The Spectacular pillow-shaped rocks at Muriwai, on Auckland’s west coast, are an example of a volcanic product that is infrequently seen yet occurs in 70 per cent of the Earth’s crust. Pillow lavas are masses of interconnected tendrils of rock, looking something like Medusa on a bad-hair day. They result only when molten rock—at a searing 1200° C—meets the cold embrace of seawater. Having once foolishly leapt from a Finnish sauna into a lake while visiting Lapland, I shudder at the thought of such a transition. It is the extremely rapid chilling that gives pillow lavas their unique tangled appearance.
Usually the lava is exuded through vents in the ocean floor, but pillows can also form when a terrestrial river of lava flows into the sea. More rarely, a volcano may erupt beneath an ice sheet or glacier, the heat creating a meltwater pond into which lava is discharged.
Most pillows tell of submarine volcanic activity. Undersea volcanoes have long been known to exist. Early mariners sometimes spoke of mysterious churnings of the sea, followed by the violent emergence of entire islands from beneath the waves. In September 1952, the undersea volcano of Myojin began to erupt some 400 km south of Tokyo. Kaiyo Maru V was dispatched to investigate, and at 12.21 P.M. on September 24, as the vessel lay directly over the submerged crater, Myojin erupted. Thirty-one people were killed, and rescuers later found pieces of floating wreckage with lumps of rock embedded in them.
When an extremely hot fluid meets a cold fluid, a display of considerable violence might intuitively be expected but doesn’t There are several reasons for the placid nature of the event, one being that a thin layer of steam tends to develop at the interface of the two fluids. This effectively insulates the water from the lava. Similarly, a drop of water as it dances on the surface of a very hot frying pan is kept from touching the metal directly by a cushion of steam.always occur. The genesis of pillow lavas, in contrast to the eruption of a volcano like Myojin, is a surprisingly gentle affair. So gentle, indeed, that scuba divers were able to swim among actively forming pillows, and even touch them, when streams of lava entered the ocean during the 1969–74 eruptions of Mauna Ulu, Hawaii.So, how do pillow lavas assume their distinctive form? It seems that as a tongue of lava emerges from an undersea vent or feeder tube, the water cools its surface so rapidly that the crystals usually associated with terrestrial volcanic rocks have no time to grow. Instead, the outer few centimetres of lava are rapidly converted to volcanic glass, forming a rind. This insulates the lava within, enabling it to remain molten.
As more molten rock issues forth from the supply vent, the pressure within the glass-jacketed lobe of lava quickly mounts, causing it to rupture. Its contents spurt out a short distance before fresh glass seals the wound. In this way, the lava advances in sporadic bursts of briefly glowing crimson.
Sometimes, subsidiary lobes emanate from the original, like sausage meat escaping its skin on the barbeque. The glassy rind remains quite flexible, and therefore the concavi-ties of one pillow match the convexities of its neighbours, the whole arrangement somewhat resembling a pile of water-filled balloons.
Eventually, the supply of lava is exhausted and the insulation afforded by the glassy rind permits the remaining lava within the lobes to cool slowly, result‑ing in normal crystalline volcanic rock. The final formation is an interconnected mass of elongate basalt lobes “stacked” in a pile, each with a glassy black skin. The term pillow alludes to the appearance of the lobes when seen “front on” in the two-dimensional exposure of a cliff face—that is, from the direction in which the lava once flowed.
Volcanic output under the sea is three times greater than that on land, and there are thought to be over a million active submarine volcanoes. These are formed under a variety of tectonic circumstances, but most occur along mid-ocean ridges. These great rifts run like sutures across the ocean floor and are where new oceanic crust is produced. The upper portion of oceanic crust is usually formed of basaltic pillow lava overlain by a thin mantle of sediment. Pillow basalt is thus the most common form of volcanic rock on Earth.
In some places very old oceanic crust has been uplifted through continental crust, where it is known as ophiolite. The Dun Mountain Ophiolite Belt is a narrow ribbon of 280-million-year-old oceanic crust that runs sinuously from one end of New Zealand to the other. It is mostly buried beneath younger rock, but where visible it often contains pillow lavas produced on an ancient mid-ocean ridge. Two sites where very old pillows can be seen are beside Roding River, near Nelson, and at Bald Hill, in Southland.
Ophiolite is the “jam in the sandwich” where two bodies of rock formed on opposite sides of an ocean have been brought together by collisional tectonics. Its presence indicates that an entire ocean has closed up—a notion that, prior to the 1960s, would have been met with derisive laughter. However, since the emergence of plate tectonic theory, such happenings are regarded as commonplace in the history of the Earth.
Muriwai is considered one of the very best places in the world to see pillow lavas because so many aspects of their origin are on display. Viewing them makes for a pleasant escape from suburban Auckland, especially if one is armed with a copy of the excellent guidebook Ancient Undersea Volcanoes, by geologist Bruce Hayward.
Muriwai’s 17-millionyear-old pillows were not formed along a mid-ocean ridge. They owe their existence to a chain of volcanic islands that then stretched from the Manukau Harbour to the Hokianga, forming at the submerged base of one of these volcanoes.
The cliffs at Maori Bay contain several spectacular bulbs of volcanic andesite up to 50 m in diameter. They look like giant fans because fractures radiate from their centres. Each fan provides a cross-sectional view of a feeder tube that once supplied fresh lava to the pillows that surround it. The fractures formed at the conclusion of proceedings: as the remnant lava within the feeder tubes cooled and contracted, it cracked with geometrical precision.
The pillows themselves range in diameter from 0.5 to 3 m. With their glassy rinds and radial fracture patterns, they resemble the petrified blossoms of some hideous plant, but this is only because the cliffs happen to have formed in such a way that the lava lobes have been sectioned more or less at right angles to the direction in which they grew. Further south, at Collins Bay, pillows can be seen side on, giving a much clearer view of their lobate form.
The enormous feeder tubes were in turn supplied with lava by steep fissures that extended to great depth. When the supply from beneath ceased, the lava within these fissures solidified to form dykes. Examples of such dykes can be seen in the bays south of Muriwai. Protruding from their upper terminations are really big pillows, up to 5 m in diameter, that are thought to have formed as lava was discharged into water-saturated seafloor sediment. The sediment is believed to have provided support for the growing pillows, enabling them to attain their larger than usual size. Because dykes are wall-like features, it is probably more accurate to say that these big pillows resemble long rolls that formed where a fissure opened into the sediment.
Pillows can also be seen elsewhere in New Zealand. The Red Rocks Scientific Reserve, near Wellington, contains pillows that are among the oldest rocks in the North Island, having formed some 250 million years ago. At Pakotai, in Northland, 1400 tonnes of copper ore was mined from among pillows between 1947 and 1951. Pillows are frequently associated with rich ore deposits because massive hydrothermal systems associated with submarine volcanoes commonly introduce metals into the rocks as they form. At Cape Wanbrow, Oamaru, spectacular pillows set in cream-coloured limestone lie beneath deposits of more explosive origin that attest to a volcanic eruption of Myojin’s fury. This occurred 34 million years ago, but lasted only weeks, and the pillows visible today represent the more placid early stages of the volcano’s development.