Volcanoes Chapter 4 Volcanoes and Plate Tectonics Volcanic Eruptions

Volcanism-the process in which magma is formed and moves towards the surface of the earth Magma-melted rock; occurs deep in the crust Lava-magma that erupts and flows on the earth’s surface Vent-the opening through which molten rock flows; most commonly called volcanoes Fissure-long cracks in the earth’s crust through which lava flows

What is a volcano? A volcano is a mountain that forms when molten rock erupts or flows as lava from an opening in Earth’s surface.

How do volcanoes form? A volcano forms when molten rock erupts or flows as lava from an opening in earth’s surface and builds up a volcanic cone. These openings are called vents. Volcanoes release molten rock, ash, and poisonous gases. All these products result from melting in the mantle or in the crust.

The Geography of Volcanism Three major zones of volcanic activity convergent plate boundaries divergent plate boundaries hot spots

Where The Volcanoes Roam! If you were to plot the major volcanoes that have erupted over the past 50 years, a distinct pattern is shown. Volcanoes tend to follow the same global patterns of earthquake zones along plate boundaries. Some areas we know volcanoes exist are: Convergent Boundaries (Subduction Zones) Divergent Boundaries (MOR/Rift Valleys)

Divergent Boundaries Along mid-ocean ridges, a ridge of underwater volcanoes is formed— Iceland’s volcanoes are part of this kind of ridge. Volcanic activity is pronounced- Active but quiet Lava- Basaltic

Convergent Boundaries Where a plate collides with another plate Friction+ Energy of earth’s interior Magma- hotter and lighter than surrounding rocks, erupts Volcanic activity- Intense and sudden Active and violent volcanoes Lava- Granitic

If both the convergent plates are continental No Volcanism magma is likely to remain buried deep in the crust, and does not erupt. Eg. Himalayas

Convergent & Divergent Boundaries

Pacific Ring of Fire When the overriding plate is an oceanic one, volcanoes erupt.

How come the most volcanic place on Earth is nowhere near a plate margin?

What are Hotspot Volcanoes? Photo: Tom Pfeiffer / The Hawaiian island chain are examples of hotspot volcanoes. A hot spot is an area of great heat in the mantle. Concentration of radioactive elements. Breaching the surface in the middle of a tectonic plate.

If a tectonic plate moves over a fixed hotspot, a chain of volcanoes or volcanic islands are formed. The volcanoes get younger from one end to the other.

Hot Spots Earth tremors indicate that Hawaii does tap the upper mantle. The Hawaiian chain is formed from a hot spot in the mantle.

The Birth of an Island There is yet another island forming on the Hawaiian Island chain → Loihi It’s top is about 3,000 feet below the water’s surface. It will reach the surface in about 10,000 to 100,000 years!

Hot Spots Loihi building, but still 1000 m below surface will eventually replace the Big Island

Classification of Volcanoes As volcanoes erupt they tend to form conical pile called volcanic cones; these cones fall into 3 categories Shield-broad based volcanoes with gentle slopes due to less viscous lava; lava travels farther Composite/Stratovolcanoes-eruptions change between quiet and explosive; create large, broad, & tall cones, more viscous lava, does not flow farther away. Fissure Volcanoes- : lava erupts out of cracks in the earth’s crust -a few metres to a few hundred kilometres.Rather than building a cone, the lava forms flat topped hills in a series of steps.

Three Types of Volcano Profiles

Volcanoes: Explosive Composite cones (stratovolcano) pointed, steep-sided, tall volcanoes “Composite”: layers of pyroclastics and lava Lava is highly Viscous Explosive and dangerous; found in subduction zones

Landforms: Effusive Shield volcanoes large, rounded volcanoes with a gentle slope central vent flat terrain (or can not build up dome) constructed by a series of basalt flows over time Lava is less Viscous.

Fissure erruptions

Deccan Traps, India The Deccan Traps are flood basalts similar to the Columbia River basalts of the NW United States. They are one of the largest volcanic provinces in the world. It consists of more than 6,500’ of flat-lying basalt lava flows and covers an area of nearly 200,000 2 mi. The volume of basalt is estimated to be 12,275 3 mi. ( Mt. St. Helen produced 1 3 km of volcanic material). Photo: Lazlo Keszthelyi, Jan. 28, 1996

Volcano Classification Active: has erupted in recorded history or in constant eruption mode; found at all boundaries. Dormant: has not been seen to erupt in history, but shows evidence of recent activity; convergent boundaries. Extinct: no sign of recent or historic activity; mostly passed over hot spot

Hot Spot Active Volcanoes

Active Volcanoes

Subduction Volcanoes

Volcanic Explosivity Index This scale assigns a number to an eruption based on things that can be observed, The explosivity, The height of an eruptive plume and The volume of the materials erupted. VEI Scale- from 1 to 8

Eruption of Mount St. Helens

The World's Deadliest Volcanoes Just as seismologists measure the size of an earthquake using the Richter scale Volcanologists use an eruption magnitude scale called the Volcanic Explosivity Index, or VEI to classify eruptions.

Landforms associated with Volcanism Bubbling mud pools Geysers Hot springs

Bubbling Mud Pools

Geysers

Hot Springs

Geysers & Hot Springs

How Magma Flows – Viscosity is the internal resistance to flow; the higher the viscosity, the thicker the magma.

The primary factors which determine the nature of volcanic eruptions: These determine the magma's mobility of viscosity. (Molasses is more viscous than water.) The thicker, or more viscous, the magma, the more resistant to flow. 1. Magma composition. 2. Magma temperature. 3. Amount of dissolved gasses.

Modern Caldera-Crater Lake

Sky light View of an active lava tube as seen through the collapsed roof: sky light.