Click to Enlarge Water can erode soil and rocks when it freezes and thaws. The best use of ice is as a fishing platform. Make sure the ice is safe before venturing out!   All About Ice by Tom Cwynar There's no limit to the wonders of frozen water. It might pop into your head some chilly winter afternoon that you are sliding or skating on the very same stuff that the fish below are swimming in. Ice is just frozen water, but it's nothing like water. It doesn't trickle or spray, and it doesn't move aside when you jump in it. Instead, it's as hard as your noggin. Ice is weird. Whack it with a hammer and it shatters, but if you apply steady pressure to it, it bends. If you freeze 100 liquid ounces of water, you'll end up with almost 10 percent more ice, but it will weigh the same. That's because water expands in volume when it freezes. It's one of the few substances on earth that acts like that. This and other unique properties of water make it possible for all life to survive. Glacial Ice Ice covers about 10 percent of the globe and about 8 percent of the land. If all the ice melted, the oceans would rise more than 200 feet. About 80 percent of the world's freshwater is locked up in ice. That's more than four times the amount of water in all the lakes and rivers on earth. Most of the ice on solid ground is contained in glaciers, which can be more than two miles thick. The ice built up from snowfall after snowfall that never melted. In some places, the weight of the ice has pushed down the land beneath as much as a half mile. During the last ice age, glaciers covered more than 30 percent of the total land area. Missouri was at the southern edge of the glaciers. Scientists believe Missouri and states to our north are still rising slowly after being pressed down by glaciers thousands of years ago. Even the hardest ice can flow. That's how glaciers travel. Gravity makes glaciers move downhill like extremely slow molasses. If the pressure above is great enough, they can even flow uphill and around ridges and big rocks. Sometimes glaciers pick up the pace-what's called a glacial surge. The glacier speed record is almost four miles in three months. That's about 15 feet an hour. Many glaciers end at an ocean, Big chunks of glacial ice fall into the water, where they ride ocean currents as icebergs. Icebergs float like ice cubes in your soda. They are much bigger underwater than above water. Ships have to avoid running into them. The Titanic sunk after scraping one. Pond and Lake Ice Missouri ponds and reservoirs sometimes ice over in the winter. They freeze at the top because that's where the water is coldest in winter. Once ice forms, it works like a blanket to insulate the water from the colder air. This keeps ponds and lakes from freezing from top to bottom. If the air is calm when the pond freezes, the ice cover is transparent-you can see through it! If it's windy when the pond freezes, the moving water keeps breaking apart new ice into a kind of slush that eventually freezes together into white or gray ice. As ice thickens, most of the buildup takes place on the bottom of the ice. Snow can also compact and form new ice on the top. Snow also insulates the water from the cold. Heavy snow can make the ice sag and weaken. Current slows ice formation, and when ice does form over current or near a spring, the ice is usually not as thick. Ice usually is thinner near the middle of the pond or lake than near the edge. In spring, however, sunlight may warm the banks, melting or thinning the near-shore ice. Current flow can also keep ice thin. And vegetation, docks or logs in the water may absorb warmth from the sun to cause early melting. Some people believe that no ice is safe to walk on. Most authorities agree that three inches of clear, solid ice will support someone walking, fishing or skating. Add another inch if you're with a group. Double the figures for slush ice or old ice. Better yet, stay off any ice that isn't solid. Because ice expands and contracts it can crack both when making new ice and when melting. On a very cold day the ice can crack a lot, but that doesn't mean the ice is weakening. If the cracks in the ice are dry, the ice is probably safe. When pond or lake ice melts it gets thinner, and it rots from the inside. The weakened ice cover can completely break apart and disappear from a lake in a few hours if the wind is blowing. Ice in the Ground Frost is frozen dew. At night, the ground cools faster than the air above the ground. When moist air contacts a freezing surface, such as the grass, the bicycle you left outdoors or the picnic table, it forms ice crystals. When the ground seems rock hard in the winter, it's usually because the water in the soil has frozen. In the far north, the frozen ground is known as permafrost, which means permanently frozen subsoil. Permafrost might be thousands of feet deep in some places. Remember how water expands when it freezes? When water in the soil freezes and thaws it can break up the soil and rocks. This mechanical weathering makes the ground more vulnerable to erosion. In moist areas like near rivers, needles of ice penetrate the soil, lifting it up and moving it around. Until the needles thaw, the ground will be crunchy underfoot. Water also finds its way into cracks in rocks. When it freezes, the expansion can split the rock apart. It's thought that ancient engineers used the freezing method to wedge off pieces of rock. The same process of freezing and expanding water accounts for frost heaves of the ground or pavement. Ice in the Sky Hailstones, created by water being swept up to colder levels of the atmosphere, are not the only ice in the sky. Our upper atmosphere contains many ice crystals that reflect solar radiation, keeping the planet from getting too hot or too cold. Scientists estimate that about 80 billion gallons of water are frozen into deep craters on the moon, and some of Jupiter's moons are covered in ice. In fact, some scientists speculate that most of the earth's water accumulated over billions of years as comets shed some or all of their ice in our atmosphere. Watch Water Expand Fill a plastic soda container about three-quarters full. Use a felt tip to mark the level of the water. Put the container-leave the lid off-in the freezer overnight. When you pull the soda bottle from the freezer the next morning, the ice level will be about 10 percent higher than the water level. More Ice Facts Frostbite is when the water in our tissues freezes. (Our bodies are about 60 to 75 percent water.) Dry ice is actually frozen carbon dioxide gas. Dry ice has a surface temperature of -109 degrees. When dry ice melts, it turns into gas. Black ice occurs on roads when rain strikes the pavement and freezes or snowbanks melt and the water freezes on the road. It's clear because the ice has no trapped air inside. Ice crystals expel brine as salt water freezes, leaving the ice pure. Eskimos get their fresh water from sea ice.