Transcript How Does Earth Work?

Formation of Magma and Igneous Rocks
Basalt lava flow - Hawaii
Formation of Magma and Igneous Rocks
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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 liquid (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
How do we classify igneous rocks into groups?
• 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
How do we classify igneous rocks into groups?
• 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 bubbles (vesicles)
• Lava is therefore degassed magma
How do we classify igneous rocks into groups?
• Plutonic (intrusive) igneous rocks
• Form from magma that cools below Earth’s surface
• Have crystals that are big enough to see with the
unaided eye (texture called phaneritic)
How do we classify igneous rocks into groups?
• Composition – primary classification method
• As magma cools, minerals will solidify at various
temperatures.
• Minerals that form depend on the chemical
composition of the magma.
• Most magma is largely SiO2 (~45 to 80%) with
oxides of Al, Mg, Fe, Ca, Na, K – and other minor
elements.
• So most igneous rocks are made of silicate
minerals, but have varying proportions of these
elements and thus different proportions of minerals.
How do we classify igneous rocks into groups?
• 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)
generally form from low SiO2 magmas, light colored minerals (e.g.
quartz, K-feldspar) form from high SiO2 magmas.
How do we classify igneous rocks into groups?
Common Igneous Rock Minerals
More SiO2
Quartz
Feldspar
Muscovite
Felsic
Biotite
Amphibole
Pyroxene
Olivine
Mafic
Less SiO2
How do we classify igneous rocks into groups?
• Texture – secondary classification method
• Crystal size is related to cooling rate:
• If it cools fast the crystals will be small
• Called aphanitic (from the Greek - invisible)
• Usually extrusive, but not always (very shallow intrusions)
• Obsidian (volcanic glass) cools so fast there are no crystals
How do we classify igneous rocks into groups?
• Texture – secondary classification method
• Crystal size is related to cooling rate:
• If it cools slowly the crystals will be larger
• Called phaneritic (from the Greek - visible)
• Generally intrusive rocks
How do we classify igneous rocks into groups?
• Texture – First Special Case
• Porphyritic (porphyry): intrusive (slow cooling)
magma erupts and cools quickly. Produces larger,
visible crystals in a fine-grained (aphanitic)
groundmass.
Fig 4.2d
How do we classify igneous rocks into groups?
• Texture – Second Special Case
• Pyroclastics: Magma that is explosively ejected as red-hot
fragmental material and cools quickly, often before it hits the
ground.
• Bombs – large pieces (what you would call a large rock or boulder)
• Lapilli – medium sized (what you would call a rock)
• Ash – fine material (what you would call sand or dust)
How do we classify igneous rocks into groups?
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
How do we classify igneous rocks into groups?
• 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 (silicic) are
compositional categories of magma/rocks.
• Rapid cooling leads to aphanitic rocks and slow
cooling produces coarser grained phaneritic rock.
Porphyritic texture contains both (rock called
porphyry).
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)