Oceanographic researchers often use a simple Styrofoam cup as an illustrative tool to demonstrate the immense forces found in the deep ocean. When a cup is attached to a deep-sea submersible or remotely operated vehicle (ROV) and sent thousands of meters beneath the surface, it returns dramatically reduced in size. This physical transformation, where the cup retains its original shape but is miniature, is a direct result of the extreme hydrostatic pressure encountered in the ocean’s abyssal zones. This phenomenon is a tangible example of how everyday materials react to the crushing weight of the water column.
What Makes Styrofoam Vulnerable?
The material commonly referred to as Styrofoam is technically Expanded Polystyrene (EPS), a lightweight plastic foam. EPS is not solid, but rather a matrix of small, interconnected beads fused together during manufacturing. Its physical structure is defined by its closed-cell composition, meaning the material is largely composed of tiny pockets of trapped gas.
EPS is overwhelmingly air, typically comprising 95% to 98% of its total volume. This high percentage of air, contained within numerous closed cells, makes the material highly susceptible to changes in external pressure. The foam has significant internal empty space that can be manipulated by external force.
The polymer walls of the polystyrene matrix are thin and inherently flexible, designed to hold the air pockets at standard atmospheric pressure. While this structure is strong enough for surface uses, it offers almost no resistance to the deep ocean’s crushing environment. The gas-filled voids make the material’s bulk density extremely low compared to solid polystyrene.
Understanding Hydrostatic Pressure
The force responsible for the cup’s shrinkage is hydrostatic pressure, which is the pressure exerted by a fluid at rest due to the force of gravity. In the ocean, this pressure increases linearly with depth because the total weight of the water column above any given point increases. At the sea surface, the pressure is approximately one atmosphere (atm), or 14.7 pounds per square inch (psi).
For every 10 meters a submersible descends, the hydrostatic pressure increases by roughly one additional atmosphere. This pressure gradient leads to astonishing forces at deep-sea exploration depths. For instance, at 3,000 meters, the pressure exceeds 300 atmospheres, equivalent to over 4,400 pounds pressing on every square inch of the cup’s surface.
This uniform force presses in on the cup from all directions simultaneously. This isotropic compression affects the object’s entire surface area, which is why the cup shrinks evenly instead of collapsing or crumbling. The pressure at 3,800 meters, the average depth of the ocean, is approximately 380 times greater than the pressure felt at sea level.
The Process of Densification
As the Styrofoam cup is lowered, the increasing hydrostatic pressure acts directly on the closed-cell structure, initiating a process called densification. The external force begins to crush the numerous air pockets trapped within the Expanded Polystyrene matrix. This physical action is governed by Boyle’s Law, which states that for a fixed amount of gas held at a constant temperature, the gas volume is inversely proportional to the pressure applied.
The air inside the cells is compressed significantly as the pressure mounts, causing the volume of the gas to decrease dramatically. This compression forces the flexible polystyrene cell walls inward, eliminating the empty space that previously made the material so light. The process is complete when the cup reaches its maximum depth, resulting in a miniature version of the original.
The final product maintains its original shape but is often reduced to one-quarter or one-eighth of its initial size. Because the air has been forced out and the polymer matrix has been permanently compressed, the shrunken cup becomes notably denser, harder, and heavier than it was at the surface. The physical rearrangement of the polymer chains under pressure is irreversible, meaning the cup will not re-expand when returned to the surface.