Continuation of the second notes
Shield-Building Stages -
Declining Stage -
Rejuvenated Stage -
Hawaiian shield volcanoes can go through several different lifestages, each
with different characteristics.
However, all volcanoes do not pass through each stage, and they can stop at any given stage.
Three primary lifestages of Hawai'i shield volcanoes:
Shield-building stage is divided into two phases:
Submarine Shield-Building Stage
Lo'ihi is in the submarine shield-building stage
Volcanoes in the submarine shield-building stage tend to erupt alkalic basalts.
Alkalic basalts are richer in Na and K, compared to Ca.
Geologists are not sure why alkalic basalts erupt early in the lifestages of Hawaiian shield volcanoes, but many suspect that it is related to the relative position of the volcano to the center of the hot spot.
Volcanoes nearer to the edge of the hot spot tend to erupt alkalic basalts.
This might be related to the degree of melting of the mantle rock.
Near the edge of the hot spot the temperature would be lower, therefore the degree of melting would be less.
A lower degree of melting can generate alkalic basalts.
The submarine shield-building stage is characterized by a shield constructed
of dense pillow lavas.
No pyroclastic material is generated because, at depths greater than 1 km, the weight of the overlying water inhibits the formation of gas bubbles.
Lo'ihi has a caldera, so calderas can form early in the lifestages of Hawaiian shield volcanoes.
Near the end of the submarine shield-building stage, as the volcano nears
the surface of the ocean, the lavas become tholeiitic.
Tholeiitic basalts are the normal basalts erupted by Hawaiian shield volcanoes.
Within a few hundred meters of sea level, the eruptions become explosive as
gas bubbles form.
Once summit is above sea-level, lava is extruded and the volcano continues to grow.
The volcano has entered the subaerial shield-building stage.
Subaerial Shield-Building Stage
Frequent eruptions of tholeiitic basalts characterize the subaerial shield-building
Volcanoes in this stage are near the center of the hot spot, so the degree of melting is greater.
During this stage the caldera repeatedly collapses and refills.
Kilauea and Mauna Loa are examples of volcanoes in the subaerial shield-building
Towards the end of the subaerial shield-building stage the
The volcano is entering the declining stage.
The primary characteristic of the declining stage is the formation of an alkalic
A volcano in the declining stage has drifted towards the edge of the hot spot and once again erupts alkalic basalts.
In addition, many unusual basalts and other magmas erupt during this stage.
A possible explanation for the production of these lavas is related to the declining frequency of eruptions.
If the magma stays in the magma chamber longer, the cooling magma can differentiate.
Differentiation can result in layered magma chambers, where the composition of the magma varies significantly.
Eruptions that tap different areas of the magma chamber can erupt lavas with distinct compositions.
The magma is more viscous and gas rich, so most of the eruptions form pyroclastic
The alkalic cap fills in and buries the caldera.
Hualalai and Mauna Kea are examples of volcanoes in the declining stage.
Mauna Kea last erupted 4,000 years ago.
Hualalai last erupted in 1801.
Eventually, volcanoes drift so far from the hot spot that eruptions cease,
and the volcano enters a long period of erosion.
Kohala has not erupted for 60,000 years and is heavily dissected by stream valleys that are eroding into the caldera region.
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