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When rock is worn away by coastal erosion

or rainwater, weak areas tend to collapse while

the surrounding stronger rock survives. This

creates cave systems, which may extend for many

kilometres in limestone country, and contain

huge caverns and underground rivers. Flowing

water beneath glaciers can also erode caves

in the ice. In places like Hawaii, erupting

volcanoes may create lava tubes – tunnels that

once contained rivers of red-hot molten rock.

Caves

Delicate calcite crystals

have grown on the end

of this stalactite where

it enters a pool

Crystal clear water

reveals the banded rock

structure of this cave

182

This big stalagmite is

made of stony calcite

deposited by centuries

of dripping water

UNDERGROUND WATER

The water that creates limestone caves flows

through the cave systems as underground

streams and even rivers. In wet weather these

can fill the caves, eroding them into fantastic

shapes that are revealed as the water level

drops. In some limestone regions, such as the

Yucatan in Mexico, there is no surface water

at all because all the rivers flow underground.

In places they are open to the sky, forming

beautiful natural wells called cenotes.

CRYSTALS

Water dripping through cave

systems contains dissolved

minerals, such as calcite and

gypsum. If the water evaporates

or changes its chemical nature

slightly, the minerals may

become solid again, forming

crystals with glittering, faceted,

jewel-like shapes.

LIMESTONE CAVES

Rainwater dissolves carbon dioxide from

the air, turning it into weak carbonic acid.

In limestone country, the acidified rainwater

drains into the rock and dissolves it, creating

chains of potholes and caverns. Where water

containing dissolved rock drips from cave

ceilings, it leaves stony deposits that build

up into hanging stalactites, and stalagmites

that grow up from the cave floor.

COASTAL CAVE

On exposed rocky coasts, pounding waves

force water into cracks in the rock at such

high pressure that they blow the rock apart.

This cuts away the rock at water level. Often

the rock above collapses to form a sheer cliff,

but if the waves cut into a weaker seam they

can create deep caves and even rock arches.

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