Warm-up #49 Apr. 3

• May 18, 1980 – Mount St. Helens erupted with tremendous force– Blew off the entire north flank of the

mountain– Ejected a cubic km of ash and rock

debris– Yakima, Washington (130 km away)

was covered with ash that blocked out the sun

• Why do volcanoes like this erupt so explosively, while others like Kilauea in Hawaii are relatively quiet?

VolcanoesUnit 9

What is a volcano?

• An opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface

Factors that Affect Eruptions

• Factors that determine the intensity of a volcano include:– Magma composition– Magma temperature– The amount of dissolved gases in the

magma

VISCOSITY

• Viscosity is a substance’s resistance to flow– Ex. Maple syrup is more viscous than water.– Magma from an explosive eruption may be

thousands of times more viscous.– As lava flow cools and begins to harden, its

viscosity increases, its mobility decreases, and halts.

– Directly related to its silica content

Dissolved Gases

• The gases trapped in magma provide the force to eject molten rock from the vent, or opening on the surface.– Water vapor and carbon dioxide– Viscous magmas slow the movement of

gases going upward creating explosive bubbles and pockets

Lava Flows• Basaltic Lava– Very fluid (low silica

content)• Two forms:

– pahoehoe flows resemble twisted braids of ropes

– Aa (Ah ah) flows have rough, jagged blocks and sharp edges

Pyroclastic Materials

• Name given to particles produced in volcanic eruptions

• Fragments ejected during eruptions range in size from very fine dust and volcanic ash (less than 2 mm) to pieces that weigh several tons

Types of Volcanoes

• Three main types:– Shield– Cinder cone – Composite cone

Anatomy

Shield Volcano

• Produced by the accumulation of fluid basaltic lavas.

• Shaped of a broad, domed structure• Ex: Hawaiian Islands, Iceland

Cinder Cones

• Usually a product of relatively gas-rich basaltic magma

• Very simple shape and small– Determined by the steep sided slope

that loose pyroclastic material maintains as it comes to rest

Composite Cones

• Considered the most beautiful and most dangerous

• Large, nearly symmetrical, composed of layers of both lava and pyroclastic deposits

AssignmentYour mission is to find information and report on

a volcano, other than the ones listed above, that has erupted in the last 100 years. Your report must include:

• Type of volcano • Geographic location • Name, distance, and population of nearest

major city • Date of most recent eruption and date of most

destructive eruption • Other events associated with the last eruption

(earthquakes, floods, mudslides, etc)

Warm-up #50 Apr. 4

According to geologists what is the most devastating natural event that can take place on Earth?

What type of structure is this?

Other Volcanic Landforms

• Caldera – large depression in a volcano.– Usually formed 2 ways:

• Collapse of the top of a composite volcano after erupting.• Collapse of a shield volcano after magma chamber is

drained.

Other Volcanic Landforms

• Necks and Pipes – conduits that connect a magma chamber to the surface.– Rocks in the pipes remain standing after the

cone has been eroded called a volcanic neck

Other Volcanic Landforms

• Lava Plateaus– The greatest volume of volcanic

material is extruded from fissures.– Ex. Columbia Plateau

Plutons

• Plutons – structures that result from the cooling and hardening of magma at depth.– Intrusive igneous bodies are classified

according to shape, size, and relationship to surrounding rock layers

• Sills and Laccoliths – plutons that form when magma is intruded close to the surface.– Sills form when magma is injected along

sedimentary bedding surfaces– Laccoliths are similar to sills but magma is

more viscous

• Dikes – form when magma is injected into fractures, cutting across preexisting rock layers.– Many form when magma from a large

chamber invades fractures.

• Batholiths – largest intrusive body– Must have a surface exposure greater than

100 square km– Ex: Idaho Batholith (40,000 square km)

Assignment• http://www.mnh.si.edu/earth/main_frame

s.html• Explore the site, Review plate tectonics

and continue on to volcanoes• Once you have read about volcanoes, go

to “GeoGallery” at the bottom of the page.

• In your journals, write about 5 different types of volcanic activity (caldera, stratovolcano, pyroclastic flow, etc)

• You should have a paragraph for each

Warm-up #51 Apr. 5

• Why is a volcano fed by a highly viscous magma likely to be a greater threat to people than a volcano fed by very fluid magma?

Origin of Magma

• Controversial subject• Geologists conclude that magma

originates when solid rock (crust and upper mantle) partially melt

Role of Heat

• Rate of temperature change averages between 20°C and 30°C per km in the upper crust.

• 100 km ranges from 1400°C and 1600°C– Close to the melting point of rock.

• Sources of heat:– Friction– Crustal rocks descend– Hotter mantle rocks rise

Role of Pressure• An increase in confining pressure

causes an increase in the rock’s melting temp.

Role of Water• Water causes rock to melt at lower

temperatures

Convergent Plate Boundaries

• The basic connection between plate tectonics is that plate motions provide the mechanism by which mantle rocks melt to generate magma

Intraplate Igneous Activity• Intraplate Volcanism occurs within a

plate, not the boundary.• Most occur where a mass of hotter than

normal mantle material called a mantle plume rises toward the surface

• Ex: Kilauea in Hawaii

Ticket Out the Door

1. What are the three types of volcanoes?

Build a Volcano• Get in groups of 3-4• Grab a plate, bottle and some play-

doh• Shape a volcano around the bottle• Add:

– Half-way full of warm water & food color

– 4-6 drops of detergent– About 2 tablespoons of baking sodaLastly, add vinegar