Rock forming process, the rock cycle, and THE rocks

*Solid aggregate of mineral grains, mineral crystals, or other rocks *Some exceptions **Obsidian is made of volcanic glass **Coal is made of plant fragments *The materials forming rocks

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

 OLIVINE

Review of Minerals

 What mineral is this?

FELDSPAR

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

 PYRITE

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

 HALITE

Review of Minerals

 What mineral is this?

Review of Minerals

 What mineral is this?

 AMPHIBOLE -

HORNBLENDE

Review of Minerals

 What mineral is this?

 BIOTITE

Goals for this lab

Learn the basics of rock identification

Learn how to distinguish between 3 rock types

Observe hand samples and infer how they

might have been formed

*Solid aggregate of mineral grains, mineral

crystals, or other rocks

*Some exceptions

**Obsidian is made of volcanic glass

**Coal is made of plant fragments

*The materials forming rocks come from the

Earth’s mantle as magma, from space, from

organisms, or from the breakdown of other rocks and minerals

*Environmental changes and processes affect the rock forming materials and existing rocks

*These changes and processes produce 3 distinct groups of rocks

**IGNEOUS

**SEDIMENTARY

**METAMORPHIC

Igneous Rocks…

What is an igneous rock?

Crystalline or glassy rocks formed from the cooling and solidification of molten magma (below Earth’s surface)/lava (on Earth’s

surface)

Compose the majority of the earth

Can use the texture and mineralogy of these rocks to determine where in the Earth they formed

Igneous Rock Textures

Where the rock forms in the Earth and how

quickly it cools determines what kind of

texture it will have

Cooling Rates and Igneous Textures

 The slower the crystals form, the larger they will be.

 Deep = Hot = Slow Cooling= Large Crystals = Phaneritic

 Shallow = Cooler = Fast Cooling = Small Crystals = Aphanitic

 Cooled slowly and then abruptly brought near surface and

cooled quickly = both large and small crystals = Porphyritic

 Cooled extremely quickly = Glassy

 Cooled quickly and bubbles present = Vesicular

 Explosive welding of materials from volcanism =

Pyroclastic/Fragmental

Igneous Rock Textures

Glassy

Vesicular

Pyroclastic/Fragmental

What minerals are present also determine what

kind of Igneous Rock forms.

Chemistry changes influence rock type

Minerals in Igneous Rocks Cont.

Chemistry changes influence rock type

Mineralogy cont.

 Felsic rocks

• dominated by K-feldspar, Na Plagioclase, quartz, and biotite

• usually light in color

• typical of continental crust (Granite and Rhyolite)

• Dominated by Ca-Plagioclase, pyroxene, olivine, amphibole

• Usually dark in color

• Typical of oceanic crusts (and the Moon, Mars, and Venus!) (Basalt,

• Gabbro)

 Ultramafic rocks

• Dominated by olivine, minor amounts of pyroxene and Ca-plagioclase

• Rarely seen on Earth’s surface

• Major constituent of Earth’s Mantle

• Peridotite

Sedimentary Rocks…

Crystallization

Deposition, burial, and lithification

Sediments Grain Size

Gravel >2mm

Sand 1/16-2mm

Silt 1/256-1/16mm

Clay <1/256

Sediments cont.

How are size and angularity affected by transport?

Sediments cont.

Grain Sorting

Sedimentary Rocks

Formed by surface processes

Sediments are formed from weathering and

erosion

break up rocks into fragments of various sizes

move them

Sedimentary Rocks

 Loose sediments form sedimentary rocks through the

process of lithification

 Lithification = converts sediment into solid rock by

• Compaction = grains are squeezed together by weight of

overlying sediment into a mass denser than original

• Cementation = minerals precipitate around deposited particles and bind them together

Delta Glacier

The sedimentary stages of the rock cycle

Weathering

breaks down

rocks.

Erosion carries away particles.

Transportation moves particles downhill.

Deposition occurs when particles settle out or precipitate.

Diagenesis lithifies the sediment to make sedimentary rocks.

Burial occurs

as layers of sediment accumulate.

Sedimentary Rock Texture

Step 1 in identifying a Sedimentary Rock

 Detrital/Siliclastic – rock made of fragments of other rocks

 Biochemical/Bioclastic – composed of organically derived material

 Chemical – sedimentary rocks precipitated out of solution

Step 2 in Identifying Sedimentary Rocks

Detrital Rocks

Oil and gas

Organic Matter

Coal

Pressure

Heat to 90° - 120° C

Heat to 90° - 120° C

Bioclastic Rocks

Chemical rocks

Evaporation  Precipitation

Breccia Chert

Conglomerate Coal-Anthracite

Hematite

Sandstone

Rocksalt

Shale Siltstone

Metamorphic Rocks…

Metamorphism is the solid-state

transformation of a protolith (parent or

pre-existing rock) into texturally or

mineralogically distinct new rock as the

result of high temperature, high

pressure, or both

Metamorphism is Described by

Texture, Index minerals, Grade, and

Facies

Identifying Metamorphic Rocks

 Step 1

 Foliated textures – rocks exhibit foliation…

layering or parallel alignment of platy or flat

mineral crystals (if the rock appears layered, it is foliated) due to pressure and recrystallization

 Nonfoliated textures – rocks exhibit no layering, yet they may exhibit stretched fossils or long,

prismatic crystals that have grown parallel to the pressure field

Increasing intensity of metamorphism

Increasing crystal size

Increasing coarseness of foliation

Diagenesis Low grade Intermediate

Slaty Rock

Cleavage Phyllite Texture

Schistosity (abundant micaceous minerals)

Gneissic Banding (fewer micaceous minerals)

Migmatite

very flat

foliation layer of visible scaly glittery

platy minerals and/or linear alignment of long prismatic crystals

alternating layers or lenses of light and dark medium to coarse grained minerals

Banding

Foliated rocks are classified by the degree

of cleavage, schistosity, and banding.

wavy or wrinkled foliation of fine grained minerals giving rock metallic luster

Progression of metamorphism

Slate

Start with a shale and then hit

it with pressure and heat.

You end up with something that is really Gneiss!

Intermediate Grade

High Grade

Metamorphic Rock Textures

(Unfoliated Textures)

- Crystalline Texture – medium to coarse grained aggregate

of intergrown, equigranular, visible crystals (example:

Marble)

- Microcrystalline Texture – fine grained aggregate of

intergrown microscopic crystals (example: hornfels)

- Sandy Texture – medium to coarse grained aggregate of

fused, sand-sized grains that resemble sandstone (example: quartzite)

- Glassy Texture – homogeneous texture with no visible

grains or other structures and breaks along glossy surfaces (anthracite coal)

Identifying Metamorphic Rocks

 Step 2

other distinctive properties

 Other Distinctive Features to Note

 Stretched or Sheared Grains – deformed pebbles,

fossils, mineral crystals, that have been stretched, shortened, or sheared

 Porphyroblastic Texture – arrangement of large crystals (PORPHYROBLASTS) set in a finer-grained

groundmass (sort of sounds like porphyritic texture)

 Hydrothermal Veins – fractures filled by minerals that precipitated from hydrothermal fluids

With increasing metamorphic

grade, mineral composition

changes.

Mineral suites define metamorphic facies.

Identifying Metamorphic Rocks

 Step 3

determine the name of the rock you are identifying

 Step 4

name the rock it was before metamorphism (this is the

metamorphic rocks “parent” rock or protolith)

Sandstone: Quartzite, Metaquartzite

Metamorphism of Igneous Rocks

For most purposes, just put “meta” in front

of the protolith name.