Glac­i­ers are more than fan­cy words for snow­fields. In the moun­tains, they’re the main obsta­cles to reach­ing peaks. In Alas­ka, they calve ice­bergs into the sea and have ori­gins in ice­fields the size of Rhode Island. And they’re melt­ing, which will have some big rip­ple effects that Al Gore has warned us about. And what are ter­mi­nal and lat­er­al moraines any­way? Here’s what hap­pens to a snow­field when it comes alive and starts to move.

A Cold, Cold River
The mag­ic hap­pens when enough snow accu­mu­lates on a down­ward fac­ing slope, which grav­i­ty then starts to move it all down­hill. A glac­i­er is sim­ply a frozen riv­er. When ice accu­mu­lates to a cer­tain thick­ness, the weight of all that ice, com­bined with grav­i­ty, caus­es the bot­tom sec­tion of the glac­i­er to flow and slip along the sur­face. The result is a riv­er that grad­u­al­ly flows down­hill, at rates that may be mea­sured inch­es, or in the case of Glac­i­er Bay’s mod­er­ate­ly hyper­ac­tive Johns Hop­kins Glac­i­er, as much as 8 feet a day. As it flows down­hill, the glac­i­er hits warmer tem­per­a­tures, and the ter­mi­nus starts to melt. The rates of flow and the rate of melt deter­mine whether a glac­i­er advances or retreats. And like any riv­er, glac­i­ers have steep sec­tions and calm ones.

Crevass­es and Ice­falls and Ser­acs, Oh My!
When the glac­i­er moves down­hill, the brit­tle upper sec­tion that can’t bend ends up crack­ing. The results are crevass­es, which climbers strug­gle to cross and dread falling in. Ser­acs are ice tow­ers that form below steep sec­tions, which form ice­falls: the glacial equiv­a­lent of a water­fall. These are par­tic­u­lar­ly treach­er­ous for climbers, because they’re both dif­fi­cult ter­rain and the most prone to shift­ing, avalanch­es, and tum­bling chunks of ice and rock. The most noto­ri­ous is the Khum­bu Ice­fall on the approach to Everest.

Exam­ple of a Pied­mont Glac­i­er, Chugach Wrangell-St. Elias Moun­tain Range Glacier

Beware the Bergschrund
At the top of every glac­i­er is the bergschrund (Ger­man for “moun­tain cleft”) a large crevasse that marks where the glac­i­er breaks away from the head­wall or snow­field to begin flow­ing down­hill. If it were a riv­er, this would be the head­wa­ters. Bergschrunds are big, deep, and hard to cross. On many climbs, espe­cial­ly ice cas­cade stra­to­vol­ca­noes, they’re one of the biggest obsta­cles. The lat­er the sea­son, the wider the bergschrund yawns and the hard­er it is to cross.

Wet Win­ters, Cool Summers
If you look at a map of the most glaciat­ed moun­tains, one thing is clear: they’re near the sea, even though many coastal moun­tain ranges are low­er than the Rock­ies. Glac­i­ers form when snow accu­mu­lates rapid­ly in wet win­ters and doesn’t melt much in cool sum­mers. The moist marine air that hits the coast of Alas­ka, British Colum­bia, and the Cas­cades of Wash­ing­ton and Ore­gon, as well as Patag­o­nia and the mon­soon sweep­ing over the Himalayas are cas­es in point: we get the mas­sive glac­i­er-cov­ered Cas­cade vol­ca­noes, and the tide­wa­ter glac­i­ers of Alas­ka and Patag­o­nia and the Himalayan ice fields. Dri­er places may be cold­er, but unless they also get enough pre­cip­i­ta­tion, the glac­i­ers will be small­er, unless we’re talk­ing Antarctica.

3 Types: Val­ley, Pied­mont, Tidewater
Val­ley glac­i­ers are the ones we see in most of the moun­tains in the low­er 48: they form near the top of peaks, often where snow accu­mu­lates below a moun­tain head­wall, where it’s pro­tect­ed from the wind. Over time it accu­mu­lates enough weight to start flow­ing, and flows down­hill, carv­ing a val­ley. Most end when they reach low­er alti­tudes, releas­ing silt-laden tor­rents down steep valleys.

If a val­ley glac­i­er flows out of the moun­tains onto a flat plain, it does just what a riv­er would: spread out into a wide, flat sheet called a pied­mont glac­i­er. The most famous of these is the Malaspina Glac­i­er in Alaska’s Wrangell St. Elias Nation­al Park.

But the most dra­mat­ic are tide­wa­ter glac­i­ers, when glac­i­ers flow all the way to the sea, calv­ing ice­bergs into the ocean. The most famous of these are in Alas­ka: Glac­i­er Bay, Prince William Sound, and Kenai Fjords, but they exist else­where in the world: Green­land, south­ern South Amer­i­ca and else­where. Tide­wa­ter glac­i­ers act like “glac­i­ers on steroids, advanc­ing and retreat­ing faster than most oth­er glaciers.

Blue, Blue Baby I Love You
Glacial ice is an enchant­i­ng shade of deep blue-turquoise. You’ll notice it most on cloudy days, or when an ice­berg breaks off. Because glacial ice is super-com­pressed from mil­len­nia under the weight of all that oth­er ice, the gas­es have been squeezed out of it, unlike the clear ice in your freez­er. This dense ice absorbs the rest of the col­ors of the spec­trum, so blue is what’s left for you to see.

Exam­ple of a Tide­wa­ter Glac­i­er, Colum­bia Glac­i­er, Colum­bia Bay, Valdez, Alaska

The Moraine Drag
Glac­i­ers are enor­mous bull­doz­ers push­ing mas­sive piles of rock and soil in slow motion. On the sides and end of the glac­i­ers, you’ll find ridges of rocks and debris from pre­vi­ous advances and retreats. The ones on the sides are called lat­er­al moraines; at the end of the glac­i­er, they’re called ter­mi­nal moraines. Where two glac­i­ers meet, they form a stripe of debris in the mid­dle of the new glac­i­er, called medi­al moraines. The land­scape around glac­i­ers is full of past moraines, some exposed rock and grav­el some over­grown with vegetation.

Is Al Gore Right?
Yes, he is. Most glac­i­ers in the world are melt­ing faster than they are accu­mu­lat­ing new ice. As a case in point, North Sister’s Col­lier Glac­i­er has been steadi­ly shrink­ing. Cli­mate change is very real. So are the effects of melt­ing glac­i­ers. Small­er glac­i­ers mean an increas­ing risk of sum­mer low flows, mean­ing less water for crops, peo­ple, and fish down the road. And the more glac­i­ers melt, the more the earth warms, because the white sur­face of ice reflect heat, while dark rocks absorb it. And as the vast glac­i­ers of Green­land melt and release cold water into the Atlantic Gulf Stream, a warm cur­rent that keeps Europe fer­tile for its lat­i­tude. Cli­mate change could hit the UK par­tic­u­lar­ly hard.

Dan­ger­ous but Beautiful
Glac­i­ers are one of the most stun­ning­ly beau­ti­ful things on earth. Their col­or and shape are amaz­ing, espe­cial­ly when seen at sun­rise on a high peak. The blue of crevass­es, the strange forms of ser­acs and the inde­scrib­able col­ors of melt­wa­ter ponds are one thing. But a few hours spent on an Alaskan glac­i­er will reveal that they have their own moun­tains, lakes, canyons, and rivers. Need­less to say, glac­i­ers should be approached with skill, care, and aware­ness of the con­di­tions. But there are few more irre­sistible things on earth.