About 11 per cent of an iceberg is visible above the waterâ€™s surface.

# Backyard bergs

Follow these instructions to make your own icebergs and learn about the buoyancy of ice.

• 25 May 2007 | Updated 9 November 2012

An iceberg is a floating piece of freshwater ice that has broken off the seaward end of a glacier or polar ice sheet. The separation or calving of icebergs from glaciers mostly occurs during spring and summer in Greenland and Antarctica.

Only about 11 per cent of an iceberg is visible above the water’s surface because ice is only slightly less dense than water. The ice only just floats; the larger part of the ice sinks beneath the surface. You can make your own backyard bergs to test how much ice floats above the water and how much hides underneath.

Note: This simplified activity only uses height, and not volume, to demonstrate that usually more of an iceberg is underwater than above water.

### What you need

• balloon
• plastic bag (zip-lock bags work well)
• rubber band
• large bowl, bucket or fish tank
• tray of ice-cubes
• ruler
• calculator.

### What to do

This activity involves making two icebergs: one in a balloon, and one in a plastic bag. You could make lots of other different shaped icebergs using a variety of balloons, bags and containers.

1. Fill the balloon with water until it is about the size of a grapefruit.
2. Tie off the end of the balloon and place it in the freezer.
3. Repeat steps 1 and 2 using the plastic bag, except this time, seal the top using the rubber band. Be careful not to overfill the bag.
4. Wait for 12 to 24 hours to fully freeze your icebergs.
5. Fill the bowl, bucket or fish tank with cool water.
6. Add the tray of ice-cubes and stir until they have melted.
7. Take your icebergs from the freezer and remove the balloon or bag.
8. Place the icebergs on the sink and measure the height of each one.
9. Place the icebergs in the bowl, bucket or fish tank and measure how much of your iceberg is floating above the water.
10. Put your results into this easy calculation: (height above water) divided by (total height) multiplied by 100 = percentage of ice above the water. The answer should fall somewhere between 11 per cent (1/9 of the height) and about 12.5 per cent (1/8 of the height) of your iceberg floating above the water. (Note: to make this calculation easy, only height is taken into account and not the volume of ice or water, which is more complicated).

### Colourful bergs

Most icebergs look white, because they are full of tiny bubbles. All wavelengths of visible light are reflected off the bubbles in the ice in equal amounts, which makes the iceberg look white.

Icebergs can also be blue, green, brown or black. In blue icebergs, the ice is compressed so much that the air bubbles are pushed out. Without air bubbles, blue light is reflected and the other wavelengths of light are absorbed. Sometimes icebergs can appear striped blue and white; this happens when crevasses in the parent glacier fill up with meltwater that refreezes so fast, that no bubbles form.

The unusual and vivid green of some icebergs is a result of algae growing in the ice. When you see a green iceberg, you are actually looking at what was once the underwater side of the iceberg. It has rolled over, exposing the previously underwater sections to view.

Brown or black icebergs are just dirty. Dust, rocks and dirt can accumulate in the glacier as it travels over the land. When an iceberg breaks off the glacier, it can have dirt layers deep within the ice giving it a brown or black appearance.

### Keeping track of icebergs

The US National Ice Centre monitors all icebergs and ice conditions in the Antarctic, Arctic, Great Lakes and Chesapeake Bay regions. The data is collected by polar orbiting satellites, and information about iceberg size and position is relayed to ships. Thanks to this early warning system, collisions with icebergs have become a very unusual occurrence.

By Philippa Rowlands

For more hands-on activities, join CSIRO's Double Helix Science Club.