Lab 4: Magmatism and Igneous Rocks

rock vesicles:

bubbles frozen into a lava, tells that gas has been dissolved in magma.

example of an intermediate, intrusive rock

diorite

Felsite Porphyry

felsic, lighter/intermediate color, pophyritic and larger phenocrysts of k-feldspar and na-plagiocase

Pumice

felsic, lightweight, contains vesicles

low viscosity

flows quickly

Gabbro

mafic, phaneritic, has minerals; crystals of pyrocene and ca-rich plagiocase feldspar

Basalt Porphyry

mafic, porphyry, contains vesicles filled with phenocrysts of the mineral olivine

vesicles in intrusive igneous rock signifies:

magma was near surface before cooling

Stock

main tube through which magma exits the volcano

Pluton

small frozen magma chamber

The three major tectonic settings in which melting occurs

subduction zones, hot spots and mid-ocean ridges

example of a felsic, extrusive rock

rhyolite

When lava crystallizes it is known as:

extrusive igneous rock

Rhyolite

felsic, aphanitic and contains rock fragments

Graphic Granite

felsic, contains minerals plagiocase and quartz, looks striped (lines of quartz)

high viscosity

flows slowly

example of an mafic, intrusive rock

gabbro

Obsidian

glassy and conchoidal fracture

example of a felsic, intrusive rock

granite

Andesite Porphyry

has an intermediate color and porphyritic-phenocrysts of amphibole, variety hornblende

mafic lavas:

have higher melting temps and lower viscosity. they form broad shaped shield volcanoes

felsic lavas:

have lower melting temps, higher viscosity and form steep volcanic domes that are more likely to erupt explosively.

phaneritic texture

igneous rock texture in which minerals are easily visible without magnification

Diorite

intermediate color, phaneritic and minerals of amphibole (black) and light-colored/white plagiocase

When magma is erupted at the surface it is called:

lava

Intrusive igneous rock

when magma crystallizes before making it to the surface

Basalt

mafic, aphanitic

Porphorytic Texture

two or more distinct grain sizes, slow cooling forms large crystals called phenocrists, subsequent rapid cooling results in fine-grained crystals

Extrusive rocks cool and crystallize faster than intrusive rocks so:

very-fine grained or aphanitic texture

Where are mafic magmas formed?

Mid-ocean ridges

amorphous solid

A solid made up of particles that are not arranged in a regular pattern (glassy, obsidian)

What is the cause of melting at mid-ocean ridges and hot spots?

Decompression melting (where mantle material is upwelling).

Which minerals are first to melt in a rock?

Felsic minerals high in silica (those with the lowest melting temperature).

What happens to melting temperature when water is introduced?

It is lowered.

Is the temperature at which mantle peridotite melts higher in the upper or lower mantle?

Lower because the pressure is higher.

When a rock begins to crystallize, which minerals are the first to crystallize?

Mafic minerals high in iron and magnesium (those with the highest melting temperatures).

What is mantle rock also known as?

Peridotite

Sources of heat within Earth

Radioactive decay: heat produced by radioactive decay of elements like uranium and potassium, Primordial Heat: heat leftover from the formation of the planet

Where are felsic magmas formed?

Subduction zone volcanic arcs

Where are intermediate composition magmas formed?

Subduction zone volcanic arcs.

Why are pyroclastic flows so dangerous?

They move extremely fast and are extremely hot. Hot gases act as lubricant. Consist of lava, hot ash and gases.

Batholith

a large frozen magma chamber

Viscosity

a liquids resistance to flow

Caldera eruption

an explosive eruption resulting from the roof of a giant magma chamber collapsing in on its self (yellowstone)

example of an intermediate, extrusive rock

andesite

example of a mafic, extrusive rock

basalt

Scoria

mafic and aphanitic, contains vesicles

mafic rocks are:

more rich in iron and magnesium and are generally dark in color

felsic rocks are:

more rich in silica and are generally light in color

Most intrusive rocks are:

phaneritic because they cool slowly allowing large crystals to form

Fine-Grained Granite

phaneritic, light-colored and contains quartz, k-feldspar and mica

Coarse-Grained Granite

phaneritic, light-colored, contains quartz, k-feldspar, and mica.

Dike

planar intrusion of magma cross cutting surrounding rock layers

Sill

planar intrusion of magma parallel to surrounding rock layers

During fractional crystallization:

the liquid portion is always more silica rich than the solid portion.

Fractional crystallization:

the process by which minerals crystallize in a cooling magma chamber and are separate from the remaining magma by density.

What causes melting in subduction zones?

water is released from the down-going slab which creates water vapor and lowers the melting temperature of the overlying mantle.