Rock Cycle The Rock Cycle is a group of changes, this change does not necessarily have to be a chemical change. Igneous rock can change into sedimentary

Rock Cycle

The Rock Cycle is a group of changes, this change does not necessarily have to be a chemical change. Igneous rock can change into sedimentary rock or into metamorphic rock. Sedimentary rock can change into metamorphic rock or into igneous rock.Metamorphic rock can change into igneous or sedimentary rock. Almost all of rock today that we have on earth is made up of all the same stuff as the rocks that dinosaurs and other ancient life forms walked, crawled, or swam overWhile the stuff that rocks are made of has stayed the same, the rocks themselves, have notOver time rocks are recycled into other rocksMoving tectonic plates are responsible for destroying and forming many types of rocks

Types of rocks

Rock divisions occur in three major families based on how they formed: igneous, sedimentary, and metamorphic.

Each group contains a collection of rock types that differ from each other on the basis of the size, shape, and arrangement of mineral grains.

Just remember 3 types of rocks=3 divisions. (igneous, sedimentary, and metamorphic)

Igneous Rocks

INTRUSIVE IGNEOUS ROCKS – When igneous rocks are formed by magma that cools BENEATH Earth’s surface, they are called intrusive igneous rocks

EXTUSIVE IGNEOUS ROCKS – When igneous rocks are formed by LAVA ON Earth’s surface, they are called extrusive igneous rocks

2 Types of Igneous Rocks

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Types of Igneous Rock

Igneous Rocks II

PYROCLASTS OR PYROCLASTIC:

Extrusive igneous rock texture referred to as pyroclastic. This texture results from a very explosive eruption, which sends not only lava flying through the air, but also fragments of the volcano itself. All airborne volcanic fragments, referred to as pyroclasts

Extrusive Igneous Rock - Lava (Hawaii)

•The rock fragments thrown out during volcanic eruption are called Pyroclasts.

Pyroclasts come in many sizes: the smallest are called ash, slightly larger are lapilli, and the biggest are called blocks or bombs.

•Based of shape and size they are categorized

ash lapilli

A large pyroclasts are known as volcanic bombs. Typical shape with head and a tail. Bombs can result into severe damage if they strike.

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Batholith exposedby erosion

Batholith

DikeSedimentaryrock layers

LopolithLaccolith

Sill

Xenoliths

Larger concordant

Some terminology:

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Xenoliths ofsurroundingrocks insill

Xenoliths ofsurroundingrocks in sill

Dike

Heatedrockzone

Bottom of lava flow

Top oflava flow

Lavaflow

Heated rockzone

Top of sill

Vesicles

Bottomof sill

Xenoliths ofunderlying rocks

in lava flow

Eroded lava fragmentsin overlyingsedimentary rock

Heated rockzone

Sill

How to recognize dikes and sills

When classifying a rock sample geologists observe the rock’s color and texture and determine its mineral composition.Texture: the size, shape, and pattern of the rock’s grain.Color: the apparent color of the rock, on the inside and the outside.Mineral composition: The minerals that make up the different parts of a rock.

Characteristics of magma

Crystallization of magma• Texture in igneous rocks is determined by

the size and arrangement of mineral grains

• Igneous rocks are typically classified by– Texture

– Mineral composition

Texture

Crystal Size:Fine-grained: < 1mm

Medium-grained: 1-2 mmCoarse-grained: larger than 2 mmPegmatitic: very coarse-grained, approx.

5cm to as large as a house

Porphyry: igneous rock with large crystals (PHENOCRYSTS) set in a matrix of fine-grained crystals (GROUNDMASS)

Different Types of Texture

Fine-Grained, Coarse-Grained, Rounded Grain, Jagged Grain, Nonbanded, Banded

Mafic Magma

Mafic (Basaltic) magma typically has a higher temperature than felsic magma.Higher-temperature magmas commonly have:

lower viscositylower volatile contentless explosive tendency (lava rather than pyroclastic material)

Igneous compositions

Silica content influences a magma’s behavior

• (Felsic) Granitic magma– High silica content

– Extremely viscous

– Liquid exists at temperatures as low as 700oC

Igneous textures

Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals

Factors affecting crystal size• Rate of cooling

– Slow rate promotes the growth of fewer but larger crystals

Igneous textures

Factors affecting crystal size • Rate of cooling

– Fast rate forms many small crystals

– Very fast rate forms glass

• Amount of silica (SiO2) present

• Amount of dissolved gases

Igneous textures

Types of igneous textures• Aphanitic (fine-grained) texture

– Rapid rate of cooling of lava or magma– Microscopic crystals– May contain vesicles (holes from gas bubbles)

• Phaneritic (coarse-grained) texture– Slow cooling– Crystals can be identified without a

microscope

Aphanitic texture

Phaneritic texture

Igneous textures

Types of igneous textures• Porphyritic texture

– Minerals form at different temperatures as well as differing rates

– Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass

• Glassy texture– Very rapid cooling of molten rock– Resulting rock is called obsidian

Porphyritic texture

Glassy texture

Igneous textures

Types of igneous textures• Pyroclastic texture

– Various fragments ejected during a violent volcanic eruption

– Textures often appear to more similar to sedimentary rocks

• Pegmatitic texture– Exceptionally coarse grained– Form in late stages of crystallization of

granitic magmas

Pyroclastic Rock - Superheated Flows

A Spectacular Pegmatite Vein of Feldspar and Quartz

very small crystals

visible crystals

Extrusive

Intrusive

Mafic FelsicIntermediate

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Igneous Rock Classification

Igneous rocks may be classified on the basis of what minerals they containA distinction is made between essential minerals (those that must be present) and accessory minerals (often present, but not necessary)

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• Example: Granite Essential: Feldspar, Quartz Accessory: Biotite, Hornblende

• Accessory minerals may become part of the rock name: Hornblende andesite, because hornblende is not an essential mineral


Felsic rocks crystallize from warm melts Mafic from hot melts

Granite Diorite

Gabbro

Extr.Intr.

