HSPT Practice Test 10

¹From the deck of a ship in polar waters, sea ice and icebergs can look like the same thing: white, cold, and floating. In fact, they form in very different ways, and the difference matters to scientists who study the oceans.

²Sea ice forms when the surface of the ocean itself freezes. Because seawater is salty, it must cool to about 28 degrees Fahrenheit, a few degrees below the freezing point of fresh water, before ice crystals begin to form. As the crystals grow, they push most of the salt out. The salt left behind makes the surrounding water saltier and therefore denser, so that water sinks toward the ocean floor. This sinking helps drive slow currents that carry water around the globe. Sea ice is far thinner than most icebergs, and much of it melts each summer and refreezes each winter.

³An iceberg, by contrast, begins on land. Snow that falls year after year on a glacier is packed into thick ice. The glacier creeps slowly toward the coast, and where it meets the sea, chunks break off and fall into the water. Scientists call this breaking-off process calving. Because the ice formed from snow, an iceberg is made of fresh water, not salt water. Some icebergs tower hundreds of feet above the waves, yet roughly nine-tenths of an iceberg's mass lies hidden below the surface.

⁴The two kinds of ice also affect sea level differently. When floating ice melts, the water level barely changes, just as the level in a glass of iced water hardly rises as the ice cubes melt. The floating ice was already displacing its own weight of water. Ice that sits on land is another matter. When a glacier sheds icebergs or melts, water that had been stored on land is added to the ocean, and sea level rises.

⁵Both kinds of ice reflect sunlight back into space and help keep the polar regions cold. When either one shrinks, darker ocean water absorbs more of the sun's heat, which can lead to further melting.