JC24: Dating volcanoes on the Mid-Atlantic Ridge


About the cruise...

Dating volcanoes on the Mid-Atlantic Ridge

RRS James Cook cruise JC24, 23 May - 28 June 2008


This cruise on bpoard the RRS James Cook is attempting to answer one of the fundamental questions of marine geology: how fast does the crust grow?  60% of the Earth’s surface is covered by oceanic crust, which is formed at mid-ocean ridges as tectonic plates separate.  Studies of magnetic stripes – which record the episodic reversing of the Earth’s magnetic poles – tell us that, on the northern Mid-Atlantic Ridge, the crust grows at an average of just over 1 cm (about half an inch) per year.  But is this a steady rate, or does it grow in fits and starts? 

Previous studies have shown that right over the plate boundary along most of the Mid-Atlantic Ridge there are elongate ridges tens of kilometres long, a few kilometres wide and a few hundred metres high, composed of hundreds of individual volcanoes.  One such ridge would be about the size and shape of the Malvern Hills near the Welsh/English border.  The volcanoes in these ridges are formed as the plates grow from molten rock (magma) derived from the Earth’s upper mantle, beneath the crust.  Were all or most of the volcanoes constructed over very short periods of time, with periods of little or no activity in between?  Or will we find a continuous distribution of ages suggesting a steady state production of crust?  And what of their relations to each other?  Perhaps we will find that all the lavas have a similar chemical composition, suggesting they all came from the same batch of magma.  Or perhaps particular clusters of volcanoes formed together from one batch of magma , but are unrelated to their neighbours which formed from another.

By answering these questions, we will obtain a much fuller picture of how often and how rapidly the mantle melts and new crust is produced.

Why can’t we use magnetic reversals to date the individual volcanoes?  The problem is that the last magnetic reversal was 800,000 years ago, and volcanoes are born, grow and become extinct in less time than that.  We will be using a variety of ways to date them, including some novel ones!

The map on the right shows where we'll be doing our research - right in the middle of the Atlantic Ocean, on the Mid-Atlantic Ridge.

The first step on the expedition is to use TOBI (Towed Ocean Bottom Instrument - see pictures below) to build up images of the seafloor surface so that suitable sites can be selected for detailed study with Isis. TOBI produces sidescan sonar images, which look a bit like aerial photos on land; however, light only travels a very short distance in water, so instead TOBI uses sound to do the imaging.  The TOBI images will enable the scientists to identify areas where different types of volcano occur on the Axial Volcanic Ridge, and we will then investigate those in detail using Isis.

Isis is a Remotely Operated Vehicle (ROV), similar to those used for example to work underwater on structures such as oil platforms.  Isis has several video and still cameras, various other instruments, two robot arms that we will use for picking up rock samples, and an echosounder that will produce very high resolution topographic maps.  It is towed on a cable beneath the ship, and has its own on-board propulsion system so that its pilots can “fly” it around on the seafloor.  One Isis dive may last up to about 1.5 days.

Above, from left: TOBI; cartoon showing how TOBI works (click to enlarge); the ROV Isis

More on...

JC24 cruise diary (from 23 May 2008)
Other Classroom@Sea cruises
Plate tectonics and seafloor spreading
Seafloor surveying
National Oceanography Centre, Southampton
Dept. of Earth Sciences, University of Durham

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May 2008