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Chapter 18 Volcanic Activity. Section 18-1 Magma. Objectives: Describe factors that can affect the formation of magma Compare and contrast the different types of magma. Volcanic Eruptions.

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section 18 1 magma
Section 18-1Magma


  • Describe factors that can affect the formation of magma
  • Compare and contrast the different types of magma
volcanic eruptions
Volcanic Eruptions
  • In the last 10,000 years more than 1500 major volcanic eruptions have been recorded…where and why do these eruptions occur?
how magma forms
How Magma Forms
  • Recall that magma is a mixture of molten/melted rock, suspended mineral grains, and dissolved gasses found deep beneath the Earth’s surface.
how magma forms1
How Magma Forms
  • Magma usually forms at temperatures between 800 and 1200 C (1600 to 2400 F)
  • The temperature, and the pressure on the rock along with the amount of water in the rock all determine at what temperature the rock will melt.
the inner earth
The Inner Earth

Crust (lithosphere)

Upper Mantle

Lower Mantle

Outer Core

Inner Core

rock review
Rock Review
  • Sedimentary (sediments)
  • Igneous (cooling magma/lava)
  • Metamorphic (heated solid rock)
pressure and temperature
Pressure and Temperature
  • As pressure increases on the rock inside the Earth, the temperature of the rock increases.
  • So, the deeper down into the Earth that you go, the higher the temperature.
water and magma
Water and Magma
  • If rock contains spaces that hold water, the rock will melt at a lower than normal temperature because the water found inside the rock helps it liquefy as it heats up.
types of magma
Types Of Magma
  • Basaltic
  • Andesitic
  • Rhyolitic
basaltic magma
Basaltic Magma
  • Typically forms when rocks in the upper mantle begin to melt, it tends to be fast moving and relatively quiet as it erupts due to its low gas content.
andesitic magma
Andesitic Magma
  • Andesitic magma is made in subduction zones and is typically about 60% silica, it tends to move and erupt at a medium pace.
rhyolitic magma
Rhyolitic Magma
  • Rhyolitic magma is thick and slow moving…it is filled with gas and water and tends to be very explosive because pressure builds within it.
  • Viscosity refers to the rate or speed at which something will flow.
  • If something has a high viscosity, the thicker and slower it will flow
  • If something is said to have a low viscosity, the thinner and faster it will flow.
section 18 2 intrusive activity
Section 18-2Intrusive Activity


  • Explain how magma affects overlying crustal rocks
  • Compare and contrast intrusive igneous rock bodies
density and movement
Density and Movement
  • Because molten rock is nearly liquid compared to the solid rock around it, it is less dense and wants to move upward.
  • What happens as the magma flows upward into cooler crust? The process is called “intrusion”.
  • The magma can force the solid rock to push apart and create fissures
  • The magma can contact the upper solid rock and cause pieces of it to melt and fall into the magma pool
  • The magma can immediately melt the rock into which it flows

As the flowing magma cools inside of the solid rock, it crystalizes and forms blocks, ribbons or veins of new “intrusive” igneous rock.


Batholiths are the specific name given largest formations of plutons. They are usually found in large mountain ranges. They cut across rock layers.


Laccoliths tend to form near the Earth’s surface where they cause the rock above to push upward in a dome shape.


Sills are intrusions that form parallel to the existing rock, they tend to stay underground and flow like a “spill”.

plate movement
Plate Movement
  • Underground plutons can be brought to the surface as tectonic plates converge and push upward exposing the rock within it.
  • Most igneous rock on the surface of the Earth is not from volcanic activity but from the slow gradual process of plate tectonics.
section 18 3 volcanoes
Section 18-3Volcanoes


  • Describe the major parts of a volcano
  • Compare and contrast shield, cinder-cone, and composite volcanoes
  • Contrast the volcanism that occurs at plate boundaries
anatomy of a volcano
Anatomy of a Volcano
  • The magma that erupts to the Earth’s surface is then called lava.
  • The lava erupts through an opening in the crust called a vent.
  • The lava will cool and solidify around the vent forming a mountain that is called a volcano.
anatomy of a volcano1
Anatomy of a Volcano

At the top of a volcano around the vent is a bowl-shaped depression called a crater

types of volcanoes
Types of Volcanoes
  • The appearance of a volcano depends on two factors:
  • The type of material that forms the volcano
  • The type of eruptions that occur
types of volcanoes1
Types of Volcanoes
  • Shield
  • Cinder-cone
  • Composite
shield volcanoes
Shield Volcanoes
  • A wide volcano/mountain with gently sloping sides and a nearly circular base
  • It results from the slow eruption of basaltic lava that builds up in layers
  • Hawaiian Islands
cinder cone volcanoes
Cinder-Cone Volcanoes
  • Very steep mountain/volcano that results from material being ejected straight up into the air and falling back down around the vent
  • Magma/lava contains some gases that make the eruptions explosive
composite volcanoes
Composite Volcanoes
  • Larger versions of cinder-cone that have been made of layers of lava and solid material. The magma/lava that makes them up is full of gas that builds up until it finally explodes violently.
  • Mt. St. Helens
volcanic material
Volcanic Material

Rock fragments thrown into the air during an eruption are called tephra.

  • Classified by size
    • Dust
    • Ash
    • Lapilli
    • Volcanic blocks (angular)
    • Volcanic bombs (rounded)
pyroclastic flows
Pyroclastic Flows
  • Violent eruptions that send out a wave of gas, ash, and tephra that can travel up to 400 mph.
where do volcanoes occur
Where do Volcanoes Occur?

The distribution of volcanoes around the world is not random, most occur at plate boundaries.

  • 80% convergent
  • 15% divergent
  • 5% non-boundary “Hot Spots”
convergent volcanism
Convergent Volcanism

Plates coming together, this forms the Pacific Ocean “Ring of Fire” and the “Mediterranean Ring” in the Atlantic.

divergent volcanis m
Divergent Volcanism
  • Plates spreading apart also create a way for magma to escape…these are called rift zones.
  • Mid-Atlantic Ridge
hot spots
Hot Spots
  • The Hawaiian Islands are not formed along a plate boundary but instead are found in an area of the mantle that is usually hot called a “hot-spot”
  • As the plates move over the hot spot volcanoes form.