Galápagos hotspot
Encyclopedia
The Galápagos hotspot is a volcanic
hotspot
in the East Pacific Ocean
responsible for the creation of the Galapagos Islands
as well as three major aseismic ridge systems, Carnegie
, Cocos and Malpelso which are on two tectonic plates. The hotspot is located near the Equator
on the Nazca Plate
not far from the divergent plate boundary with the Cocos Plate
. The tectonic setting of the hotspot is complicated by the Galapagos Triple Junction
of the Nazca and Cocos plates with the Pacific Plate
. The movement of the plates over the hotspot is determined not solely by the spreading along the ridge but also by the relative motion between the Pacific Plate and the Cocos and Nazca Plates.
The hotspot is believed to be over 20 million years old and in that time there has been interaction between the hotspot, both of these plates, and the divergent plate boundary, at the Galapagos Spreading Centre. Lavas from the hotspot do not exhibit the homogeneous nature of many hotspots; instead there is evidence of four major reservoirs feeding the hotspot. These mix to varying degrees at different locations on the archipelago and also within the Galapagos Spreading Centre.
In 1963, Canadian geophysicist J. Tuzo Wilson came up with the "hotspot" theory to explain why although most earthquake and volcanic activity happened at plate boundaries, there is some that was located well away from any boundaries. The theory claimed that small, long-lasting, exceptionally "hot" areas of magma are located under certain points on Earth. These places, dubbed "hotspots," provide localized heat and energy systems (thermal plumes) that sustain long-lasting volcanic activity on the surface. This volcanism builds up seamounts that eventually rise above the ocean current, forming volcanic islands.
As the islands slowly moved away from the hotspot, by the motion of sliding plates as described by the theory of plate tectonics
, the magma supply is cut, and the volcano goes dormant. Meanwhile, the process repeats all over again, this time forming a new island, on and on until the hotspot collapses. The theory was developed to explain the Hawaiian-Emperor seamount chain
, where historic islands could be traced to the northwest in the direction that the Pacific Plate is moving. The early theory put these fixed sources of heat for the plumes deep within the Earth; however, recent research has led scientists to believe that hotspots are actually dynamic, and able to move on their own accord.
and Nazca
plates; the hotspot interacts with both plates and the spreading ridge over the last twenty million years as the relative location of the hotspot in relation to the plates has varied. Based on similar seismic velocity gradients of the lavas of the Carnegie, Cocos and Malpelos Ridges there is evidence that the hotspot activity has been the result of a single long mantle melt rather than multiple periods of activity and dormancy.
In Hawaii the evidence suggests that each volcano has a distinct period of activity as the hotspot moves under that portion of the Pacific plate before becoming dormant and then extinct and eroding under the ocean. This does not appear to be the case in the Galapagos, instead there is evidence of concurrent volcanism over a wide area. Nearly all Galapagos Islands show volcanism in the recent geological past, not just at the current location of the hotspot at Fernandina. The list below gives the last eruption dates for the Galapagos volcanoes, ordered from West to East.
The movement of the Nazca and Cocos plates have been tracked. The Nazca plate moves at 90 degrees at a rate of 58±2 km per million years. The Cocos Plate moves at 41 degrees at a rate of 83±3 km per million years. The location of the hotspot over time is recorded in the oceanic plate as the Carnegie and Cocos Ridges.
The Carnegie Ridge is on the Nazca plate is 600 km (373 mi) long and up to 300 km (186 mi) wide. It is orientated parallel to the plate movement, and its eastern end is approximately 20 million years old. There is a prominent saddle in the ridge at 86 degrees West where the height drops much closer to the surrounding ocean floor. The Malpelo Ridge, which is 300 km (186 mi) long was once believed to be part of the Carnegie Ridge.
The Cocos Ridge is 1000 km long located on the Cocos plate and is orientated parallel to the plates motion from the 91 degree west transform fault at the Galapagos Spreading Centre towards the Panamanian cost. The north eastern end of the ridge dates from about 13-14.5 million years ago. However the Cocos Islands to the northern end of the ridge are only 2 million years old, and were therefore created at a time well after the ridge had moved away from the hotspot.
