Chapter 5, Problem 1GST

(a)

To determine

Whether the Crater Lake is connected with a divergent boundary, convergent boundary, or intraplate volcanism.

Answer to Problem 1GST

The Crater Lake is related to a convergent boundary.

Explanation of Solution

The convergent boundary envisages the subduction of the denser oceanic slab below the overriding plate whose outcome is the partial melting of the mantle rocks lying above, triggered by the rising volatiles derived from the rocks of sinking plate subjected to high temperature and pressure. The hot melt thus produced forms the magma that ascends due to buoyancy through the crust.

Many volcanic eruptions are related to composite cones whose summit collapse produces deep and large calderas. Composite volcanoes are produced from the convergence of an oceanic plate and a continental plate.

When the crater collapses down on losing the previously existing support as a result of an explosive eruption just like the one encountered by Mount Mazama in Oregon, a caldera is created, which when filled with water forms the Crater Lake.

(b)

To determine

Whether Kilauea is associated with a divergent boundary, convergent boundary, or intraplate volcanism.

Answer to Problem 1GST

The shield volcano Kilauea of Hawaii is associated with an intraplate volcanism.

Explanation of Solution

The caldera seen on a shield volcano such as Kilauea is created in response to the loss of fluid lava from the magma chamber beneath during eruption and consequent collapse of the volcano’s peak slowly in the absence of a support that once existed. Kilauea belongs to Hawaii, which is an area of mantle plumes and hotspots. Thus, it may be associated with intraplate volcanism.

The voluminous outbursts of fluid basaltic lavas away from the adjacent plate boundary may result in intraplate volcanism. The column of hot, melted mantle rocks ascending to the surface, deep from the core-mantle boundary, called as mantle plumes, are believed to be serving as magma in intraplate volcanism. The surface expression of the decompression melting at the mantle plume is called as a hot spot.

The plumes are hotter and more buoyant than the nearby mantle materials, and thus acts like lava lamps with two immiscible liquids where the denser liquid at the bottom on heating rises as blobs due to buoyancy. Mantle plumes resemble blobs having a globular head and a slender tail when it ascends. This head on reaching close to the lithosphere triggers the decompression melting of overlying rocks and hence brings forth heavy outbursts of lava for a period of $1\text{million}\text{years}$.

(c)

To determine

Whether Mount St. Helens is associated with a divergent boundary, convergent boundary, or intraplate volcanism.

Answer to Problem 1GST

Mount St. Helens is related to convergent plate boundary.

Explanation of Solution

North America visualized its largest volcanic eruption on 18^th May, 1980 when Mount St. Helens erupted so violently blowing out the whole northern fringe of the volcano. The eruption lowered the volcano that had its head more than $2900\text{meters}$ above the sea level by $400\text{meters}$.

Mount St. Helens is located in the Ring of Fire.

The Ring of Fire known for active volcanism encloses majority of the world’s active volcanoes. The Ring of Fire lies in the circum-Pacific belt that is the convergence zone where the denser oceanic plate subducts beneath the less dense plate. Thus, Mount St. Helens is concerned with a convergent plate boundary.

(d)

To determine

Whether East African Rift is associated with a divergent boundary, convergent boundary, or intraplate volcanism.

Answer to Problem 1GST

The East African Rift is related to divergent plate boundary.

Explanation of Solution

The divergent boundaries are characterized by the movement of plates away from each other causing the upwelling and melting of the mantle materials leading to the construction of a new seafloor. They are also known as spreading boundaries or constructive boundaries.

The volcanism concerned with the divergence of plates involves rocks of basaltic composition. As the plates get separated away, the confining pressure on the rocks declines causing the decompression melting of the mantle rocks lying below the diverging plates. This gives rise to the hot melt or magma of basaltic composition that buoyantly rises toward the rift to produce the new oceanic ridge.

