Igneous Textures

Question 1

What are the textures based on crystalline forms?

Answer 1

1. Euhedral/Idiomorphic
2. Subhedral
3. Anhedral/xenomorphic
4. Hypiodiomorphic-granular texture

Question 2

Texture based on crystalline forms that have complete crystal faces that are not impinged upon by other crystals, developed under circumstances such as slow cooling of magma.

Answer 2

Euhedral

Question 3

Texture based on crystalline forms that have partially complete crystal forms

Answer 3

Subhedral

Question 4

Texture based on crystalline forms where crystals that lack any observable faces; they have had to take the shapes of whatever open spaces were available between the already crystallized minerals.

Answer 4

Anhedral

Question 5

A mix of euhedral, subhedral, and anhedral.

Answer 5

Hypidiomorphic-granular texture

Question 6

What are the textures based on the degree of crystallinity?

Answer 6

1. Holocrystalline (wholly crystalline)
2. Hypocrystalline (partially crystalline)
3. Holohyaline (glassy)

Question 7

Crystalline igneous textures that contains - small crystals less than 1 mm in diameter that are not generally discernible to the naked eye and are associated with volcanic rocks that cool quickly on Earth’s surface.

Answer 7

Aphanitic

Question 8

(Under Aphanitic texture)
Igneous texture - with crystals too fine to be identified even with a petrographic microscope, but are visible with an electron microscope and can be identified by X- Ray Diffraction (XRD).

Answer 8

Cryptocrystalline

Question 9

(under aphanitic texture)
-Igneous texture containing minerals that can only be discerned using a petrographic microscope called ________.
-______Crystals too small for the naked eye, but large enough to be identified with a petrographic microscope

Answer 9

-Microcrystalline
-Microlites

Question 10

Crystalline igneous textures that is characterized by crystals
with diameters ranging from 1 to 30 mm but is subdivided into three: fine (1 to 3 mm in diameter), which commonly develop in shallow, plutonic dikes and sills; medium (3 to 10 mm in diameter); or coarse (10 to 33 mm in diameter), which are associated with larger or deeper intrusions. Enumerate its sub classifications.

Answer 10

Phaneritic
-Fine-grained - 1 mm to 3 mm.
-Medium-grained - 3 mm to 10 mm.
-Coarse-grained - 10 mm to 30 mm.

Question 11

Crystalline igneous textures that consists of two distinctly
different crystal sizes due to a two- stage cooling process, where larger phenocrysts had started to grow and developed finer groundmass as it cool rapidly when it approached the Earth’s surface. What are those two different crystal sizes?

Answer 11

Porphyritic
-Phenocrysts (Large crystals)
-Groundmass (fine-grained)

Question 12

What are the two types of porphyritic textures?

Answer 12

a. Porhpyritic-Phaneritic (phenocrysts are distinctly larger than the groundmass)
b. Porphyritic-Aphanitic (phenocrysts are embedded in a cryptocrystalline, microcrystalline, or a glassy texture)

Question 13

Crystalline igneous texture that is characterized by large crystals averaging more than 30 mm in diameter, display large early formed euhedral crystals surrounded by later formed subhedral crystals, and develop most commonly in granitic plutons with high volatile contents. What are the rocks with this texture called?

Answer 13

Pegmatitic
-Pegmatite

Question 14

The number of new “seed” crystals that develop per volume per unit time, commonly expressed in nuclei/cm³/s.

Answer 14

Crystal nucleation rate

Question 15

What are the factors affecting crystal nucleation?

Answer 15

1. Cooling rate
2. Ion availability
3. Diffusion

Question 16

What are the non-crystalline igneous textures?

Answer 16

1. Holohyaline/glassy
2. Vitrophyric
3. Perlitic
4. Vesicular
5. Pyroclastic

Question 17

An igneous rock texture primarily composed of glass that develops in lava that solidifies without experiencing significant crystallization.

Answer 17

Holohyaline

Question 18

Occurs when melts come
into contact with air and water, rapidly absorbing heat, causing the melt to solidify rapidly before crystals could nucleate and grow

Answer 18

Quenching

Question 19

An igneous rock texture
that contains recognizable phenocrysts in a glassy groundmass.
What do you call the rocks with this texture?

Answer 19

Vitrophyric
-vitrophyre

Question 20

A process when glasses crystallize into a solid state by growing on pre-existing microlitic or cryptocrystalline nuclei?
What are the round masses that develop from this process?

Answer 20

Devitrification
-Spherulites (Ex. Snowflake Obsidian)

Question 21

An igneous rock texture characterized by a cloudy appearance and _______ cracks.
-What do you call SiO2-rich volcanic rocks with this texture?

Answer 21

Perlitic
-perlites

Question 22

An igneous rock texture that
are dominated by vesicles and develop due to exsolution and entrapment of gas bubbles in lava as it cools and solidifies.

Answer 22

Vesicular

Question 23

A depth where volatiles exsolve from the liquid as a separate phase.

Answer 23

Level of exsolution

Question 24

A process that occurs above the level of exsolution where volatiles nucleate as small bubbles.

Answer 24

Vesiculation

Question 25

Encountered when bubbles constitute 70-80% of the magma volume, and magma changes from a liquid with suspended gas bubbles to a buoyant gaseous mixture containing liquid blobs.

Answer 25

Fragmentation Bubbles

Question 26

A modifier for igneous rocks containing
a. 5-30% vesicles?
b. >5% vesicles?

Answer 26

a. Vesicular
b. Vesicle Bearing

Question 27

An igneous rock texture primarily made up of cemented or welded pyroclasts.

Answer 27

Pyroclastic

Question 28

What are rock particles of varying sizes ejected by volcanic eruptions?
Enumerate its types.

Answer 28

Pyroclasts
-Tephra (Airborne)
-Lithic (ROck)
-Vitric (Glassy)
-Crystals (MIneral and crystalline)