Formation of Magma and Igneous Rocks Basalt lava flow - Hawaii

Formation of Magma and Igneous Rocks

Basalt lava flow - Hawaii

Formation of Magma and Igneous Rocks

• How do we classify igneous rocks into groups?• Where do we find igneous rocks?• How and why do rocks melt?• How does magma generation relate to plate tectonics?• What makes igneous rock compositions so diverse?• Why are there so many different types of volcanoes and

volcanic eruptions?• What are the types of volcanic hazards?• Why don’t all magmas erupt?

Why study igneous rocks?

• Igneous rocks result from the solidification of molten silicate mixtures (magma)

• Why are igneous rocks and processes important?• Location of economically important minerals, ore

deposits associated with igneous intrusions• Gives us insight into Earth’s dynamic internal

processes• Helps us understand volcanic hazards• Continents built largely by addition of igneous rocks

What are igneous processes?

• Igneous processes involve the melting of rock to form magma and its solidification into rock.

• Igneous rocks form when magma crystallizes below Earth’s surface or erupts onto the Earth’s surface.

• Geologists study processes forming igneous rocks through field observations and laboratory studies that include geochemical analyses and experiments (melting and crystallization can be done in the laboratory).

How do we classify igneous rocks into groups?

• There are two primary classes of igneous rocks that we see:

Volcanic (extrusive) Plutonic (intrusive)

• Each has members that vary in:• Composition – the elemental composition of the

rock as a whole (controlled by minerals present)• Texture – defined by the size (or range in size) of

crystals that comprise the rock

• Volcanic (extrusive) igneous rocks: • Form from flowing lava at the surface - or - • Form from explosively ejected red-hot fragmented

magma (pyroclasts) that tend to form glassy deposits• Are composed of very small size crystals due to rapid

cooling (this texture is called aphanitic)


Basalt

• Gases are also present in magma (up to 5-6%)• Mainly water vapor, some CO2 + sulfur compounds

• Gas escapes when magma nears the surface• Escaping gas creates voids (vesicles)

• Lava is therefore degassed magma


• Plutonic (intrusive) igneous rocksPlutonic (intrusive) igneous rocks• Form from magma that cools below Earth’s surfaceForm from magma that cools below Earth’s surface• Have crystals that are big enough to see with the Have crystals that are big enough to see with the

unaided eye (texture called phaneritic)unaided eye (texture called phaneritic)


• CompositionComposition – primary classification method – primary classification method• As magma cools, minerals will solidify at various As magma cools, minerals will solidify at various

temperatures.temperatures.• Minerals that form depend on the chemical Minerals that form depend on the chemical

composition of the magma.composition of the magma.• Most magma is largely silica (45 to 80%) with Most magma is largely silica (45 to 80%) with

oxides of Al, Mg, Fe, Ca, Na, K – and other minor oxides of Al, Mg, Fe, Ca, Na, K – and other minor elements.elements.

• So most igneous rocks are made of silicate So most igneous rocks are made of silicate minerals, but have varying proportions of these minerals, but have varying proportions of these elements and thus different proportions of minerals.elements and thus different proportions of minerals.


• The chemistry of a magma controls the minerals present.

• Higher temperature minerals (first to crystallize) are on the right, lower temperature minerals (last to crystallize) are on the left.

• Note that dark colored minerals (e.g. olivine, pyroxene) form from low SiO2 magmas, light colored minerals (e.g. quartz, K-feldspar) form from high SiO2 magmas.


Common Igneous Rock Minerals

Quartz

Feldspar

Muscovite

Biotite

Amphibole

Pyroxene

Olivine

Felsic

Mafic

More SiO2

Less SiO2


• Texture – secondary classification methodTexture – secondary classification method• Crystal size is related to cooling rate:Crystal size is related to cooling rate:

• If it cools fast the crystals will be smallIf it cools fast the crystals will be small• Called Called aphaniticaphanitic (from the Greek - invisible) (from the Greek - invisible)• Usually extrusive, but not always (very shallow intrusions)Usually extrusive, but not always (very shallow intrusions)• Obsidian (volcanic glass) cools so fast there are no crystalsObsidian (volcanic glass) cools so fast there are no crystals


• Texture – secondary classification methodTexture – secondary classification method• Crystal size is related to cooling rate:Crystal size is related to cooling rate:

• If it cools slowly the crystals will be largerIf it cools slowly the crystals will be larger• Called phaneritic (from the Greek - visible)Called phaneritic (from the Greek - visible)• Generally intrusive rocksGenerally intrusive rocks


• Texture – First Special CaseTexture – First Special Case• Porphyritic: intrusive (slow cooling) magma erupts Porphyritic: intrusive (slow cooling) magma erupts

and cools quickly. Produces larger, visible crystals and cools quickly. Produces larger, visible crystals in a fine-grained (aphanitic) groundmass.in a fine-grained (aphanitic) groundmass.

Fig 4.2d


• Texture – Second Special CaseTexture – Second Special Case• Pyroclastics: Magma that is explosively ejected as red-hot Pyroclastics: Magma that is explosively ejected as red-hot

fragmental material and cools quickly, often before it hits the fragmental material and cools quickly, often before it hits the ground.ground.

• Bombs – large pieces (what you would call a large rock or boulder)Bombs – large pieces (what you would call a large rock or boulder)• Lapilli – medium sized (what you would call a rock)Lapilli – medium sized (what you would call a rock)• Ash – fine material (what you would call sand or dust)Ash – fine material (what you would call sand or dust)


Pyroclastics – different size, shape and color

Fig 4.4


Composition and Texture Finally Come Together – The Big Picture

Composition and Texture Finally Come Together – The Big Picture


• Composition and texture are used to classify and name igneous rocks.

• Pyroclastic deposits are formed by volcanic explosions and are classified by fragment size and degree of consolidation.

• Ultramafic, mafic, intermediate, and felsic are compositional categories of magma/rocks.

• Rapid cooling leads to aphanitic rocks and slow cooling produces coarser grained phaneritic rock. Porphyritic texture contains both.

Where do we find igneous rocks?

• Divergent and convergent plate boundaries• Continental margins above subduction zones• Island arc volcanic chains above subduction zones• Divergent boundaries: mid-ocean ridges, continental rifts

• Hot spots• Hawaii – in an ocean basin• Yellowstone – on the N. American continent• Northern part of the E African Rift Valley (sometimes

continental rifts, or divergent boundaries within continents, are started by hotspots)

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Formation of Magma and Igneous Rocks Basalt lava flow - Hawaii