(Update 17/12/2017)
Volcano talk
The earth;
Formation 4bya from exploded stars.
Meteorites 1b years bring water (and nitrogen?)
Settled with core of Iron, magma of silicates,
Core (2000km), magma (6000km), crust (50km), (atmosphere, magnetosphere)
Volcanoes are when the magma breaks through the crust.
Types of volcano:
Ocean trench: Formed as the continental plates drift apart.
Subduction: Formed as one plate slides under another. Much of the dust plume is from the crust and not the magma. Mount Saint Helens,
Field: Liquid lava flows from many vents and covers all the old hills and valleys. These basalt plains.
Caldera: The centre of some field volcanoes fall in on the gap underneath, causing a valley or lake.
Freestanding: A traditional volcano rising above the plains. Mt Macedon.
Ages:
Atherton tablelands 30 million years ago
Glasshouse mountains (Brisbane) 23m
Aberfeldy, Thompson R. 30m.
Melbourne, Warragul. 6m?
Mt Macedon, Trentham 350m?
Volcanic plains 2m
Mount Porndon 300k
Mount Elephant 180k (and Mt Buninyong?)
Tower Hill (Warrnambool) 40k
(Greenstone dacite from Phillip Is., Heathcote, Kangaroo Is. are ocean trench volcanoes (over 60m?)
There is very little on the surface. It was highly valued as tools by the aborigines)
Also:
Scoria is formed if the vent comes through limestone and water when the steam “froths” the rock.
Thick wet limestone causes an explosive eruption to form a “maar” crater, and “tuff” instead of scoria.
If the crater is below the water table it forms a lake, such as Lake Bullen Merri and Lake Gnotuk.
Olivine is the magma which slows down in the vent during eruption and crystallizes from the mix.
Lots of olivine in Mount Shadwell, but not much in Mount Elephant.
Often other base rock is worn from the vent and brought up with the lava.
Gemstones of ruby, sapphire, diamond, zircon are scraped from the base of the crust in explosive eruptions. e.g. Mt Barrington near Dubbo.
Granite also is magma but cools as a “pluton” below the surface, and does not erupt to the surface.