Stack (geology)

A stack or sea stack is a coastal geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by wave erosion. Stacks are formed over time by wind and water, processes of coastal geomorphology.

Stacks form when a headland is progressively eroded by hydraulic action and abrasion. The force of the sea exploits structural weaknesses in the rock, such as joints, bedding planes, and faults, causing cracks to expand into caves and eventually collapse into free-standing pillars. Coastal stacks provide critical predator-free nesting sites for seabirds, and many are prominent destinations for rock climbing.

Isolated, steep-sided oceanic islets typically of volcanic origin are also loosely referred to as "stacks" or "volcanic stacks".

Formation

Stacks typically form in horizontally bedded sedimentary or volcanic rocks, particularly on limestone and chalk cliffs. The medium hardness and compressive strength of these rocks provide moderate resistance to marine erosion, while a harder overlying layer may act as a protective capstone. Cliffs made of weaker materials, such as claystone or heavily fractured rock, tend to slump and erode too rapidly to maintain vertical pillars, whereas harder rocks such as granite erode through different structural mechanisms.

The formation process follows a sequence of coastal landform progression:

Fissure exploitation: The formation process usually begins when the sea attacks structural weaknesses, such as steep joints or small fault zones in a cliff face.