Summary - Igneous Rock Texture and CompositionThere are also ultramafic rocks from the mantle

Note Rock color chart

Extr.

Intr.

very small crystals

visible crystals

Extrusive

Intrusive

Mafic FelsicIntermediate

Igneous Compositions

Igneous rocks are composed primarily of silicate minerals

• Dark (or ferromagnesian) silicates– Olivine

– Pyroxene

– Amphibole

– Biotite mica

Igneous Compositions

Igneous rocks are composed primarily of silicate minerals

• Light (or nonferromagnesian) silicates– Quartz

– Muscovite mica

– Feldspars



• Granitic magma– High silica content

– Extremely viscous

– Liquid exists at temperatures as low as 700oC


Granitic versus basaltic compositions• Basaltic composition

– Composed of dark silicates and calcium-rich feldspar

– Designated as being mafic (magnesium and ferrum, for iron) in composition

– More dense than granitic rocks

– Comprise the ocean floor as well as many volcanic islands


Granite


Naming igneous rocks – granitic (felsic) rocks

• Granite– Phaneritic– Over 25 percent quartz, about 65 percent or

more feldspar– May exhibit a porphyritic texture– Very abundant as it is often associated with

mountain building– The term granite covers a wide range of

mineral compositions


Naming igneous rocks – intermediate rocks

• Diorite– Plutonic equivalent of andesite

– Coarse grained

– Intrusive

– Composed mainly of intermediate feldspar and amphibole

Diorite


Naming igneous rocks – basaltic (mafic) rocks

• Gabbro– Intrusive equivalent of basalt

– Phaneritic texture consisting of pyroxene and calcium-rich plagioclase

– Makes up a significant percentage of the oceanic crust

Gabbro - a maficigneous rock. A large-grain version of Basalt



• Rhyolite– Extrusive equivalent of granite

– May contain glass fragments and vesicles

– Aphanitic texture

– Less common and less voluminous than granite

Rhyolite


Naming igneous rocks – intermediate rocks

• Andesite– Volcanic origin


– Often resembles rhyolite

Andesite


Granitic versus basaltic compositions• Basaltic composition

– Composed of dark silicates and calcium-rich feldspar

– Designated as being mafic (magnesium and ferrum, for iron) in composition

– More dense than granitic rocks

– Comprise the ocean floor as well as many volcanic islands


Naming igneous rocks – basaltic (mafic) rocks

• Basalt– Volcanic origin


– Composed mainly of pyroxene and calcium-rich plagioclase feldspar

– Most common extrusive igneous rock



• Basaltic magma– Much lower silica content

– Fluid-like behavior

– Crystallizes at higher temperatures

Basalt

Basalt (Scoria)

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Lava flow

• Basaltic lava flow• Location probably near Raton, New

Mexico

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Columnar basalts Dark, fine grained volcanic rocks, chiefly basalt and andesite, are exposed at the northern edge of the Fraser Valley. These rocks formed as lavas, shallow intrusions, and volcanic ash deposits. Most volcanic rocks are resistant to erosion and form prominent hills in the Fraser Valley Basalt with columnar jointing,

near Whistler, British Columbia

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Pillow basaltsPillow basalts form during underwater eruptionsThis basalt was found in Marin County, California

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Pillow basalt• Pillow lava forms when

eruptions are underwater or when lava flows enter a body of water

• Abundance of pillows and pillow deltas indicates rivers and lakes were common features during the formation of the Columbia River Flood Basalt Province

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Pillow deltas

• Pillows can be up to 5 m long and about 0.5 to 1 m thick

• They are elongate in the direction of flow and dip 20-30 degrees

• Dips decrease higher up in the sequence

Figure 4.23



• Obsidian– Dark colored– Glassy texture

• Pumice– Volcanic– Glassy texture– Frothy appearance with numerous voids

Obsidian


Extrusive products can include:• Pumice

– Volcanic

– Glassy texture, very light weight, mostly air

– Frothy appearance with numerous voids (extrusive foam)

– Forms when lavas have a lot of water and other volatiles

Common with intermediate compositions

Ash and pumice layers

Volcanic Terms

Pyroclastic rock: explosively erupted as hot but nevertheless solid particles

Tuff: explosively erupted, fine-grained volcanic rock whose grains are commonly welded together

Vesicles: cavities in a volcanic rock, once occupied by gas bubbles


Naming igneous rocks – pyroclastic rocks• Composed of fragments ejected during a

volcanic eruption• Varieties

– Tuff – ash-sized fragments

– Volcanic breccia – particles larger than ash

Summary - Igneous Rock Texture and CompositionThere are also ultramafic rocks from the mantle

Note Rock color chart

Granite RhyoliteFigure 4.7 A

DioriteAndesite

Figure 4.7 B

Gabbro Basalt

Figure 4.7 C

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Basalt

Vesicular basaltGas in magma is trapped by rapid cooling, leaving vesicles

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Flood BasaltsTwickenham flows of the Colombia River Basalt Group, Washington StateNote columnar basalt near top of picturePhoto: M.L. Bevier

Figure 4.21 B

Documents

Rock Cycle The Rock Cycle is a group of changes, this change does not necessarily have to be a chemical change. Igneous rock can change into sedimentary