Chapter 12 Volcanoes Volcanoes and Earths Moving Plates

Chapter 12 Volcanoes

Volcanoes and Earth’s Moving Plates A. Volcano – a mountain built from the products of eruption with a central vent 1. Vent – an opening in earth’s surface which molten rock is released 2. Lava – molten rock that has extruded onto the Earth’s surface 3. Magma – molten rock below the Earth’s surface.

Volcanoes and Earth’s Moving Plates B. Volcanoes can kill people, destroy property, and disrupt the environment 1. Lava and pyroclastic flows can bury cities and towns in their paths. 2. Sulfurous gases from volcanoes can create acid rain, which can kill organisms and pollute water.

Volcanoes and Earth’s Moving Plates C. Parts of a Volcano • Magma Chamber. Open space that contains a large quantity of magma • Crater- Circular depression at the top of the mound • Tephra- Pyroclastic Debris Cloud

Other Features 4. Volcanic Neck– Forms when volcano stops erupting – Magma hardens in vent – Cone erodes Devils Neck in Wyoming

Where do Volcanoes form? • At DIVERGENT Boundaries – Plates are moving apart, forming rifts. Lava flows from these rifts and cools quickly by sea water – Forms New seafloor or can build up to form Islands Example Mid Atlantic Ridge

Mid-Atlantic Ridge

Where do Volcanoes Form? • At CONVERGENT Boundaries – – Plates are moving together Magma forms when another plate subducts and melts Magma is forced upward Tend to be more violent • Example “ Ring of Fire” chain of volcanoes over the Pacific ocean http: //geology. about. com/ od/platetectonics/tp/All. About-Convergent-Plate. Boundaries. htm

Where do Volcanoes Form? • Island arcs are volcanic islands that form parallel to ocean trenches in subduction zones.

Where do Volcanoes Form? • At HOT SPOTS – Not formed at plate boundaries – Some areas between Earths mantle and core are unusually Hot – Hot Rock at these areas is forced towards crust • Example: Hawaiian Islands • Why are Hawaiian Island in a row? https: //www. geolsoc. org. uk/Plate-Tectonics/Chap 3 -Plate-Margins/Mid-plate/Hawaiian-Islands

Types of Volcanoes A. The amount of water vapor and other gases present is one factor that determines whether a volcanic eruption will be quiet or explosive. 1. Gases can be trapped in magma by pressure of surrounding magma and rock; eventually they cause explosive eruptions. 2. Magma at convergent plate boundaries can contain a lot of water vapor that can cause explosive eruptions.

Types of Volcanoes B. The composition of magma is a second factor affecting the nature of a volcano’s eruption. 1. Low-silica magma, called basaltic, is fluid and produces a quiet, nonexplosive eruption. a. Pahoehoe lava runs down the side of a volcano. b. Aa lava is a stiff, slow moving lava. 2. High-silica magma called granitic and intermediate silica magma called andesitic produce explosive eruptions.

Types of Volcanoes • Shield Volcanoes – Broad Volcano with gently sloping slides – Buildup of layers of basaltic lava – Example Hawaiian Islands

Types of Volcanoes • Cinder Cone Volcano – Small cone shaped volcanoes – Forms from explosive lava that falls back to the ground and hardens

Types of Volcanoes • Composite Volcano – Also called stratovolcano – Found mostly at convergent boundaries – Most explosive – Steep sides

Igneous Rock Features A. Many intrusive igneous features form underground are later exposed. 1. Batholiths – rock bodies formed when magma bodies that are being forced upward from inside Earth cool slowly and solidify before reaching the surface 2. Dike – magma that hardens after being forced into a crack cutting across rock layers; sill – magma that hardens after being forced into a crack parallel to rock layers

Igneous Rock Features B. A volcanic neck forms when the cone is eroded away, leaving the solid igneous core. 1. Caldera – large depression formed when the top of a volcano collapses 2. Weathering and erosion wear down surface rock and expose igneous rock features.