Part One: Volcanoes- Volcanism at plate tectonic boundaries

Plate boundaries

Volcanoes are formed at plate boundaries of the earth’s crust. Plate boundaries mark the sites where two plates are either moving away from one another, moving toward one another, or sliding past one another. There are three types of boundaries:


1) Divergent plate boundaries -- Plates diverge from one another at the site of thermally buoyant mid-oceanic ridges. Oceanic crust is created at divergent plate boundaries.
→gives rise to Sea-floor spreading/ Spreading center volcanism

2) Convergent plate boundaries -- Plates converge on one another at the site of deep oceanic trenches. Oceanic crust is destroyed at convergent plate boundaries.
→gives rise to Subduction zone volcanism

3) Transform plate boundaries -- Plates slide past one another.
→ lack of significant volcanism

Additionally, Intraplate volcanism/ hotspot describes volcanic eruptions within tectonic plates.

Each of these three volcano-tectonic environments is depicted in the following diagram:

Fig 1.1. Volcanism at divergent and convergent plate margins.

Courtesy of USGS.

Volcanic Environments

1) Spreading Center Volcanism/ Sea-floor spreading

Spreading center volcanism occurs at the site of mid-oceanic ridges, where two plates diverge from one another. As the plates are pulled apart, hot magma rises up the earths’ crust.

As the hot asthenosphere rises to shallow levels, it decompresses and melts to produce basalt magmas. These magmas pond in crustal magma chambers where they are tapped by vertical fractures to allow the rapid rise of magma to the surface. Fissure eruptions often result in erupting basalt generating vast lava fields. The lava quenches quickly against the bottom waters to produce characteristic bulbous shapes called pillow basalt.

As basaltic lava erupts at the surface continuously for millions and millions of years, it is constantly accreted onto the edge of the spreading plates as it cools into a hardened basalt layer. The sea floor spreads and new oceanic crust is generated as a result, hence the process is commonly termed as sea-floor spreading. Oceanic crust is youngest near the ridge, but it becomes progressively older away from the spreading center due to divergence of the plates over time. This age progression is demonstrated in the image below.

Fig. 1.2. Age of the Atlantic oceanic crust. The crust near the continental margins (blue) is about 200 million years old. It gets progressively younger toward the mid-Atlantic ridge, where oceanic crust is forming today. Courtesy of NOAA.

2) Subduction Zone Volcanism

Subduction zone volcanism occurs where two plates are converging on one another. Subduction occurs when one plate containing oceanic lithosphere descends beneath the adjacent plate, consuming the oceanic lithosphere into the earth's mantle. As the descending plate bends downward at the surface, it creates a large linear depression called an oceanic trench.

The crustal portion of the subducting slab contains a significant amount of surface water, as well as water contained in hydrated minerals within the seafloor basalt. As the subducting slab descends to greater depths, it encounters greater temperatures pressures which cause the slab to release water into the mantle wedge overlying the descending plate. Water lowers the melting point of the mantle, causing it to melt. The magma produced varies from basalt to andesite in composition.

The magma rises upward to produce a series of volcanoes parallel to the oceanic trench, also known as volcanic islands. The chain of volcanic islands is called an island arc.

Fig.1.3. Formation of an island arc

If the oceanic plate subducts beneath continental plate, then a belt of volcanoes called a volcanic arc. E.g. the Cascade volcanic arc of the U.S. Pacific northwest, and the Andes volcanic arc of South America.

Fig.1.4. Formation of a volcanic arc

The most volcanically active belt on Earth is known as the Ring of Fire, a region of subduction zone volcanism surrounding the Pacific Ocean.

3) Intraplate Volcanism/ Hotspot

The interior regions of plates with voluminous volcanism are called hotspots. Most hotspots are thought to be underlain by a large plume of anomalously hot mantle. These mantle plumes appear to be generated in the lower mantle and rise slowly through the mantle by convection. They rise as a plastically deforming mass with a bulbous plume head fed by a long, narrow plume tail. As the head impinges on the base of the lithosphere, it spreads outward into a mushroom shape. Such plume heads have diameters between ~500 to ~1000 km. Decompressional melting of this hot mantle source can generate huge volumes of basalt magma.

Intraplate seamount chains can be attributed to volcanism above a mantle hotspot to form a linear, age-progressive hotspot track. Mantle plumes appear to be largely unaffected by plate movements. As lithospheric plates move across stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the mantle plume in the direction of plate movement. Thus, a linear belt of inactive volcanic islands and seamounts will be produced. E.g. The "Big Island" of Hawaii.

Fig.1.3 How Hawaii is formed above the stationary mantle plume, and becomes progressively older to the northwest.
Courtesy of the USGS.


:) Steph