Why are hotspots characterized by non explosive volcanism

Hot spots appear to result from plumes of hot mantle material upwelling toward the surface, independent of the convection cells though to cause plate motion.

Hot spots tend to be fixed in position, with the plates moving over the top. As the rising plume of hot mantle moves upward it begins to melt to produce magmas. These magmas then rise to the surface producing a volcano. But, as the plate carrying the volcano moves away from the position over the hot spot, volcanism ceases and new volcano forms in the position now over the hot spot.

This tends to produce chains of volcanoes or seamounts former volcanic islands that have eroded below sea level. Volcanism resulting from hotspots occurs in both the Atlantic and Pacific ocean, but are more evident on the sea floor of the Pacific Ocean, because the plates here move at higher velocity than those under the Atlantic Ocean.

A hot spot trace shows up as a linear chain of islands and seamounts, many of which can be seen in the Pacific Ocean. The Hawaiian Ridge is one such hot spot trace.

Here the Big Island of Hawaii is currently over the hot spot, the other Hawaiian islands still stand above sea level, but volcanism has ceased. Northwest of the Hawaiian Islands, the volcanoes have eroded and are now seamounts.

The ages of volcanic rocks increase along the Hawaiian Ridge to the northwest of Hawaii. The prominent bend observed where the Hawaiian Ridge intersects the Emperor Seamount chain has resulted from a change in the direction of plate motion over the hot spot.

Note that when the Emperor Seamount chain was produced, the plate must have been moving in a more northerly direction. The age of the volcanic rocks at the bend is about 50 million years. Yellowstone appears to be over a continental hot spot that has produced a chain of volcanoes as the North American Plate moves southwestward over the hot spot.

Examples of questions on this material that could be asked on an exam. Natural Disasters. Volcanic Landforms, Volcanoes and Plate Tectonics.

Volcanic Landforms Volcanic landforms are controlled by the geological processes that form them and act on them after they have formed. Shield Volcanoes A shield volcano is characterized by gentle upper slopes about 5 o and somewhat steeper lower slopes about 10 o. Most shields were formed by low viscosity basaltic magma that flows easily down slope away form the summit vent.

Most shield volcanoes have a roughly circular or oval shape in map view. Vents for most shield volcanoes are central vents, which are circular vents near the summit.

Hawaiian shield volcanoes also have flank vents, which radiate from the summit and take the form of en-echelon fractures or fissures, called rift zones, from which lava flows are emitted. This gives Hawaiian shield volcanoes like Kilauea and Mauna Loa their characteristic oval shape in map view. Stratovolcanoes also called Composite Volcanoes Have steeper slopes than shield volcanoes, with slopes of 6 to 10 o low on the flanks to 30 o near the top.

The steep slope near the summit is due partly to thick, short viscous lava flows that do not travel far down slope from the vent. Long periods of repose times of inactivity lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings about possible eruptions.

Cinder Cones also called Tephra Cones Cinder cones are small volume cones consisting predominantly of tephra that result from strombolian eruptions. They usually consist of basaltic to andesitic material. They are actually fall deposits that are built surrounding the eruptive vent. Slopes of the cones are controlled by the angle of repose angle of stable slope for loose unconsolidated material and are usually between about 25 and 35 o. They show an internal layered structure due to varying intensities of the explosions that deposit different sizes of pyroclastics.

On young cones, a depression at the top of the cone, called a crater, is evident, and represents the area above the vent from which material was explosively ejected. Craters are usually eroded away on older cones. If lava flows are emitted from tephra cones, they are usually emitted from vents on the flank or near the base of the cone during the later stages of eruption. Cinder and tephra cones usually occur around summit vents and flank vents of stratovolcanoes.

Skip to main content. Toggle menu Go to search page. Search Field. You are here Home » Types of Volcanoes. Shield Volcanoes. Plateau or Flood basalts are extremely large volume outpourings of low viscosity basaltic magma from fissure vents. The basalts spread huge areas of relatively low slope and build up plateaus. Many of these outpourings appear to have occurred along a zone that eventually developed into a rift valley and later into a diverging plate boundary. Examples of questions on this material that could be asked on an exam.

