Oceanic crust in all its glory

One of the world’s best exposures of pillow lavas graced the cover of Geotimes, EARTH’s predecessor, in 1975. Credit: American Geosciences Institute. One of the world’s best exposures of pillow lavas graced the cover of Geotimes, EARTH’s predecessor, in 1975. Credit: American Geosciences Institute.
The classic ophiolite sequence is a cross section through both the upper mantle and the oceanic crust. Credit: K. Cantner, AGI. The classic ophiolite sequence is a cross section through both the upper mantle and the oceanic crust. Credit: K. Cantner, AGI.

Our loop drive through the high Hajar Mountains boasted excellent exposures of every oceanic crustal layer. One good exposure of the sequence of mantle rocks is found in the hills surrounding Oman’s capital city Muscat. They consist of a variety of peridotite called harzburgite, which is the pyroxene-depleted residue left behind after basaltic magma is extracted. That extracted magma forms the oceanic crust, whose characteristic rock sequence is, from bottom to top: 1) dark, layered gabbro overlain by massive gabbro (the intrusive equivalent of basalt that comprises solidified magma chambers); 2) sheeted basaltic dikes (the conduits that transported lava from the magma chambers up to the ocean floor); and 3) bulbous pillow lavas, which form when basalt erupts directly into seawater.

Even after seeing this fantastic sequence, we were compelled to make a detour to Wadi Jizzi, a two-hour drive north of Muscat, in search of what is quite possibly the world’s most astounding outcrop of pillow lavas: the famed “Geotimes” outcrop, so named after its photo graced the cover of Geotimes (EARTH’s predecessor) in 1975. The stunning outcrop features tube-shaped blobs of basalt, each taller than a person, intertwined with one another like a mass of wriggling black worms. The pillows form when seawater quenches the exterior of the submarine lava flows while the hot interiors continue to expand as they fill with lava supplied by the sheeted dikes below. A similar process is creating pillow lavas along the edge of Hawaii’s Big Island today.

Terri Cook and Lon Abbott

Terri Cook (www.down2earthscience.com) is a science and travel writer based in Colorado and an EARTH roving correspondent. Lon Abbott is a geology professor at the University of Colorado at Boulder.

Sunday, May 6, 2018 - 06:00