Igneous Environments

Extrusive

Igneous rocks that cool at or near the surface are refered to as extrusive (volcanic) rocks.. These rocks cool rapidly, too fast to grow big crystals.

• Mafic – erupt easily
• Intermediate – harder to erupt
• Felsic - explosive

 

Volcanic Features

• Vents
  • Crater or summit eruption: Magma rises to the surface via a conduit
  • Fissure eruption: Magma is squeezed through cracks and produces a “curtain of fire.” These vents can evolve into discrete vents.
    
    
• Craters - A bowl-shaped depression atop a volcano. Craters are up to 500 m across; 200 m deep.
  
  
• Calderas - Basically a very large crater that is 1-10s of kilometers across. They form when a volcano erupts so violently that the magma chamber empties. The weight of the overlying rock collapses into the chamber, forming the cauldera.

 

Volcano Types

• Shield - Broad, slightly domed, and very large in size. Primarily made of basaltic (fluid) lava. Eruption style is peaceful. Example: the Hawaiian Islands
  
  
• Stratovolcano - Formed in association with subduction zones. Most are adjacent to the Pacific Ocean as a result. These volcanoes are formed by layers of andesitic / rhyolitic lavas & pyroclastics. The high silica content of the magma generates explosive eruptions.
  
  
• Cinder cone - These volcanoes are built from ejected lava fragments, and have a very steep slope angle. They frequently occur in groups. Example: Pu'u O'o Cinder cone.
  
  
• Lava Domes - Sometimes a volcano erupts so much that most of the magma is ejected. What is left behind is a bulbous mass of congealed lava. This lava is very difficult to erupt. Most lava domes are associated with explosive eruptions of gas-rich magma. Examples: Mt. Unzen and Mt. St. Helens.
  
  
• Caldera - Steep walled depression at the summit formed by collapse. Produces andesitic to rhyolitic lavas and pyroclastics and erupts explosively. Examples: Crater Lake, Yellowstone, Santorini.
  
  
• Fissure eruptions and lava plateaus - Basaltic lava extruded from crustal fractures. Incredibly large volumes of lava can pour out of fissures over millions years. The large amount of volcanism can affect global climate. Eruption of the Deccan Traps between 65 - 60 million years ago released a large amount of sulfur dioxide, which lowered the global temperature of the planet by about 2 °C.