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Sunday, 4 March 2007

ERUPTION DYNAMICS



This section examines the variability of volcanic environments and the physical and chemical controls on eruption dynamics. Environments of volcanism are discussed in terms of plate tectonic theory.



VOLCANO-TECTONIC ENVIRONMENTS



  • DISTRIBUTION OF ACTIVE VOLCANOES
    The earth is a dynamic planet. Its rigid outer surface layer is broken into several tectonic plates which are in constant motion relative to one another. As demonstrated in the world map below, most of the ~550 active volcanoes on earth are located along the margins of adjacent plates.
World map showing plate boundaries(blue lines), the distribution of recent earthquakes(yellow dots) and active volcanoes(red dots)
  • PLATE MOTION, MANTLE CONVECTION, AND MAGMA GENERATION
Tectonic plates are composed of lithosphere, the rigid outer portion of the earth. With a thickness of about 100 km, the lithosphere is composed of an upper layer of crust (~7 km thick under the oceans, and ~35 km thick under the continents) and a lower, denser layer of the earth's upper mantle. The lithosphere is underlain by the asthenosphere, a hot, mobile layer of partially molten rock lying within the earth's upper mantle.


The rigid lithospheric plates are driven by convection within the mobile asthenosphere. Hot mantle rises beneath mid-oceanic ridges, and cold, denser mantle descends at oceanic trenches. Lateral motion of the lithospheric plates above these circular convection cells is analogous to rigid blocks riding above a rotating conveyor belt.

Volcanic eruptions above these lithospheric plates are driven by the ascent of magma (molten rock) from deep beneath the surface.
They vary from mafic, intermediate, to felsic as their silica (SiO2) content increases. Mafic (basaltic) magmas are generated directly from the mantle, either within the asthenosphere or within the overlying mantle lithosphere. Many mafic-to-intermediate (basaltic-to-andesitic) magmas appear to be derived from the melting of hydrated lithospheric mantle. More differentiated, intermediate-to-felsic magmas, on the other hand, are partly derived from the melting of continental crust by hot, mafic magmas that either pond at the crust-mantle boundary, or intrude into the overlying continents where they reside in magma chambers located at various crustal levels.

Volcanism is typically widespread along plate boundaries. Although volcanism in the interior of plates is less common, these intraplate regions can also generate voluminous eruptive products. The regional volcano-tectonic processes associated with plate-boundary environments and intraplate environments are described in more detail below.

  • VOLCANISM AT PLATE TECTONIC BOUNDARIES

Plate boundaries mark the sites where two plates are either moving away from one another, moving toward one another, or sliding past one another. Adjacent plates are delineated by three types of boundaries defined by this relative motion:
  • Divergent plate boundaries -- Plates diverge from one another at the site of thermally buoyant mid-oceanic ridges. Oceanic crust is created at divergent plate boundaries.
  • Convergent plate boundaries -- Plates converge on one another at the site of deep oceanic trenches. Oceanic crust is destroyed at convergent plate boundaries.
  • Transform plate boundaries -- Plates slide past one another.
Although volcanism is abundant at divergent and convergent plate boundaries, there is a distinct lack of significant volcanism associated with transform plate boundaries. Spreading center volcanism occurs at divergent plate margins, and subduction zone volcanism occurs at convergent plate margins. Intraplate volcanism describes volcanic eruptions within tectonic plates. Each of these three volcano-tectonic environments is depicted in the following diagram:





posted by the geog bloggers at 8:47:00 pm | 0 Comments

Monday, 26 February 2007

Eruption Products

PART ONE - LAVA FLOW TYPES

BASALTIC LAVA


Where do basaltic lava flows erupt from?

  • Shield volcanoes
  • Rift zones
  • Cinder cones

Fluid lava flows can be subdivided into:

  • Pahoehoe Lava -- Surfaces are smooth, billowy, or ropy.
  • A'a Lava -- Surfaces are fragmented, rough, and spiny, with a "cindery" appearance

Pahoehoe Lava



Charateristics of Pahoehoe Lava

  • Relatively thin (1-2m)
  • Very fluid
  • Low Viscosity

Pahoehoe Lava can be further divided into 3 different types:

  • Ropy Pahoehoe - lava surface is bunched up or wrinkled and resembles a coiled rope
  • Sheylly Pahoehoe - contains a billowy flow top with a frothy vesicular surface skin, only a few centimeters thick, overlying large cavities, generally 5-30 centimeters thick
  • Slabby Pahoehoe - contains a series of closely spaced slabs, a few meters across and a few centimeters thick, broken and tilted by mass movement, or drainage, of the underlying lava.

A'a Lava



Characteristics of A'a Lava

  • Cindery fragments broken during the churning action of flow advancement.
  • Thicker than and less viscous than pahoehoe lava.

ANDESITIC LAVA




Characteristics of Andesitic Lava

  • Blocky nature of lava flows
  • High viscosity
  • Relatively high thickness (some thick enough to form lava domes)

DACITIC TO RHYOLITIC LAVA




  • As silica content and polymerisation increases, the viscosity of dacitic and rhyolitic lava increases.
  • Although dacitic-to-rhyolitic lavas typically erupt from stratovolcanoes, they are not as abundant as andesite lava.
  • Viscous dacitic-to-rhyolitic lavas generally ooze out of the volcano's central vent to form symmetrical lava domes

Characterisitics of Dacitic to Rhyolitic lava

  • High viscosity
  • High gas content



posted by the geog bloggers at 11:26:00 pm | 1 Comments

Friday, 23 February 2007

Volcanic Landforms

VOLCANIC LANDFORMS

Volcanic landscapes contain diverse landforms. The most recognizable of these include volcano edifices, calderas, and lava domes. Each of these landforms can vary markedly in size, shape, composition, and eruptive history.

DIFFERENT TYPES OF VOLCANOES:




We shall look into two types of volcanoes: Shield volcanoes and stratovolcanoes.

1) Shield volcanoes:

- Broad, low-profile features with basal diameters that vary from a few km to over 100 km
- Gentle lower slopes, steeper upper slopes
- Low viscosity of basalt lava leads to their broad shape


Three-dimensional image of the Alcedo shield volcano on Isabella Island, Galapagos

2) Stratovolcanoes:
- Also known as composite cones
- Gentle lower slopes, rise steeply towards the summit to produce an overall morphology that is concave in an upward direction.
- May contain several eruptive centres, caldera or amphitheatre as a result of a lateral blast.


Mt. Mayon


posted by the geog bloggers at 10:04:00 pm | 0 Comments

Friday, 9 February 2007

Eruption Products Overview

An Overview of Eruption Products





posted by the geog bloggers at 11:31:00 pm | 1 Comments

Friday, 26 January 2007

The First Post

Welcome to the the blog on volcanoes!

We will be focusing on...
  • Eruption Dynamics
  • Volcanic Landforms
  • Eruption Products
  • Eruption Types
  • Historical Eruptions


posted by the geog bloggers at 10:10:00 pm | 0 Comments