Exploring a mid-ocean-ridge rhyolite dome

April 4, 2015

Ryan Portner writes: Today we dove on the Alarcón Rise mid-ocean ridge. With few exceptions, mid-ocean ridges typically lie very deep beneath the sea surface. They are a locus of active volcanoes, hydrothermal vents and chimneys, and an abundance of life. Mid-ocean ridges occur where two tectonic plates move away from one another. As the plates spread apart, basalt lava comes up to the surface from deep in the Earth. Basalt is a silica-poor volcanic rock and forms the majority of the 72,420-kilometer (45,000 mile)-long mid-ocean ridge system.

The purpose of today’s dive was to explore a very unusual geologic feature—a mid-ocean-ridge rhyolite dome. Rhyolite is a silica-rich volcanic rock that typically forms on continental crust but not on mid-ocean ridges. In fact, the occurrence of rhyolite on the mid-ocean ridge system is so unusual that such a feature had never been discovered until this one was found by MBARI in 2012. Rhyolite has only been described from areas of the mid-ocean ridge system that are exposed above sea level, such as in Iceland and the Galapagos. In those areas, it is thought that unusually thick oceanic crust helps form the unusual rocks. Such an explanation cannot be the case for Alarcón Rise, which is by and large a normal mid-ocean ridge. Samples and ROV observations, guided by AUV maps collected by MBARI several weeks ago, will provide the necessary framework to understand how the Alarcón Rise rhyolite dome formed.

One of the challenges of working on the rhyolite dome is its highly rugged nature. During our dive we ascended and descended cliffs and overhanging walls composed of vast piles of very large boulders and blocks. The control room was buzzing with anticipation and excitement. Frequent communication between the scientists, the ROV pilots, and ship crew were essential in positioning the ROV to acquire samples and make key observations. It was a highly successful dive that will result in many hours of sample preparation and analysis back in the lab.

Contact between a very rugged rhyolite flow above a smooth, basaltic lava lobate sheet flow that flowed in around it.

Contact between a very rugged rhyolite flow above a smooth, basaltic lava lobate sheet flow that flowed in around it.


The surface of these rhyolite lava pillows is rough and corrugated. This is because the high silica content makes the rhyolite lava very viscous (pasty).

The surface of these rhyolite lava pillows is rough and corrugated. This is because the high silica content makes the rhyolite lava very viscous (pasty).


In contrast to the rhyolite lava pillows in the previous image, the surface of these basalt lava pillows is relatively smooth, and the pillows are narrower. This is because basalt lava flows more easily due to its lower silica content and viscosity. Bright yellow stalked crinoids are common on Alarcón Rise.

In contrast to the rhyolite lava pillows in the previous image, the surface of these basalt lava pillows is relatively smooth, and the pillows are narrower. This is because basalt lava flows more easily due to its lower silica content and viscosity. Bright yellow stalked crinoids are common on Alarcón Rise.


A sample of gravel-sized talus from a fault scarp on the rhyolite dome is collected with a scoop-bag.

A sample of gravel-sized talus from a fault scarp on the rhyolite dome is collected with a scoop-bag.


Ryan Portner sieves a gravel sample from the rhyolite dome. The gravel contains fragments from upslope lava flows, so will give us a wider sample of the eruptions that have occurred here than collections of individual rocks will.

Ryan Portner sieves a gravel sample from the rhyolite dome. The gravel contains fragments from upslope lava flows, so will give us a wider sample of the eruptions that have occurred here than collections of individual rocks will.


ROV Pilot Mark Talkovic sprays water on the spooled ROV tether to supplement the system that should be keeping it from overheating. Although 3,800 volts of electricity are sent down the tether, only about 3,000 volts make it to the ROV at the far end of the tether. That means a lot of heat generated in the process must be dissipated, especially while the ship is working in this hot climate.

ROV Pilot Mark Talkovic sprays water on the spooled ROV tether to supplement the system that should be keeping it from overheating. Although 3,800 volts of electricity are sent down the tether, only about 3,000 volts make it to the ROV at the far end of the tether. That means a lot of heat generated in the process must be dissipated, especially while the ship is working in this hot climate.

—Ryan Portner