The current model for the interaction of the hotspot and the spreading centre between the Cocos and Nazca plates attempts to explain the ridges on both plates; the split between the Carnegie and Malpelo Ridge and subsequent volcanic activity away from the hotspot. There have been eight major phases in the last 20 million years.
(1) 19.5 million years - 14.5 million years ago: the hotspot was located on the Nazca plate, forming a combined Carnegie and Malpelo Ridge. The type of lava erupted was a mix of plume material and depleted upper mantle, similar to the type of lava found in the central Galapagos islands at the current time.
(2) From 14.5 million years to 12.5 million years ago: the Galapagos Spreading Centre moved south and the ridge overlay the southern edge of the hotspot. Less material is erupted over the Nazca plate resulted in the saddle being formed in the Carnegie Ridge. The movement of the location of the Galapagos Spreading Centre starts to rift the Malpelo Ridge away from the Carnegie Ridge. The majority of the hotspot lavas are created on the Cocos plate resulting in the formation of the Cocos Ridge. The lavas formed here are similar to the types erupted on the western shield volcanoes of the Galapagos, which are predominantly plume.
(3) 12 million years to 11 million years: The Galapagos hotspot is centred under the Galapagos Spreading Centre. plume-type lavas are now abundant on the Cocos Ridge.
(4) 9.5 million years ago: the rifting between the Carnegie and Malpelo Ridges ends.
(5) 5.2 million years ago to 3.5 million years ago: the Galapagos Spreading Centre has another ridge jump, moving northwards with the plume now erupting on the Nazca plate, similar to the present orientation.
(6) 3.5 million to 2 million years ago: A short-lived east–west trending spreading centre is formed north of the Galapagos Spreading Centre. This new rift fails but leads to post abandonment volcanic activity and the subsequent formation of the Cocos Islands and surrounding seamounts. Around the hotspot plume lavas predominate.
(7) 2.6 million years ago: a major transform fault occurs north of the Galapagos hotspot. This results in widespread volcanism in the northern Galapagos along the Wolf Darwin Lineament and around Genovesa Island
.
(8) Present : The Galapagos hotspot is south of the spreading centre and there is geochemical zonation of the plume.
The four types are:
PLUME – this is magma associated with the plume itself and is similar to magmas from other ocean islands within the Pacific. It has the characteristics of intermediate Strontium (Sr), Neodymium (Nd) and Lead (Pb) ratios. The PLUME lavas are found predominately in the west of the islands, around Ferdinandina
and Isabela Islands
, which is near to the current position of the hotspot. The PLUME lavas erupted on Fernandina and Isabela are relatively cool. Analysis shows that they are as much as 100 degrees Celsius cooler than those in Hawaii. The cause of this is not fully understood but may be due to cooling in the lithosphere or being relatively cool at formation in the mantle. They are then found in lower quantities in a horseshoe pattern north and south of the central islands mixing with the other reservoirs as it progresses east. PLUME lavas are also found in the lavas from the Galapagos Spreading Centre due to convection and mixing of all of these lavas. In the upper mantle convection currents bring in mantle material at shallow angles from the south of the Galapagos Spreading Centre. These convection current will draw in some PLUME type magma to the spreading centre where it is then erupted.
DGM – (Depleted Galapagos Mantle), this has similar characteristics to ocean ridge basalts throughout the Pacific and the Galapagos Spreading Centre. Partial melting of the upper mantle as a result of the spreading centre will leave mantle material depleted in some compounds. It has low Sr and Pb isotope ratios and high Nd ratios. DGM is found in the central islands of the Galapagos such as Santiago
, Santa Cruz
, San Cristobal
and Santa Fe
. It fills in the centre of the horseshoe formed by the PLUME lavas to the west, north and south.
