Intrusive and extrusive volcanic activity?

Question

Definition, similarities and differences?

Answer 1

The words say it all.



Intrusive volcanic rocks "intrude" the surrounding rock but never make it to the surface. Hot magna rises because it is lighter (less dense). It moves upward through fissures and by melting the surrounding (country) rock. As this magma is never exposed to the air it cools slowly, crystals have time to grow and the result is a fairly coarse grained rock. The most common example is granite. Bodies of intrusive rocks are called "batholiths".

Extrusive volcanics are "extruded" to the surface as lava. As with intrusives the magma rises and when it reaches the surface it comes out as and eruption or a flow. This depends on the composition of the magma - sticky silica rich magma (andesitic or rhyolitic) results in explosiv eruptions while silica poor magma (basaltic) generally just flows. Contrast the violent Mt St Helen's eruption with the lava flows in Hawaii (andesitic versus basaltic). These rocks are fine grained (small crystals) because they cool quickly.

Hope this helps

  

Answer 2

Intrusive rocks are the builders of the continental crust and mountain belts.

Extrusive rocks are the volcanics that created the oceanic crust.

Granite (intrusive) is the igneous rock most common on the continents, basalt (extrusive) is most common on the ocean floor, and andesite (extrusive, usually on or near continental margins) is the building material of young volcanic mountain ranges.

A coarse grained texture is the most significant difference between plutonic (intrusive) rocks and extrusive rocks which are likely to be fine-grained.