Though most of the spreading centers are situated along an oceanic ridge axis, there are some cases of exceptions where continental plates get separated away giving rise to expansive ejection of fluid basaltic lava and numerous active volcanoes. The East African Rift belongs to this category.

(e)

To determine

Whether Yellowstone is associated with a divergent boundary, convergent boundary, or intraplate volcanism.

Answer to Problem 1GST

The Yellowstone is associated with an intraplate volcanism.

Explanation of Solution

The voluminous outbursts of fluid basaltic lavas away from the adjacent plate boundary may result in intraplate volcanism. The column of hot, melted mantle rocks ascending to the surface, deep from the core-mantle boundary, called as mantle plumes, are believed to be serving as magma in intraplate volcanism. The surface expression of the decompression melting at the mantle plume is called as a hot spot.

The Yellowstone calderas lie over a hotspot and thus may be associated with intraplate volcanism. The plumes are hotter and more buoyant than the nearby mantle materials and thus acts like lava lamps with two immiscible liquids where the denser liquid at the bottom on heating rises as blobs due to buoyancy. Mantle plumes resemble blobs having a globular head and a slender tail when it ascends. This head on reaching close to the lithosphere triggers the decompression melting of overlying rocks and hence brings forth heavy outbursts of lava for a period of $1\text{million}\text{years}$.

(f)

To determine

Whether Mount Pelee is associated with a divergent boundary, convergent boundary, or intraplate volcanism.

Answer to Problem 1GST

Mount Pelee is related to a convergent plate boundary.

Explanation of Solution

Mount Pelee, which is an active stratovolcano, is located in the Ring of Fire. The Ring of Fire known for active volcanism encloses majority of the world’s active volcanoes.

The Ring of Fire lies in the circum-Pacific belt that is the convergence zone where the denser oceanic plate subducts beneath the less dense plate. Thus, Mount Pelee is concerned with a convergent plate boundary.

(g)

To determine

Whether Deccan Traps is associated with a divergent boundary, convergent boundary, or intraplate volcanism.

Answer to Problem 1GST

The Deccan Traps is associated with an intraplate volcanism.

Explanation of Solution

The voluminous outbursts of fluid basaltic lavas away from the adjacent plate boundary may result in intraplate volcanism. The column of hot, melted mantle rocks ascending to the surface, deep from the core-mantle boundary, called as mantle plumes, are believed to be serving as magma in intraplate volcanism. The surface expression of the decompression melting at the mantle plume is called as a hot spot.

The plumes are hotter and more buoyant than the nearby mantle materials and thus acts like lava lamps with two immiscible liquids where the denser liquid at the bottom on heating rises as blobs due to buoyancy. Mantle plumes resemble blobs having a globular head and a slender tail when it ascends. This head on reaching close to the lithosphere triggers the decompression melting of overlying rocks and hence brings forth heavy outbursts of lava for a period of $1\text{million}\text{years}$. Large mantle plumes known as super plumes may cause the formation of expansive basalt plateaus as seen in the case of Columbia plateau, Deccan plateau, Siberian traps, and Ontong Java plateau.

(h)

To determine

Whether Mount Fujiyama is associated with a divergent boundary, convergent boundary, or intraplate volcanism.

Answer to Problem 1GST

Mount Fujiyama is related to a convergent plate boundary.

Explanation of Solution

Mount Fujiyama, which is an active and characteristic composite cone, has alternate layering of cinders and ash, which are erupted interbedded lava flows with an almost symmetrical and large structure. The area with the largest number of composite volcanoes is situated in an active zone bordering the Pacific Ocean, which is given the name ‘Ring of Fire’. Mount Fujiyama is also located in the ‘Ring of Fire’.

The ‘Ring of Fire’ known for active volcanism encloses the majority of the world’s active volcanoes. The ‘Ring of Fire’ lies in the circum-Pacific belt that is convergence zone where the denser oceanic plate subducts beneath the less dense plate. Thus, Mount Pelee is concerned with a convergent plate boundary.