FLO – (Floreana), characteristic of that islands lavas. It is thought that this reservoir came from subducted ocean crust that has been entrained by the mantle plume. It has enriched Sr and Pb ratios and is enriched with trace elements. FLO is associated principally with the island of Floreana and shows up on the mixing of lavas within the Galapagos along the southern side archipelago and is diluted to the east and north of there.
WD – (Wolf Darwin) is unique in the Pacific and resembles material from an Indian Ocean Ridge system. It is found on the Wolf
and Darwin Island
s and the seamounts that connect them along the Wolf Darwin Lineament. It has a unique Pb ratio. WD is located along the northern side of the archipelago and dilutes to the east and south.
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
hotspot
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
in the East Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
responsible for the creation of the Galapagos Islands
Galápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
as well as three major aseismic ridge systems, Carnegie
Carnegie Ridge
The Carnegie Ridge is an aseismic ridge on the Nazca Plate that is currently being subducted beneath the South American Plate. The ridge is thought to be a result of the passage of the Nazca Plate over the Galapagos hotspot...
, Cocos and Malpelso which are on two tectonic plates. The hotspot is located near the Equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
on the Nazca Plate
Nazca Plate
]The Nazca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction along the Peru-Chile Trench of the Nazca Plate under the South American Plate is largely responsible for the...
not far from the divergent plate boundary with the Cocos Plate
Cocos Plate
The Cocos Plate is an oceanic tectonic plate beneath the Pacific Ocean off the west coast of Central America, named for Cocos Island, which rides upon it.-Geology:...
. The tectonic setting of the hotspot is complicated by the Galapagos Triple Junction
Galapagos Triple Junction
The Galapagos Triple Junction is a geological area in the eastern Pacific Ocean several hundred miles west of the Galapagos Islands where three tectonic plates - the Cocos Plate, the Nazca Plate and the Pacific Plate - meet. It is an unusual type of triple junction in which the three plates do not...
of the Nazca and Cocos plates with the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
. The movement of the plates over the hotspot is determined not solely by the spreading along the ridge but also by the relative motion between the Pacific Plate and the Cocos and Nazca Plates.
The hotspot is believed to be over 20 million years old and in that time there has been interaction between the hotspot, both of these plates, and the divergent plate boundary, at the Galapagos Spreading Centre. Lavas from the hotspot do not exhibit the homogeneous nature of many hotspots; instead there is evidence of four major reservoirs feeding the hotspot. These mix to varying degrees at different locations on the archipelago and also within the Galapagos Spreading Centre.
Hotspot theory
As the islands slowly moved away from the hotspot, by the motion of sliding plates as described by the theory of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, the magma supply is cut, and the volcano goes dormant. Meanwhile, the process repeats all over again, this time forming a new island, on and on until the hotspot collapses. The theory was developed to explain the Hawaiian-Emperor seamount chain
Hawaiian-Emperor seamount chain
The Hawaiian–Emperor seamount chain is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, and the Emperor Seamounts, a vast underwater mountain region of islands and intervening seamounts, atolls, shallows, banks and reefs along a line trending...
, where historic islands could be traced to the northwest in the direction that the Pacific Plate is moving. The early theory put these fixed sources of heat for the plumes deep within the Earth; however, recent research has led scientists to believe that hotspots are actually dynamic, and able to move on their own accord.
Tectonic setting
The Galapagos hotspot has a very complicated tectonic setting. It is located very close to the spreading ridge between the CocosCocos Plate
The Cocos Plate is an oceanic tectonic plate beneath the Pacific Ocean off the west coast of Central America, named for Cocos Island, which rides upon it.-Geology:...
and Nazca
Nazca Plate
]The Nazca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction along the Peru-Chile Trench of the Nazca Plate under the South American Plate is largely responsible for the...
plates; the hotspot interacts with both plates and the spreading ridge over the last twenty million years as the relative location of the hotspot in relation to the plates has varied. Based on similar seismic velocity gradients of the lavas of the Carnegie, Cocos and Malpelos Ridges there is evidence that the hotspot activity has been the result of a single long mantle melt rather than multiple periods of activity and dormancy.
