Volcanic Environments

Volcanic environments are one of the Earth’s most spectacular natural events. Hot molten material from inside the Earth explodes onto the surface and into the atmosphere. It can destroy and build up at the same time.

Molten: metal or rock that is in liquid from due to great heat.

Volcanos form when magma is pushed through an opening in the Earth’s crust. This material often builds up into a cone-shaped mountain as the lava cools.

Magma: the hot molten rock beneath the surface of the earth.

Lava: the molten material that flows out from a volcano.

Volcano Categories

• Active Volcanoes: currently erupting or likely to erupt in the future. Active volcanoes have erupted in the last few hundred years.

• Dormant Volcanoes: currently inactive but may erupt in the future. These volcanoes have not erupted for up to 10,000 years.

• Extinct Volcanoes: unlike to erupt again. These volcanoes have not erupted for more than 10,000 years.

There are about 1300 active volcanoes on land as well as many undersea volcanoes. Australia has no active

volcanoes.

Mt. Kilauea, Hawaii

(Key-Low-Way-Ah)

Mount Etna, Italy

Mount Fuji, Japan last erupted 1708

Underwater volcano, off the coast of Indonesia

Dormant Volcano: Blue Lake, Mount Gambier ; last eruption 4,500 years ago

Extinct: Mount Macedon

Structure of the Earth

Layers of the Earth

Layers of the Earth: Core

The core is the central layer of the Earth. It is made up of the inner core and outer core. The inner core is believed to be solid due to the surrounding pressure. The outer core is liquid made up mainly of iron.

Layers of the Earth: Mantle

The largest layer of the Earth is the mantle. It makes up about 80% of the Earths volume. The upper mantle provides the magma for volcanoes. The Earth’s core pushes molten rock up towards the surface. As it cools it sinks back down. This movement causes convection currents.

Asthenosphere

• The asthenosphere is the zone beneath the relatively rigid lithosphere (top of the Mantle) where the fluid molten rock has maximum plasticity (easily moulded) or flowing ability.

• The asthenosphere is the main source of magma for Earth’s volcanoes.

Layers of the Earth: CrustThe crust extends as far as down as 50km (possibly 70 km depending on reference) the average thickness of the crust is between 30-40km under the continents. Under the oceanic plates it can be as little as 6 km thick. The crust floats on the soft mantle underneath. The crust is broken into a mosaic of plates, that can move past or bump into each other.

How is an egg similar to the Earth?

Plate Tectonics

The Earth is moving

Tectonic: from the Latin for building. A tectonic plate is a massive slow moving piece of the Earth’s surface.

Plate Tectonic Key Ideas

1. The Earth’s surface is covered with several tectonic plates.

2. The ocean floors are always moving; in a repeating process

of new material rising from the centre and old material

falling at the edges.

3. The plate move in different directions due to the convection

current underneath them.

4. The convection currents get their energy from radioactive

sources within the mantle of the Earth

• The lithospheric plates float like rafts on the deeper, molten layer of mantle.

• Movement is due to convection currents. These currents redistribute the huge amounts of heat generated by the formation of the planet and natural radioactive decay.

• The movement of the convection currents is very slow, measuring only millimetres per year.

Converging Boundary: Continental- Continental

Convergent boundaries – where plates collide into each other one plate is destroyed as it subducts under the other.

Converging Boundary: Continental- Oceanic (a) & Oceanic- Oceanic (b)

Subduction is a process in which the edge of a lithospheric plate slides underneath the edge of an adjacent plate.

Diverging Boundary: Continental- Continental

Divergent boundaries- where adjacent plate move away from each other as new oceanic lithosphere is created.

Diverging Boundary: Oceanic- Oceanic

One of the best known divergent boundaries is the Mid-Atlantic Ridge, which spreads apart at a rate of about 2.5 cm per year.

Computer-generated detailed topographic map of a segment of the Mid-Oceanic Ridge. "Warm" colours (yellow to red) indicate the ridge rising above the

seafloor, and the "cool" colours (green to blue) represent lower elevations.

Transform Boundary

Transform boundaries- where the plates slide past each other, neither creating nor destroying the plates. Most transform boundaries are oceanic but the rare example is the San Andreas fault in California on land.

Hotspots• Hotspots are relatively hotter zones of the

mantle that rise up through the lithosphere to form volcanoes.

• As the plate moves the old volcano becomes extinct. The hotspot creates a younger volcano. Forms a chain like Hawaii

Factors affecting types of volcanic activity

Type of eruption

Central vent eruptionFissure eruption

Reading: Geography Environments, page 35

Volcanic Material

LavaPyroclastic rocks

Rock in liquid form

Broken solid fragments

Pahoehoe (ropey appearance)

Aa (thick rocky solidified lava).

Lava viscosityViscosity is the ‘thickness’ of a fluid, the ability to resist flow. Water is low viscosity, honey is high viscosity. The higher the lava viscosity the more explosive the eruption.

• High viscosity lava usually involves basaltic lava.

• Lower viscosity lava usually involves granitic magma.

Types of volcanic material

• Tephra (Greek for ash) is the material exploded into the atmosphere-hot ash, rocks and lumps of semi-solidified lava from the volcano. This may aerially bombard human settlements and cause loss of life or property damage.

Tephra

• Tephra is classified by size. Ranging in size from dust particles to much larger lava bombs and blocks.

• Pumice is classified as tephra as it is a rock the cools quickly in the atmosphere.

Types of volcanic material

Pyroclastic Eruptions are the fast-moving clouds of lava, ash and volcanic gases that may suffocate and incinerate people and animals. Pyroclastic eruptions and mudflows account for the most deaths caused by volcanoes.

Types of volcanic material

• Poisonous gases such as carbon monoxide, sulphur dioxide, carbon dioxide and hydrochloric acid may be released during eruptions. These suffocate and kill people and animals. Also affects climate change.

What determines how explosive the eruption will be?

Viscosity of lavaThe amount of pressure Gas contentChemical composition of the lava

References:

• Geography environments, VCE Geography 1&2 GTAV, 2012

• Big Ideas, Geography level 6, Oxford, 2010• Thinking Geography