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Constructive & Destructive Margins

Constructive: Plates move away from each other (N.American plate & Eurasian plate)

Features: Mid ocean ridges, Sea floor spreading & Rift valleys

Lava extruded is alkaline and free flowing (basaltic lava), volvanoes have gentle sides forming shiled volcanoes, vent does not block so eruptions are frequent and non-violent.

Destructive:  3 types (Oceanic subducted under continental - two continents meet creates fold mountains and marks the end of an ocean - two oceanic plates meet the older denser plate subducts.

Features: Deep ocean trenches, Deep focus earthquakes, Island arcs & Volcanoes

Lava reaches the surface through fissures in continental crust, it absorbs silica and becomes acidic and viscous, it is slow moving so forms steep sided cones (composite volcanoes) lava can solidify and block the vent forming a plug, pressure must build up before eruption e.g. Mount St. Helens.

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Intrusive features

  • Dike: a vertical feature where the magma has pushed into a joint in the rock and hardened e.g. Monte Somma, Italy
  • Sill: horizontal feature where magma has hardened in the bedding plane of a rock strata e.g. Wind sill N. England
  • Laccolith: dome shaped igneous feature where the magma has forced rock strata upwards creating a bulge
  • Batholith: igneous mass of magma that has cooled and solidified underground, larger and more formless than laccoliths
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Extrusive features

  • Geysers: surface water works its way down to around 2000M where it contacts hot rocks, the boiling of this water results in an explosion of steam from the surface vent
  • Fumaroles: Water is superheated and it turns to steam as pressure drops when it emerges from gorund and releases this steam and gases such as CO2 & Sulfur dioxide through vent
  • Solfatara: Fumarole that releases large amounts of Sulfur (naples, italy)
  • Mudpools: Geothermally heated water reaches the surface at a spot where soil is rich in volcanic ash and clay
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