In Hawaii the evidence suggests that each volcano has a distinct period of activity as the hotspot moves under that portion of the Pacific plate before becoming dormant and then extinct and eroding under the ocean. This does not appear to be the case in the Galapagos, instead there is evidence of concurrent volcanism over a wide area. Nearly all Galapagos Islands show volcanism in the recent geological past, not just at the current location of the hotspot at Fernandina. The list below gives the last eruption dates for the Galapagos volcanoes, ordered from West to East.
| Name | Last Eruption |
|---|---|
| Darwin Island Darwin Island Darwin Island is named in honor of Charles Darwin, and is among the smallest in the Galapagos Archipelago with an area of just one square kilometer. With no dry landing sites, Darwin Island's main attractions are found in the Pacific Ocean, which is teeming with a spectacular variety of marine life... |
Extinct |
| Fernandina Fernandina Island Fernandina Island is the third largest, and youngest, island of the Galápagos Islands. Like the others, the island was formed by the Galápagos hotspot... |
2009 |
| Ecuador (volcano) | 1150 |
| Wolf Island Wolf Island Wolf Island or Wenman Island is a small island in the Galapagos Islands and was named after the German geologist Theodor Wolf, who also has the volcano Wolf on Isabela Island named after him... |
Extinct |
| Cerro Azul Cerro Azul (Ecuador volcano) Cerro Azul is a shield volcano on the south western part of Isabela Island in the Galápagos Islands, the name translates from Spanish as blue mountain... |
2008 |
| Wolf (volcano) Volcán Wolf Volcán Wolf is the highest peak in the Galapagos Islands and is situated on Isabela Island and reaches and is the highest point in the Galapagos Islands... |
1982 |
| Darwin (volcano) | 1813 |
| Alcedo Alcedo Volcano Alcedo Volcano is one of the six coalescing shield volcanoes that make up Isabela Island in the Galapagos. The remote location of the volcano has meant that even the most recent eruption in 1993 was not recorded until two years later... |
1993 |
| Sierra Negra | 2005 |
| Santiago Island Santiago Island (Galápagos) Santiago Island is an island of the Galápagos Islands. It is also known as San Salvador, named after the first island discovered by Columbus in the Caribbean Sea , or as James Island. The island, which consists of two overlapping volcanoes, has an area of 585 km² and a maximum altitude of... |
1906 |
| Pinta Island Pinta Island Pinta Island is an island located in the Galapagos Islands group, Ecuador. It has an area of 60 km² and a maximum altitude of 777 meters.... |
1928 |
| Marchena Island Marchena Island Named after Fray Antonio Marchena, Marchena Island has an area of 130 km² and a maximum altitude of 343 meters.There aren't any visitor's sites on this island, although it is possible to dive in the waters around Marchena on organised tours.... |
1991 |
| Santa Cruz Island Santa Cruz Island (Galápagos) Santa Cruz Island is one of the Galápagos Islands with an area of and a maximum altitude of .Situated in the center of the archipelago, Santa Cruz is the second largest island after Isabela. Its capital is Puerto Ayora, the most populated urban centre in the islands. On Santa Cruz there are some... |
Unknown |
| Floreana Island Floreana Island Floreana Island is an island of the Galápagos Islands. It was named after Juan José Flores, the first president of Ecuador, during whose administration the government of Ecuador took possession of the archipelago, having previously been called Charles Island... |
Extinct |
| Genovesa Island Genovesa Island Genovesa Island is a shield volcano in the Galápagos Islands in the eastern Pacific Ocean. The island occupies about , and its maximum elevation is . The horse-shoe shaped island has a volcanic caldera whose wall has collapsed, forming the Great Darwin Bay, surrounded by cliffs... |
Unknown |
| San Cristóbal Island San Cristóbal Island San Cristóbal is the easternmost island in the Galápagos archipelago, and one of the oldest geologically.Its Spanish name "San Cristóbal" comes from the patron saint of seafarers, St. Christopher... |
Unknown |
The movement of the Nazca and Cocos plates have been tracked. The Nazca plate moves at 90 degrees at a rate of 58±2 km per million years. The Cocos Plate moves at 41 degrees at a rate of 83±3 km per million years. The location of the hotspot over time is recorded in the oceanic plate as the Carnegie and Cocos Ridges.
