What types of landforms are made by Volcanic Eruptions? (Part 4 of 6)

What types of landforms are made by Volcanic Eruptions? (Part 4 of 6)


Volcanic eruptions have a big impact on
the landforms of a region. The most violent rhyolitic volcanic eruptions can produce a feature called a caldera. Calderas are a very large depression caused by the
collapse of a magma chamber of a rhyolitic volcano. Volcanic explosions
in the magma chamber throw material clear of the vent. The unsupported walls of
the vent and the top of the crater collapse into the chamber. Further magma forces its way to the surface and form small cones on the floor to caldera
which may later fill with water to form a lake. New Zealand’s largest lake Taupo
is actually an enormous caldera that has been partly filled by the lake. The present magma chamber is between 6 and 8 kilometers below the lake. Taupo is a rhyolitic super-volcano that has erupted numerous times. The most recent big eruption was 1,800 years ago and was the world’s most violent eruption than the last 5,000 years. The eruption plume reached 50
kilometres into the air, well into the stratosphere. All of New Zealand received
at least one centimeter of ash. An example being these ash deposits at
Farm Cove, Auckland, 272 kilometers away Pyroclastic flows from Taupo spread up
to 90 kilometers from the vent forming the Rangipo Desert which lies east of Ruapehu. Ash from this mighty eruption may have been the cause of red sunsets recorded by the Romans and Chinese at that time. 99% of the material erupted from Taupo is pumice and ash which is clearly evident around the shoreline of the lake. Layered cliffs of light-colored ignimbrite rock are also found around the lake. Mount Tauhara at the northern end of the lake is a relatively small lava dome formed from the 1% of magma that had lost enough gas to flow rather than explode. Other calderas in New Zealand include
Lake Rotorua and Lyttelton Harbor in the South Island. Behind me here is Mount Tarawera. Mount Tarawera is an example of a rhyolite dome. Rhyolite domes form
after a caldera has exhausted up all of gases. The result is that the magma
oozes really slowly, rather than quickly. So instead of flowing away from the crater like a normal lava flow, the lava instead piles up around the main vent. The result is a steep-sided dome just like the one you see behind me now. There are many examples of rhyolite domes in the Bay of Plenty region including Mukoia Island and Mount Ngongtaha which erupted within the Rotorua caldera. Another rhyolite dome volcano is Mount
Tarawera which lies within the Okataina caldera. The 1886 eruption of Mount
Tarawera is New Zealand’s largest and historic times. The volcano has three
dome shaped peaks, the highest being the Ruawahia dome at 1111 meters. These domes have been split down the middle by a series of fissures that run 17
kilometers northeast to southwest. The fissure includes the Waimangu Thermal
Valley which was created by the eruption. Tarawera is surrounded by a number of
lakes created or altered by the 1886 eruption. Shield volcanoes are wide-based
with gentle slopes. Their lava is runny and thin which means that it can travel a long way before cooling and solidifying. Often these eruptions are non violent and
last for years. Rangitoto which in Maori means bloody sky is a volcanic shield volcano in Auckland’s volcanic field. Rangitoto is New Zealand’s youngest volcano and was formed by an eruption 600 to 700 years ago. Today Rangitoto is 260 meters high and 5.5 kilometers wide. The summit is part of
the scoria cone but encircles a crater. Today you can visit the island and see
lava fields and lava caves. Lava caves and tubes are formed when the surface of
a lava flow cools and solidifies while the still molten interior flows through
and drains away. Stratovolcanoes also known as composite
cones are formed from alternate eruptions of ash and lava which causes a
steep-sided cone to be built off layers of ash and lava. New Zealand has several
stratovolcanoes such as Mount Ngaruahoe behind me, Mount Ruapehu, Mount
Taranaki and White Island. Stratovolcanoes like Tongariro often
have more than one vent. Tongariro is a massive complex of volcanic cones and
craters formed by eruptions from at least 12 vents over thousands of years. This is caused by cooled lava becoming solid in the main vent. The magma escapes along fissures which are small gaps to form what we call a dike. A dike is a long wall of volcanic rock that cuts across other volcanic layers. Alternatively a smaller parasite cone may form on the side of the main cone. Mount Tongariro has mineral springs and fumaroles. Fumaroles are steam vents. For example, on the other side of this mountain is a spring called Ketatahi hot springs. We also have behind me the Red Crater and the Te Maari crater. Some craters have filled with water to create the Blue Lake which you can also see and the Emerald Lakes. The Red and Te Maari craters were previously active in the 1800s but the Te Maari area has recently become
active again in 2012. Another stratovolcano on the North Island’s West
Coast is Mount Taranaki. At 2518 meters high it is the second
highest peak in the North Island. Taranaki which last erupted in 1755 has
a prominent parasite cone called Fanthams peak on the south side. Mount Taranakiis also encircled by a ring plain a circular plane built of materials washed
by rivers or lahars from its cone. The rich volcanic soils of the ring
plain are also popular with dairy farmers. Mount Ruapehu is a stratovolcano made of successive layers of andesite lava and ash deposits. At 2797 meters it is the
largest active volcano in New Zealand. Ruapehu has several small glaciers at its summit which are the only glaciers at present in the North Island. Ruapehu’s active vent lies beneath the crater lake. The mountain is surrounded by a ring plain of volcanic material from lahars, landslides and ash falls. A fumarole is a small geothermal vent from which volcanic gases and steam are emitted under pressure. The most spectacular types are geysers which occur when steam or boiling water is thrown tens of meters into the air. For example, the Pohutu geyser in Rotorua. An explosion crater is a broad low relief
volcanic crater formed by many shallow explosive eruptions and maybe later
filled with water. For example, in the Auckland volcanic field there are several explosion craters including the Orakei Basin and the Panmure Basin. These eruptions can result in the
formation of low rings of pyroclastic rock called tuff. There are tuff rings around the craters of many of Auckland’s volcanoes such as Lake Pupuke.

3 comments

  • Andrew Meraba

    This is a wonderful lesson, well presented. Thanks

    Reply
  • Didj Terminator

    sounds like you next to an eruption

    Reply
  • Todd Wheatley-DR-KNOW

    Given its broad application, everyone should understand GEOLOGY – therefore this video has been indexed and a link added by DR-KNOW / iq-2k  Information Services  – roughly 650 videos have indexed for this series

    Reply

Leave a Reply

Your email address will not be published. Required fields are marked *