Download Warm-up #49 Apr. 3

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?
Volcanoes
Unit 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
• 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
Lava Flows
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
Assignment
Your 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_frames.ht
ml
• 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
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•
•
•
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 soda
Lastly, add vinegar