1.2 Igneous Rocks and Processes Flashcards
How is magma formed?
When the heat in the earth melts rocks to form molten material
What does magma contain?
Molten rock, as well as volatiles such as gas and water
Where might magma be found?
- lower continental crust
- lower oceanic crust
- top of the mantle
- core/mantle boundary
What are some examples of igneous rocks?
- peridotite
- granite
- dolerite
- basalt
- obsidian
- andesite
Description of olivine in an igneous rock?
Green and glassy
Description of plagioclase fledspar in an igneous rock?
White
Description of orthoclase feldspar in an igneous rock?
Fleshy-pink
Description of quartz in an igneous rock?
Grey and glassy
Description of biotite mica in an igneous rock?
Black and flaky
Description of muscovite mica in an igneous rock?
White and flaky
Description of ferromagnesian minerals in an igneous rock?
Black
What are the four types of magma from which igneous rocks crystallise?
- silicic magma
- intermediate magma
- mafic magma
- ultramafic magma
What experiment can be used to show how the rate of cooling with affect crystal size/texture of the igneous rock?
- substance called SALOL - melts and crystallises at lower temperatures than magma
- drops put onto warm piece of glass and also a cold
What are the four main rates of cooling of magma to form igneous rocks?
- slow
- moderate
- rapid
- very rapid
What factors vary the type of igneous rock formed?
- type of magma
* rate of cooling
What does it mean in an igneous rock, if the crystals are aligned?
Indicates direction of flow of magma
What is porphyritic texture?
Larger crystals (phenocrysts) embedded in a fine-grained groundmass
What are the large crystals in a porphyritic texture called?
Phenocrysts
At what rate did the phenocrysts in porphyritic textured igneous rock cool?
Slowly, deep in the crust
What are the smaller, poorly formed crystals in porphyritic textured igneous rock called?
Groundmass
At what rate did the grounmass in porphyritic textured igneous rock cool?
A moderate/rapid rate, near to earth’s surface
In granite, which mineral are the phenocrysts formed from?
Feldspar
In granite, which minerals form the groundmass?
Quartz and mica
What type of magma did granite cool from?
Silicic
What is the crystal size in igneous rocks related to?
The cooling rate of the magma
How can igneous rocks be classified?
- texture
* mineralogy
How would a crystal size >3 mm be classified?
Coarse
How would a crystal size <1 mm be classified?
Fine
When magma cools in the crust, what type of igneous rocks are formed?
Intrusive
When magma cools on the surface of the crust, what type of igneous rocks are formed?
Extrusive or volcanic
What is a xenolith?
A piece of the country rock that has fallen into the magma but not completely melted
How are dykes formed?
When molten magma flows upward through vertical faults toward the surface and cools
What is a batholith?
A giant mass of intrusive igneous rock that forms when magma collects and cools deep in the earth’s crust without being exposed to the surface
How is a laccolith formed?
When magma pushes through layers of rock above it and pools in a dome shape
What is a sill?
A flat intrusion of igneous rock that forms between preexisting layers of rock
Given that magma is hot, what do igneous intrusions do to country rock?
Bakes/burns it
Why can sills and lava flows sometimes be confused?
They are both layers of igneous rock that follow the bedding
How might you identify between sills and lava flows?
- veins on upper surface of sills
- sills have rock with medium crystals, lava flows have rock with fine
- lava flow - baking only on lower surface
What does magma viscosity affect?
The type of volcanic activity and shape of volcanoes
Characteristics of granitic magma?
Sticky and viscous
Characteristics of andesitic magma?
Quite viscous, but can flow more easily than granitic
Characteristics of basaltic magma?
Can flow easily → fluid
Order of viscosity of magmas?
Basaltic < Andesitic < Granitic
What is the shape of basaltic volcanoes?
Shield volcanoes - gradual sides, convex
Type of activity in basaltic volcanoes?
Fissure eruption and central cone
Main products from basaltic volcanoes?
Basaltic lava
Examples of locations for basaltic volcanoes?
Hawaii and Iceland
What type of volcano might Ropy or Pahoehoe lava flows be formed from?
Basaltic volcanoes
What is a Pahoehoe lava flow?
Wrinkled lava surface resembling ropes - where surface cooled quicker than underlying lava flow - so hasn’t been able to move smoothly
What type of volcano might columnar jointing be formed from?
Basaltic volcanoes
What is columnar jointing?
Joints formed perpendicular to cooling surface - columns form as a result
How does columnar jointing occur?
When lava cools and contracts
What type of volcano might pillow lavas be formed from?
Basaltic volcanoes
What are pillow lavas?
When basalt lava is erupted underwater and gases can’t escape easily - lava squeezed out from front of lava flow and rounded by sea water
What type of volcanoes are andesitic volcanoes?
Composite cone volcanos
What type of volcano produces volcanic bombs (pyroclasts)?
Andesitic volcanoes
What is equicrystalline texture?
Even cooling, all crystals 1.0 – 1.5 mm
