When magma erupts beneath the ocean’s surface, a process known as submarine volcanism begins, creating unique rock formations. The deep-sea environment subjects the molten rock to immense hydrostatic pressure and a dramatic temperature difference with the surrounding seawater. The cold water acts as a coolant, causing the lava to solidify much faster than an eruption exposed to air. This rapid chilling determines the texture and structure of the resulting extrusive igneous rocks, which form the vast majority of the oceanic crust.
Primary Result: Pillow Basalt
The most common rock type to form from deep-sea eruptions is basalt, a dark, fine-grained rock with a mafic composition, which takes on a distinctive physical structure known as pillow lava. This structure occurs when the erupting lava contacts the cold seawater, instantly forming a solid, glassy crust around the molten interior. As more lava pushes through this rind, it inflates and breaks out to form a new, bulbous lobe. This process leads to a stack of interconnected, pillow-shaped masses, typically ranging from a few centimeters to a meter in diameter.
The rapid quenching by the water results in a glassy margin, often composed of volcanic glass called sideromelane, which is the identifying feature of pillow basalt. Beneath this glassy exterior, the remaining molten material cools more slowly, resulting in a fine-grained interior where small crystals have time to form. This internal structure, along with the radial fractures that develop as the lava shrinks during cooling, provides evidence of an underwater formation environment. Pillow basalts are the most abundant type of lava flow on Earth because they make up the majority of the oceanic crust.
Fragmentation Products: Hyaloclastite
While coherent pillow basalts are the primary result of submarine volcanism, the interaction between lava and water also produces fragmented material called hyaloclastite. Hyaloclastite is a volcaniclastic rock, consisting of angular shards of volcanic glass. This material forms when the thermal shock of the lava contacting the water is so intense that the rapidly chilled, brittle glass rind shatters.
The fragmentation is often non-explosive, occurring as the newly formed glassy crust shrinks and spalls off due to thermal stress. However, in shallower eruptions where water pressure is lower, the magma’s volatile gases and superheated steam can cause explosive fragmentation, known as a phreatomagmatic eruption. These glassy fragments, which are typically angular and range in size from a millimeter to a few centimeters, accumulate between the pillow basalts, forming an interpillow matrix. This glassy material often alters into a yellowish-brown mineraloid called palagonite through reaction with seawater.
Global Significance and Formation Zones
The formation of pillow basalts and hyaloclastites is linked to the planet’s largest geological features: the mid-ocean ridges. These divergent plate boundaries, where tectonic plates move apart, are the sites of constant magmatic activity and represent the primary zones of new oceanic crust generation. Approximately three-quarters of all volcanic activity on Earth occurs as deep, underwater eruptions along these global rift systems.
The outpouring of basaltic lava at these spreading centers forms the upper part of the oceanic crust, often referred to as Layer 2, which is composed almost entirely of pillow lavas. When sections of this oceanic crust are thrust up and preserved on land, they are known as ophiolites. The presence of pillow basalt within them is a definitive marker of an ancient subaqueous environment.