Physical Geology. Volcanoes and Volcanic Eruptions. Magmas and Lava Since volcanic eruptions are caused by magma a mixture of liquid rock, crystals, and dissolved gas expelled onto the Earth's surface, we'll first review the characteristics of magma that we covered previously. Viscosity of Magmas Viscosity is the resistance to flow opposite of fluidity.

Higher SiO 2 content magmas have higher viscosity than lower SiO 2 content magmas Lower Temperature magmas have higher viscosity than higher temperature magmas. Solidified Volcanic Rock. Solidified Plutonic Rock.

Intermediate or Andesitic. Pahoehoe Flows - Basaltic lava flows with low viscosity start to cool when exposed to the low temperature of the atmosphere. This causes a surface skin to form, although it is still very hot and behaves in a plastic fashion, capable of deformation.

Such lava flows that initially have a smooth surface are called pahoehoe flows. Initially the surface skin is smooth, but often inflates with molten lava and expands to form pahoehoe toes or rolls to form ropey pahoehoe. See figure 9. Pahoehoe flows tend to be thin and, because of their low viscosity travel long distances from the vent.

A'A' Flows - Higher viscosity basaltic and andesitic lavas also initially develop a smooth surface skin, but this is quickly broken up by flow of the molten lava within and by gases that continue to escape from the lava. This creates a rough, clinkery surface that is characteristic of an A'A' flow see figure 9. Lava Tubes - Once the surface skin becomes solid, the lava can continue to flow beneath the surface in lava tubes. The surface skin insulates the hot liquid lava form further cooling.

When the eruption ends, liquid lava often drains leaving an open cave see figure 9. Pillow Lavas - When lava erupts on the sea floor or other body of water, the surface skin forms rapidly, and, like with pahoehoe toes inflates with molten lava.

Eventually these inflated balloons of magma drop off and stack up like a pile of pillows and are called pillow lavas. Ancient pillow lavas are readily recognizable because of their shape, their glassy margins and radial fractures that formed during cooling see figure 9.

Columnar Jointing - When thick basaltic or andesitic lavas cool, they contract. The contraction results in fractures and often times results in a type of jointing called columnar jointing. The columns are usually hexagonal in shape.

This often happens when lavas pool in depressions or deep canyons see figure 9. Lava Domes or Volcanic Domes - result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent. Blocks of nearly solid lava break off the outer surface of the dome and roll down its flanks to form a breccia around the margins of domes.

Pyroclastic Material If the magma has high gas content and high viscosity, the gas will expand in an explosive fashion and break the liquid into clots that fly through the air and cool along their path through the atmosphere. Blocks are angular fragments that were solid when ejected. Volcanic Landforms Volcanic landforms are controlled by the geological processes that form them and act on them after they have formed.

Shield Volcanoes A shield volcano is characterized by gentle upper slopes about 5 o and somewhat steeper lower slopes about 10 o. Most shield volcanoes have a roughly circular or oval shape in map view. Long periods of repose times of inactivity lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings about possible eruptions.

Cinder Cones Cinder cones are small volume cones consisting predominantly of ash and scoria that result from mildly explosive eruptions. They usually consist of basaltic to andesitic material. They are actually fall deposits that are built surrounding the eruptive vent. Slopes of the cones are controlled by the angle of repose angle of stable slope for loose unconsolidated material and are usually between about 25 and 35 o.

On young cones, a depression at the top of the cone, called a crater, is evident, and represents the area above the vent from which material was explosively ejected. Craters are usually eroded away on older cones. Craters and Calderas Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and ash.

Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to 50 km. Calderas form as a result of collapse of a volcanic structure. The collapse results from evacuation of the underlying magma chamber. Crater Lake Caldera in southern Oregon is an 8 km diameter caldera containing a lake The caldera formed about years ago as a result of the eruption of about 75 km 3 of rhyolite magma in the form of tephra, found as far away as Canada, accompanied by pyroclastic flows that left thick deposits of tuff on the flanks of the volcano.

