This photo shows the marine debris laden shores of Kanapou Beach, an area in the main Hawaiian Islands where plastics and other debris accumulate.
Human-made products are not completely biodegradable. These products will take a long time, possibly hundreds of years, to degrade. Some products such as glass never degrade. To determine how long it will take for debris to degrade depends on several factors such as material type, size, thickness, and environmental conditions (e.g., amount of exposure to sunlight or location - on the beach or floating at sea).
While photodegradable plastics (plastics capable of being broken down by light) may break down from its first state (or created state), these plastics never completely degrade, but actually divide into tiny pieces called microplastics. Microplastics are the multi-colored pieces of plastic that can be found in a handful of sand on the beach or in the ocean. Scientists are still investigating the impact of microplastics on our ocean and marine life.
For more information:
NOAA Marine Debris Program
Diving Deeper Podcast, Episode 3 (Feb. 23, 2009) - What is marine debris?