet. Like mentioned above, winter and summer temperatures determine the existence of permafrost in an area. Winter temperatures and ground insulation such as snow and vegetation determine the thickness of the permafrost. A thick vegetation mat and a relatively thick blanket of snow keep the cold from penetrating the ground deeply, such as in the Interior. A thin organic mat and very little snow coverage such as on the North Slope allows the cold to penetrate deeply. The only factor that keeps it from going deeper and deeper into the ground is the warmth radiating from the earth’s core.

The upper part of the permafrost is called active layer; it freezes in the winter and thaws in the summer. The active layer can be a few inches to a few feet thick. The frozen soil below prevents drainage of the meltwater and rainwater, resulting in ponds of standing water. This creates an important source of water in the dry subarctic environment (it is also very good for mosquitoes!).

Permafrost is kept in place by a balance of many factors. If either nature or humans disturb this balance, permafrost melts. A forest fire can destroy the insulating vegetation mat and without mat and without its protection the ground warms up. Human activities can also result in melting of the frozen ground. For example, disturbing the insulating vegetation mat through construction or simply digging a hole, or placing warm items such as buildings or a blacktopped road on top of permafrost can cause melting. Engineers have spent years to figure out ways to safely build on permafrost. The key is to keep the ground cold. This can be achieved by placing a thick layer of gravel between the heat source and the frozen ground, or in cases of many houses, build on piles so cold air can circulate under the building in the winter. Over a billion dollars was spent during construction of the pipeline is run above ground and the pilings holding it up use a special refrigeration system to keep the ground at their base cold.

Permafrost, by definition, does not imply the presence of ice in the ground. This is “dry permafrost” such as sand or bedrock with temperatures below freezing. Most permafrost areas, however, have ice in form of ice lenses or ice wedges associated with them. You could say the landscape ‘deflates,’ areas with the most ground ice will sink the deepest. Often, lakes are formed when permafrost thaws, so-called “thaw lakes”. These lakes can start as small puddles and grow more and more since liquid water has a high heating capacity and warms the surrounding ice-rich ground. When buying a house in a permafrost zone, you might want to check if that pretty lake next to your house is not a thaw lake. If it is, chances are the house will end up in the lake sooner or later.

When the permafrost thaws, you don’t always get a thaw lake forming. Often the ground will just appear lumpy and bumpy due to some areas being more ice-rich than others. If this thawing should occur in a forest, trees will lean ever which way. We call this a “drunken forest” and there is a good example of one at mile 24 on the south side of Denali Park road, just past Sanctuary River.

On slopes, the active layer often becomes saturated during the summer melt seasons. The saturated soil slides downhill, carrying the vegetation cover with it. This process of saturated soil flowing over permafrost is called solifluction. Denali has some good examples of those as well.