The Carnegie Ridge is on the Nazca plate is 600 km (373 mi) long and up to 300 km (186 mi) wide. It is orientated parallel to the plate movement, and its eastern end is approximately 20 million years old. There is a prominent saddle in the ridge at 86 degrees West where the height drops much closer to the surrounding ocean floor. The Malpelo Ridge, which is 300 km (186 mi) long was once believed to be part of the Carnegie Ridge.
The Cocos Ridge is 1000 km long located on the Cocos plate and is orientated parallel to the plates motion from the 91 degree west transform fault at the Galapagos Spreading Centre towards the Panamanian cost. The north eastern end of the ridge dates from about 13-14.5 million years ago. However the Cocos Islands to the northern end of the ridge are only 2 million years old, and were therefore created at a time well after the ridge had moved away from the hotspot.
The current model for the interaction of the hotspot and the spreading centre between the Cocos and Nazca plates attempts to explain the ridges on both plates; the split between the Carnegie and Malpelo Ridge and subsequent volcanic activity away from the hotspot. There have been eight major phases in the last 20 million years.
(1) 19.5 million years - 14.5 million years ago: the hotspot was located on the Nazca plate, forming a combined Carnegie and Malpelo Ridge. The type of lava erupted was a mix of plume material and depleted upper mantle, similar to the type of lava found in the central Galapagos islands at the current time.
(2) From 14.5 million years to 12.5 million years ago: the Galapagos Spreading Centre moved south and the ridge overlay the southern edge of the hotspot. Less material is erupted over the Nazca plate resulted in the saddle being formed in the Carnegie Ridge. The movement of the location of the Galapagos Spreading Centre starts to rift the Malpelo Ridge away from the Carnegie Ridge. The majority of the hotspot lavas are created on the Cocos plate resulting in the formation of the Cocos Ridge. The lavas formed here are similar to the types erupted on the western shield volcanoes of the Galapagos, which are predominantly plume.
(3) 12 million years to 11 million years: The Galapagos hotspot is centred under the Galapagos Spreading Centre. plume-type lavas are now abundant on the Cocos Ridge.
(4) 9.5 million years ago: the rifting between the Carnegie and Malpelo Ridges ends.
(5) 5.2 million years ago to 3.5 million years ago: the Galapagos Spreading Centre has another ridge jump, moving northwards with the plume now erupting on the Nazca plate, similar to the present orientation.
(6) 3.5 million to 2 million years ago: A short-lived east–west trending spreading centre is formed north of the Galapagos Spreading Centre. This new rift fails but leads to post abandonment volcanic activity and the subsequent formation of the Cocos Islands and surrounding seamounts. Around the hotspot plume lavas predominate.
(7) 2.6 million years ago: a major transform fault occurs north of the Galapagos hotspot. This results in widespread volcanism in the northern Galapagos along the Wolf Darwin Lineament and around Genovesa Island
Genovesa Island
Genovesa Island is a shield volcano in the Galápagos Islands in the eastern Pacific Ocean. The island occupies about , and its maximum elevation is . The horse-shoe shaped island has a volcanic caldera whose wall has collapsed, forming the Great Darwin Bay, surrounded by cliffs...
.
(8) Present : The Galapagos hotspot is south of the spreading centre and there is geochemical zonation of the plume.
