Download Origin of magma (pg.270-273)

Objectives: the origin of MAGMA
1. Define/describe geothermal gradient and how it
applies to rock melting.
2. How does reducing confining pressure change
rock’s melting temperature?
3. Decompression melting of rock occurs when
rocks (ascend, descend)? Explain.
4. What effect do volatiles (such as water) have on
the melting temperature of rock?
5. Do all minerals melt at the same temperature?
Explain the partial melting of magma?
Bellringer:
Review: Name and describe Earth layers.
Which layers are solid? Liquid?
From which layer does magma originate?
A.
B.
C.
D.
E.
convection
Earth’s Mantle
• Earth's mantle is
thought to be
composed mainly
of iron peridotite
Origin of magma (pg.270-273)
Magma forms when SOLID rock and
minerals, located in the asthenosphere and
upper mantle, melt
Factors that form magma:
• Heat
• Pressure
• Volatiles (refers to the volatile or reactive
components of magma [mostly water vapor and
carbon dioxide])
Role of Heat
Earth’s natural temperature increases with
depth 20°-30°/ km in the upper crust.
(geothermal gradient)
In the asthenosphere (60 miles) temps. are
1200°-1400°
Yet this is NOT hot enough to melt
rock…due to the fact that the pressure at
that depth is too great.
Role of Pressure
 Pressure increase with depth.
Because of pressure melting must occur
at a higher temperature.
• Increase in pressure causes an increase in melting
temperature
• So….DECREASE in pressure can cause
decompression melting:
• when mantle rock rises, pressure decreases
and melting can occur !!
•
Role of Water in magma
Water
• Allows rock to melt at a LOWER temperature
• “Wet” rock has a lower melting point
• When an ocean plate sinks under a continent,
ocean water goes down with it.
Minerals MELT and cool / crystalize at
different temperatures to form different
igneous rocks.
Origin of magma
 Partial melting
• Rocks are MIXTURES of minerals
• Melting occurs over a range of temperatures
• Minerals with the lowest melting point melt first
producing magmas with a higher silica content
than the original rock. (felsic)
• Minerals that melt last with higher melting
points produce mafic magma (basaltic)
– (hot mantle plumes, Hawaiian Islands, basaltic rocks)
Different types of magma react differently
and give different types of eruptions…