Subsequent eruptions have built a cinder cone on the floor of the caldera, which now forms an island called Wizard Island. Larger calderas have formed within the past million years in the western United States. The Yellowstone caldera is an important example, as it illustrates the amount of repose time that might be expected from large rhyolitic systems, and the devastating effect caldera forming eruptions can have on widespread areas.

Yellowstone Caldera which occupies most of Yellowstone National Park, is actually the third caldera to form in the area within the past 2 million years. The three calderas formed at 2. Thus the repose time is on the average about , years. Tephra fall deposits from the latest eruption are found in Louisiana and into the Gulf of Mexico, and covered much of the Western part of the United States.

The eruption , years ago produced about km 3 of rhyolite in comparison, the eruption of Mt. Helens in May of produced only 0. Magma still underlies Yellowstone caldera, as evidenced by the large number of hot springs and geysers in the area.

Volcanic Eruptions In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks. When the magma reaches the Earth's surface, the gas bubble will simply burst, the gas will easily expand to atmospheric pressure, and a effusive or non-explosive eruption will occur, usually as a lava flow If the liquid part of the magma has a high viscosity, then the gas will not be able to expand very easily, and thus, pressure will build up inside of the gas bubble s.

Effusive Eruptions Effusive or Non explosive eruptions are favored by low gas content and low viscosity magmas basaltic to andesitic magmas. If the viscosity is low, non-explosive eruptions usually begin with fire fountains due to release of dissolved gases. Lava flows are produced on the surface, and these run like liquids down slope, along the lowest areas they can find. If the magma emerges along a fracture, it results in a fissure eruption, often called a "curtain of fire" Lava flows produced by eruptions under water are called pillow lavas.

If the viscosity is high, but the gas content is low, then the lava will pile up over the vent to produce a lava dome or volcanic dome. If the pressure in the bubbles is low, the eruption will produce an eruption column only a few hundred meters high, and most of the pyroclastic material will fall to close to the vent to build a cinder cone.

This type of eruption is called a Strombolian eruption , and is considered mildly explosive. Lahars Volcanic Mudflows A volcanic eruption usually leaves lots of loose unconsolidated fragmental debris. It is important to understand that lahars can occur accompanying an eruption, or can occur simply as the result of heavy rainfall or sudden snow melt, without an eruption.

Volcanic Gases Although the predominant gas erupted from volcanoes is H 2 O vapor, other gases are erupted can have disastrous effects on life. Sulfur gases in the atmosphere, along with volcanic ash, reflect incoming solar radiation back into space and have a cooling effect on the atmosphere, thus lowering average global temperatures.

The effect lasts only as long as the gases and ash remain in the atmosphere, normally a few years at the most. CO 2 gas, produces the opposite effect. It is a greenhouse gas which absorbs solar radiation and causes a warming effect. Eruptions in the past that produced huge quantities of this gas may have been responsible for mass extinction events.

As the first block, began to slide downward, the magma chamber beneath the volcano became exposed to atmospheric pressure. The gas inside the magma expanded rapidly, producing a lateral blast that moved outward toward the north. Scientists think that this volcanic chain of islands has been forming for at least 70 million years over a hot spot underneath the Pacific plate. The Hawaiian Islands were created one right after the other as the plate moved northwest—almost like an island factory.

Of all the inhabit ed Hawaiian Islands, Kauai is located farthest from the presume d hot spot and has the most eroded and oldest volcanic rocks, dated to be around 5.

Hot spots can also develop beneath continent s. The Yellowstone hot spot, for example, has produced a series of volcanic features that extend in a northeastern direction. The features stretch from the U. Over It formed from an eruption roughly Hot spots don't always create volcanoes that spew rivers of lava. These eruptions are called geyser s.

When it erupts, the water is Kilauea, above, is one of five volcanoes on the "Big Island" of Hawaii—three of them active. The Big Island sits over the Hawaiian hot spot. Photograph by James L. Amos, National Geographic. Extraterrestrial Hot Spots.

Hot Spots are Cool. Island volcanoes that form over hot spots are generally less explosive than volcanic arcs that form over subduction zones. Also called the geosphere. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.

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