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Physical Geology 101
8. Igneous Rocks I
(p. 102-104; 114-116)
What Makes a Rock?
What is the difference between a mineral and a rock?
A mineral is: _____________________________________________________________
A rock is: _______________________________________________________________
Do rocks have a chemical formula?
YES or NO
The three rock types are:

1._______________ - How do they form?: _______________________________

2._______________ - How do they form?: _______________________________

3._______________ - How do they form?: _______________________________
Magma and Lava
Inside the Earth, molten rock is called ______________.
On the surface of the Earth, molten rock is called _____________.
Why do rocks melt? ____________________________________________________
____________________________________________________________________
The rate that temperature increases with increasing depth inside the Earth is called the:
___________________________________
On average, this temperature gradient is about _____ °C increase per 1km increase in depth into the
Earth.
Is the geothermal gradient the same everywhere on Earth? ____________________
____________________________________________________________________
Contour lines of equal temperature, used to represent the changing temperature with depth inside the
Earth, are called ____________________. They show that the rate of temperature increase is highest
beneath the oceans.
Most rock types will start to melt at the Earth’s surface at temperatures of between ______°C and
_______°C. Temperatures this high are reached at a depth of about 100 km/62 mi below the continents.
So why isn't the whole inside of the Earth a liquid below 100 km?
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Physical Geology 101
1. The _______________ also increases with depth, which affects melting temperatures. As
pressure increases, melting temperature _____________.
2. Also, the presence of ___________ in the magma tends to lower the melting temperature.
Pressure is the most important control on the melting temperature of particular rock types.
The Origin of Magma
In rocks, different minerals have different melting temperatures at a particular pressure.
Which mineral will start to melt first as a rock gets heated up inside the Earth?
________________________________________
This process of melting different minerals at different times as a rock gets heated up is called
________________________.
e.g., the mineral _______________ tends to melt before other minerals.
The chemistry of the melted portion is very different to the chemistry of the solid minerals left behind. If we
raise the temperature high enough though, eventually the whole rock will melt.
If we were to somehow remove the partial melt before all of the rock gets melted, we would leave behind
bits of rock with a different chemistry to the rock that we started out with. The melted part would also have
a different chemistry to the original rock.
This process of separating the melt from the solid rock is called __________________.
It happens a lot inside the Earth because high pressures squeeze the melted part out of the solid parts
like water out of a sponge. As a result of these processes, we end up with many different types of igneous
rocks that form from the melted parts (magma) and the left-behind solid parts.
Why does magma move up towards the Earth's surface? ______________________________________
From how deep does most magma originate? _____________________
Most magma first accumulates underground in a large reservoir or pod of magma called
a_____________________. Eventually, it will erupt onto the surface as lava.
Properties of Magma and Lava
The most abundant elements in the Earth's crust are:
Si, Al, Fe, Ca, Mg, Na, K, H, O
What is the most abundant component of magma? _______________
There are four types of magma or lava (and thus igneous rocks) based on composition:

_________________: >65% silica; lots of Na, K, and Al (light-colored rocks)
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Physical Geology 101

_________________: 53 - 65% silica (light and dark colors)

_________________: 45-52% silica; lots of Ca, Fe, and Mg (dark-colored rocks)

_________________: <45% silica; lots of Fe and Mg (very dark-colored rocks)
Partial melting and fractionation of ultramafic rocks in the mantle produces mafic magma. Mafic lava then
gets erupted at ________________________, such as the mid-Atlantic ridge at the bottom of the Atlantic
Ocean, halfway between Europe and North America.
Mafic magma can also develop above mantle plumes at __________________, where jets of magma
burn holes through the crust from below.
Examples of hot spots: _________________ and _________________.
Mafic oceanic crust moves back down into the mantle at a ______________________, where it starts to
partially melt. This partial melt forms either an intermediate or a felsic magma.
We have a subduction zone right off the coast of Washington and Oregon called the Cascadia
subduction zone. Intermediate and felsic magma is produced in this subduction zone which erupts from
Cascades volcanoes, such as _____________________.
Magma contains liquid, solids (crystals) and gases all mixed together. What do we call dissolved gases
inside a magma? ____________________
How hot can magma get? _____________________________
The ability of a lava to resist flowing downhill is called the ___________________.
How fast have lavas been recorded to flow in Hawaii?:
_______ mph
The two types of lava flow on Hawaii are:
___________ (ah-ah): Description: sharp, blocky fragments
Fast or slow? ____________
_____________ (pah-hoy-hoy): Description: smooth and syrupy texture
Fast or slow? ___________
What are the main controls on how fast a lava will flow?
__________________ (hotter = faster)
and
___________________ (more silica = slower)
So what type of lava tends to flow the fastest? _________________
What is the furthest that lava flows been known to flow (in the Columbia River valley region of the Pacific
Northwest)? ___________________
Igneous Rocks
Igneous rocks can form through one of two processes:
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Physical Geology 101

through accumulation and hardening of ______________________ (hot ash).

through the process of cooling and ______________________ of minerals from magma or lava.
The type of minerals that grow is determined by the __________________ of the magma or lava.
The type of igneous rock that forms is determined by the ________________________ that crystallize
out of the magma or lava.
Do rocks that form from lava have big or small crystals?
BIG
Do rocks that form from magma have big or small crystals?
or
BIG
SMALL
or
SMALL
FINAL QUESTION:
During crystallization, what controls the size of crystals that grow?
_____________________________________________
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