Chemical structure of the Galapagos lavas
Analysis of the radioactive isotopes of the lavas on the islands of the Galapagos archipelago and on the Carnegie Ridge shows that there are four major reservoirs of magma that mix in varying combinations to form the volcanic province.The four types are:
PLUME – this is magma associated with the plume itself and is similar to magmas from other ocean islands within the Pacific. It has the characteristics of intermediate Strontium (Sr), Neodymium (Nd) and Lead (Pb) ratios. The PLUME lavas are found predominately in the west of the islands, around Ferdinandina
Fernandina Island
Fernandina Island is the third largest, and youngest, island of the Galápagos Islands. Like the others, the island was formed by the Galápagos hotspot...
and Isabela Islands
Isabela Island (Ecuador)
Isabela Island is the largest island of the Galápagos with an area of , and length of nearly 4 times larger than Santa Cruz, the next largest of the islands. This island was named in honor of Queen Isabella of Spain, who sponsored the voyage of Columbus. . By the English, it was named Albemarle...
, which is near to the current position of the hotspot. The PLUME lavas erupted on Fernandina and Isabela are relatively cool. Analysis shows that they are as much as 100 degrees Celsius cooler than those in Hawaii. The cause of this is not fully understood but may be due to cooling in the lithosphere or being relatively cool at formation in the mantle. They are then found in lower quantities in a horseshoe pattern north and south of the central islands mixing with the other reservoirs as it progresses east. PLUME lavas are also found in the lavas from the Galapagos Spreading Centre due to convection and mixing of all of these lavas. In the upper mantle convection currents bring in mantle material at shallow angles from the south of the Galapagos Spreading Centre. These convection current will draw in some PLUME type magma to the spreading centre where it is then erupted.
DGM – (Depleted Galapagos Mantle), this has similar characteristics to ocean ridge basalts throughout the Pacific and the Galapagos Spreading Centre. Partial melting of the upper mantle as a result of the spreading centre will leave mantle material depleted in some compounds. It has low Sr and Pb isotope ratios and high Nd ratios. DGM is found in the central islands of the Galapagos such as Santiago
Santiago Island (Galápagos)
Santiago Island is an island of the Galápagos Islands. It is also known as San Salvador, named after the first island discovered by Columbus in the Caribbean Sea , or as James Island. The island, which consists of two overlapping volcanoes, has an area of 585 km² and a maximum altitude of...
, Santa Cruz
Santa Cruz Island (Galápagos)
Santa Cruz Island is one of the Galápagos Islands with an area of and a maximum altitude of .Situated in the center of the archipelago, Santa Cruz is the second largest island after Isabela. Its capital is Puerto Ayora, the most populated urban centre in the islands. On Santa Cruz there are some...
, San Cristobal
San Cristóbal Island
San Cristóbal is the easternmost island in the Galápagos archipelago, and one of the oldest geologically.Its Spanish name "San Cristóbal" comes from the patron saint of seafarers, St. Christopher...
and Santa Fe
Santa Fe Island
Santa Fe Island , also called Barrington Island after admiral Samuel Barrington, is a small island of 24 km² which lies in the centre of the Galapagos archipelago, to the south east of Santa Cruz Island. Geologically it is one of the oldest, since volcanic rocks of about 4 million years old...
. It fills in the centre of the horseshoe formed by the PLUME lavas to the west, north and south.
FLO – (Floreana), characteristic of that islands lavas. It is thought that this reservoir came from subducted ocean crust that has been entrained by the mantle plume. It has enriched Sr and Pb ratios and is enriched with trace elements. FLO is associated principally with the island of Floreana and shows up on the mixing of lavas within the Galapagos along the southern side archipelago and is diluted to the east and north of there.
WD – (Wolf Darwin) is unique in the Pacific and resembles material from an Indian Ocean Ridge system. It is found on the Wolf
Wolf Island
Wolf Island or Wenman Island is a small island in the Galapagos Islands and was named after the German geologist Theodor Wolf, who also has the volcano Wolf on Isabela Island named after him...
and Darwin Island
Darwin Island
Darwin Island is named in honor of Charles Darwin, and is among the smallest in the Galapagos Archipelago with an area of just one square kilometer. With no dry landing sites, Darwin Island's main attractions are found in the Pacific Ocean, which is teeming with a spectacular variety of marine life...
s and the seamounts that connect them along the Wolf Darwin Lineament. It has a unique Pb ratio. WD is located along the northern side of the archipelago and dilutes to the east and south.