This section is divided into 2 parts; Hydrothermal Systems and Oceanic volcanism

Oceanic Volcanism

Journey to the centre of the earth

In the section on plate tectonics we have looked at some aspects of ocean volcanism eg the formation of spreading ridges and island arcs – in this section we will discuss in more detail the various oceanic environments where magmatic activity occurs and investigate the kind of rocks produced. Volcanic eruptions on the surface of the earth are driven by melting at depth - to produce magma. Magma types are termed mafic, intermediate, or felsic depending on their silica (SiO2) content.

Ocean spreading ridge volcanism

Oceanic spreading ridges are the most active volcanic zones on earth, continually producing new oceanic lithosphere. Our understanding of the composition of the oceanic lithosphere comes from:

1. Collecting samples from the sea floor,
2. seismic studies that show layers of different velocity. These layers have been subdivided into 4 – the seismic velocity increases from layer 1 (sediments) at the top to layer 4 (mantle) at the base.
3. cores collected by the Ocean Drilling Program and
4. sections of the oceanic lithosphere that have been pushed up onto the continents. These sections are termed ophiolites. One of the most famous and best studies ophiolite complexes is the Semail ophiolite, which comprises the Oman Mountains in northern Oman (virtual tour at http://www.bris.ac.uk/Depts/Geol/vft/oman.htm). But ophiolites can also be found closer to home in Papua New Guinea, New Caledonia and New Zealand and on Australias' own MacQuarie Island.

The top of the idealized ophiolite sequence consists of ocean sediments (eg chert, limestone etc). Below this are pillow basalts - tholeiite, which form when hot magma is extruded onto the ocean floor. Pillow lavas are the most abundant volcanic rock on earth. They are often altered to serpentinite by interaction with hot seawater. Sheeted or intruded dikes occur below the pillow basalts. These dikes represent the feeders to the subaqueous pillow basalts. Underlying the basalts is a layer of intrusive rock called gabbro. Gabbro is similar in composition to basalt but because it cools more slowly it has bigger crystals. The upper part of the gabbro is generally massive, but the basal part of the gabbro commonly contains layers. The layers (called cumulates) are composed of the crystals that form first and sink to the bottom of the magma. The contact between the gabbro layer and an ultramafic rock called peridotite, marks the geophysical base of the crust known as the Moho. Here the density contrast causes a marked attenuation in seismic velocity.

< Pillow lava on the sea floor (photo USGS)

< Late magmatic liquid (white) intrudes a foliated gabbro. This core was retrieved during ODP Leg 118 on the Southwest Indian Ridge

< On ODP Leg 147 (Hess Deep Rift Valley in the Pacific Ocean). Core reveals black peridotite impregnated with white magmatic liquid.

Trench related volcanism; island arcs and mountain chains

As we noted in the section on convergent boundaries, in the process known as subduction, one plate descends beneath another. Subduction zones are characterised by the formation of deep ocean trenches and the growth of chains of volcanoes on the overriding plate, parallel to the trench. The so called Ring of Fire, a region of subduction zone volcanism surrounds the Pacific Ocean.

In the past people thought that volcanoes were caused by water seeping into underground caverns, We now know this is not the case, but water does play an essential part in the formation of trench related volcanoes. The subducting lithosphere contains a lot of water – in a layer of waterlogged sediment and also in hydrated minerals within the seafloor basalt. As the subducting slab descends it encounters greater temperatures and greater pressures which cause the release of water into the overlying mantle wedge. This water has the effect of lowering the melting temperature of the mantle and generating water -rich viscous magmas that produce explosive eruptions. The rock type commonly formed contains more silica than basalt and is called andesite (named after the Andes).

< Andesite lava flow Brokeoff Volcano, California (USGS photo gallery)
The linear belt of volcanoes parallel to the oceanic trench is called an island arc (see the Bismark volcanic arc that has formed behind the New Britain Trench north of Australia). If the oceanic lithosphere subducts beneath an adjacent plate of continental lithosphere, then a similar belt of volcanoes will be generated on the continental crust. This belt is referred to as a volcanic arc, and examples include the Andes volcanic arc of South America.

Mantle Plume (hot spot) volcanism

While most volcanism coincides with the location of plate boundaries there are many well know regions of volcanism that occur no where near a plate boundary –like the Hawaiian volcanoes and Australia’s only active volcano on Heard Island. These are examples of hot spot volcanism (also sometimes referred to as intraplate volcanism).

Hot spots are caused by a rising plume that is thought to originate near the core-mantle boundary. The formation of mantle plumes is not fully understood, but they appear to be part of the Earth’s mantle convection system. Volcanoes form on the plate above the hot spot. Because the plates are moving over the hot spot, new volcanoes form and the old ones, which no longer overlie the hot spot, become extinct. In this way hot spot trails are formed.

< The Hawaiian Island – Emperor Seamount chain have formed above a hot spot. The hot spot has remained stationary for the last 70 million years. However during this time the Pacific Plate has moved over it, carrying with it the old volcanoes that formed above the hot spot. You can calculate the history of plate movement by determining the age of the rocks in the hot spot chain.

In some cases hot spot volcanism can produce huge amounts of magma and form what are termed large igneous provinces (LIP). One such LIP, Kerguelen Plateau – Broken Ridge, was recently studied by the Ocean Drilling Program (Leg 183). They discovered that the earliest volcanism associated with the Kerguelen hot spot began about 130 million years ago when Africa, Antarctica, Australia, India and Madagascar were just beginning to break apart, leading to the formation of the Indian Ocean. Northward movement of the Indian plate over the hot spot plume formed a 5000-km-long hot spot track known as the Ninetyeast Ridge. Changing tectonic plate motions about 40 million years ago resulted in the Antarctic Plate moving over the hot spot. Since then the Kerguelen Archipelago, Heard and McDonald Islands, and a chain of submarine volcanoes between these islands have been constructed The Kerguelen hot spot continues to erupt today at Heard and McDonald islands.

Basalts formed at hot spots are similar to those formed at mid ocean ridges but they tend to have less silica higher concentrations of Na and K. Consequently they are often called alkali basalts.

Hydrothermal Systems