Christmas Island is the peak of a basalt volcanic seamount which rose steeply 5,000 metres from the ocean floor about 60 million years ago. The highest point of the island is Murray Hill, which rises to 361 m above sea level.
The island’s emergence is a result of a series of geological uplifts over 10 or so million years. At each stage layers of coral reefs built up over the basalt core, leaving today an almost continuous limestone cap. The ocean eroded cliffs at each uplift, forming the stepped terraces and inland cliffs of the island’s central plateau.
Evidence of the island’s volcanic origins can be seen at The Dales and at Dolly Beach — where the underlying volcanic basalt is exposed.
The island has geologically significant subterranean cave systems, including anchialine cave systems (a subterranean water body with connections to the ocean), which provide habitat for endemic fauna, and its geological features are significant for illustrating geological and evolutionary processes.
The soils of Christmas Island are derived from two sources - limestone (terra rossa soils) or basaltic extrusive rocks (krasnozem soils).
The island's 73 km coastline is an almost continuous sea cliff ranging in height up to 20 metres. At 13 places, breaks in the cliff give way to shallow bays and small sand and coral beaches. The largest of these bays forms the island's port at Flying Fish Cove.
Although it has a high rainfall, Christmas Island lacks permanent surface water. A few perennial streams flow at the Dales, Ross Hill Gardens, The Ravine, Jones Spring, Freshwater Spring, Dolly Beach, Hosnies Spring and Waterfall.
Nearly all rainfall goes quickly underground to join a karst drainage system. Underground water accumulates in the caves and sinkholes at the interface of limestone and the underlying volcanic rock strata. Here it either flows along the interface, or flows down fractures in the volcanic rock. The flows along the limestone-volcanic rock interface emerge in some places as springs.