classification of rocks Flashcards
Where do sedimentary rocks come from?
pre-existing rocks/ pieces of once living organisms
When are sedimentary rocks clastic?
When they form from the lithification (compaction) of (mechanical weathering, pieces of bedrock, …) rock and mineral fragments
*indiv. grains can be angular or rounded
ex. quartz, clay, feldspar
When are sedimentary rocks crystalline?
When they precipitate out of solution
ex. dolomite, calcite, halite, gypsum
When are sedimentary rocks bioclastic?
When they are formed from the accumulation of organic material or biologic activity
may contain remnants of plants, corals, shell, or fossil fragments
When are metamorphic rocks crystalline?
When the dominant pressure is heat
What happens in foliation?
When platy or elongated minerals align themselves parallel to the axis of pressure, resulting in a layered appearance or foliation
Types of foliation
slaty, phyllitic, schistose, gneissic
metamorphic rocks
When is foliation slaty?
If the minerals are microscopic, the rock may not look foliated. But, it will manifest physically in the rock’s tendency to break along in parallel liines
ex. slate
metamorphic
When is foliation phyllitic?
If the minerals are barely visible to the naked eye, their allignment results in an obv but not clearly-defined folitation
ex. phyllite
metamorphic rocks
When is foliation schistose?
If the minerals are visible to the naked eye, their layering is more distinct
*usually composed of platy minerals like chlorite, graphite, biotite, and muscovite
ex. schist
metamorphic rocks
When is foliation gneissic?
If the minerals are visible and elongated, the rock exhibits a coarsely-branded appearance due to the alignment of minerals (like amphibole, feldspar, and quartz)
ex. gneiss
metamorphic rocks
How are crystalline textures created?
It happens when the parent rock is exposed with enough heart that it induces recrystallization of the existing minerals
*usually composed of 1 type of mineral
“smashed texture”
ex. quartzite, marble
When are igneous rocks crystalline?
When they are formed from cooled magma or lava
When are igneous rocks pyroclastic?
When they are made of consolidated eruption products like volcanic ash
What is the difference between different crystalline textures?
They depend on the rate of cooling and where it took place
What are intrusive or plutonic rocks?
- these rocks form from slowly cooled magma in the mantle
- large minerals are formed
- texture is phaneritic
- if cooled very slow at depth, the texture is pegmatitic (massive minerals)
What are extrusive or volcanic rocks?
- these rocks are formed from rapidly cooled magma on crust
- mineral cystals need magnifying tools to be seen
- texture is aphanitic
- if lava is expelled and cooled underwater, texture is glassy
- if gases escape while volcanic rock is forming, texture is vesicular
When are igenous rocks porphyritic?
- when conditions during the cooling of magma change relatively quickly (slow to fast cooling)
- shows at least 2 distince crystal sizes
When are igneous rocks pyroclastic?
- these rocks are formed by the lithification of erupted volcanic material
- composed of rock fragments (pyroclasts)
igneous rocks
What is the classification of rocks with light colors (white, light gray, tan, and pink)?
a felsic composition (granitic) - rich in silica
igneous rocks
What is the classification of rocks with dark colors (black and brown)?
a mafic composition (basaltic) or ultramadic composition (magnesium oxide) - poor in silica, but rich in iron and magnesium
igneous rocks
What is the classification for rocks with an intermediate color (gray or consisting of equal parts of light and dark minerals)?
an intermediate composition (andesitic) - 5 to 20% quartz
igneous rocks
What are exceptions to the color index?
a. obsidian - volcanic glass and dark, but is mostly felsic in compositon
b. dunite - ultramafic composition, but is green bc it’s composed almost entirely of green mineral (olivine)
