Volcanoes and Volcanic Hot Spots



Where an opening in the Earth‘s crust ejects molten rock, gas, and dust into a landform



Highly associated with plate boundaries



Magma rises to the surface due to a decrease in pressure and density



Plates collide, one subducts under the other

Two types of volcanoes at Sub-boundaries: 1.) Oceanic-continental subduction

2.) Oceanic-oceanic subduction



At a subduction boundary, volcanoes ALWAYS form on the overriding plate

The Cascade Mountain Range



At a subducton boundary, volcanoes ALWAYS form on the overriding plate

Aleutian Islands near Alaska



Not all volcanoes occur over plate boundaries



Hot spots are areas of volcanic activity that result from plumes of hot magma that rises from within the Earth’s mantle



Source is very deep: 670-2900 Km beneath the surface

 Mantle

ejects magma as lava on the surface creating volcanoes  The plate shifts and moves over the hot spot again

 Some

volcanoes erupt violently while others erupt real slow

 The

major reason is the amount of silica in the magma

 Silica

= Si on the periodic table

 Silica

(Si) is a chief ingredient in all magmas.  The amount of silica determines its viscosity  Viscosity = a materials resistance to flow

 High

silica content  Pressure builds up resisting flow  Extremely resistant to flow  Therefore…high 

viscosity!

Example: maple syrup

 Low

silica content  Pressure is released and doesn’t build up  Flows over time, not very resistant  Therefore…low 

viscosity!

Example: water, alcohol

Basaltic magmas contain the least amount of silica  The pressure doesn’t build up, so they flow easily over time Basaltic Flow  Low viscosity 

 Magma

forms at rifts/mid-ocean ridges and hotspots in the ocean

Examples:  Mid-ocean

ridge  Kilauea near Hawaii  Ethiopian rift zone

Hawaiian Island Hot Spot

Mid-Ocean Ridge

Andesitic magma contains a higher amount of silica than basaltic magma  Pressure builds up more, so it’s much more explosive Gas and Dust  High viscosity 

Explosive!

 Magma

forms at subduction zones primarily at plate boundaries

Examples:  Mount

Saint Helens  Andes Mountains

Most explosive eruption in U.S History  Occurred in the state of Washington  Intense andesitic magma with high pressure built up exploded ERUPTION! 

 Rhyolitic

magma contains the highest amount of silica  Pressure builds up and it is release explosively  High viscosiity  Similar

to Andesitic magma

 Magma

occurs under hot spots which lie underneath a continental plate

Example:

Yellowstone

Pahoehoe

Aa



Lava makes it to the water and cools very rapidly into pillow shapes

Pillow



Ejected rock from the volcano’s summit



Mixes with gas and dust as a superheated cloud that travels several hundred miles an hour



In A.D 79 Mount Vesuvius buried the city of Pompeii under as from a pyroclastic flow

Oh yea!!!

 Every

volcano has its own internal and external structure

 Split

into three main classes: 1.) Shield volcanoes 2.) Cinder Cone volcanoes 3.) Composite volcanoes

Lava tends to flow for a long time depositing in layers  Broad bases or gentle sloping sides  Support huge heights of volcanoes 

Total of 9170 meters or 27,000 feet +/ Less explosive because of its low viscosity magma  Damage extensive over a longer period of time 

Simplest design of a volcano  Molten lava is thrown in the air, but solidifies before it hits the ground  Forms a cone shape at the base  Form in groups along larger volcanoes 

 Develop

from accumulated layers over successive eruptions over time  Material includes hardened lava and pyroclastic flow materials  Mount Saint Helens 1980 volcano  Quiet volcanoes, then explosive  Pressure rebuilds

When a volcano’s top collapses in on itself after an eruption, you get a caldera  A crater shaped basin is created 