Chapter 12

Volcanoes

Sec 12-1 What are volcanoes?

A volcano is an opening in Earth that erupts gases, ash, and lava. A volcanic mountain forms when layers of lava ash build up along side of the volcano.

Kilauea (kee low Ay ah) in Hawaii is today’s most active volcano.

When eruptions occur they can cause major destruction, like the burning down of homes and forests and covering land in ash.

The flow of lava down the side of a mountain is called pyroclastic flow.

Formation of a volcano Deep in the Earth heat and pressure cause

surrounding rock to melt and become magma. Because this magma is less dense then the

rock around it, it is forced upward toward the surface.

After a long time the magma reaches the surface and flows through a vent, or an opening in the earth.

As the magma flows out it becomes lava and hardens very quickly causing igneous rock to form around the vent , causing a steep wall, called a crater to form around the vent.

Volcano

Where do volcanoes occur?

They occur in 3 places:1. Where plates move apart.2. Where plates move together.3. And at hot spots.

Where do volcanoes occur?

Where plates move apart is called a divergent plate boundary. A major one is the Mid-Atlantic ridge.

When these plates move apart they form deep cracks called rifts. When these form lava can then rush through them creating a volcano.

Ex: The Surtsey Island in Iceland was formed this way.

Where do volcanoes occur?

Where plates move together is called a convergent plate boundary. Here plates tend to slide above or below each other and cause the most violent eruptions.

Ex: Soufriere Hills in Montserrat

Soufriere Hills in Montserrat,West Indies

Where do volcanoes occur?

Some areas at the boundary between the Earths mantle and core are hot enough to melt rock and cause the magma to come to the surface.

These are called hot spots and no plate movement is involved.

Ex: The Hawaiian IslandsH.W. pg 335 ques.1-4 & pg 354 ques

4,6,7,

Kilauea volcano, Hawaii

Sec 12-2 Volcano Eruptions

Some volcanoes erupt very violently and some quietly flow. What controls this?

The amount of water vapor and other gases. And how much silica is in the magma.

If the gases (water and CO2) from the magma are under great pressure then a violent eruption occurs.

If the gases have a chance to exit the vent before the pressure is too great then the lava gently flows out.

Quiet & Explosive Eruptions

Silica poor magma or basaltic magma will cause quiet eruptions. Ex: Kilauea.

The lava will flow gently down the sides of the volcano.

Silica rich magma or granitic magma will cause explosive eruptions because of its thick nature, it traps gases.

Ex: Soufriere Hills in Montserrat

Types of Volcanoes Shield volcanoes-

broad volcanoes with gentle sloping sides. Usually made from basaltic lava and found in the Hawaiian islands.

Types of Volcanoes

Cinder Cone are caused by explosive eruptions that throw lava into the air. This solid lava that falls from the air is called tephra, and when it falls it forms steep-sided, loosely packed cinder coned volcanoes.

Types of Volcanoes

Composite volcanoes can be quiet or violent depending on the gases and silica in the magma.

When a volcanoes eruptions switch back and forth from quiet to violent, spewing tephra, they create composite volcanoes.

These usually occur over plates and an ex: is Soufriere Hills in Montserrat.

H.W. pg 343 ques. 1-4 & 354 ques. 1-3,5,8,9,11,13

Sec 12-3 Igneous rock features

Igneous rock that form inside the Earth are called intrusive. The largest rock body that intrusive rock forms is called a batholiths.

These form when large amounts of magma that is being pushed toward the surface cool before they get there.

Sills and Dikes

Magma sometimes squeezes into cracks of rocks and hardens below the surface. These are called dikes.

Magma that enters cracks that are parallel to the rocks are called sills.

Sills and Dikes

Volcanic neck

volcanic neck or lava neck, is a volcanic landform created when lava hardens within a vent on an active volcano and the surrounding volcanic mountain is eroded away.

Calderas

When an eruption causes the top of the volcanic mountain to collapse, it causes a large depression called a Caldera.

These can later on become filled with water and become lakes.

H.W. pg 356 1-20