Igneous Rock Formation Flashcards
What are the three major rock types.
Sedimentary
Metamorphic
Igneous
How are Igneous rocks formed?
-Magma or lava cools and crystallises to form minerals
-Pyroclastic materials are consolidated to form a solid from the previously loose particles
Name the two main categories of Igneous rocks
Extrusive and Intrusive
Define extrusive (Volcanic) igneous rocks
Rocks that form from materials erupted at the Earth’s surface result from the cooling of lava flows or consolidation of pyroclastic materials
Define intrusive (Plutonic) igneous rocks
Rocks that form from magma that cools below the Earth’s surface
What is the main constituent of magma?
Silica
Name the types of magma.
Ultramafic
Mafic
Intermediate
Silicic
What lava and rock types come from ultramafic /mafic magma
Ultramafic (Peridotite)
Mafic (Basaltic lava – Basalt, Dolerite, Gabbro)
Describe Bowen’s Reaction Series
Bowen’s Reaction Series shows how differentiation and partial melting work to create and modify magma composition. If a rock is re-heated, the minerals in Bowen’s Reaction Series will melt in reverse order
Where does magma originate & accumulate?
Most magma originates in the upper mantle or lower crust. Magma forms from the partial melting of ultramafic material in the asthenosphere. Magma accumulates in reservoirs known as magma chambers
Where are magma chambers found?
Beneath spreading centres or divergent boundaries (i.e. mid-oceanic ridges) magma chambers exist at a depth of only a few kilometres
At convergent boundaries, magma chambers are commonly a few tens of kilometres deep
Name the factors affecting the generation of magma
Temperature
Pressure
Water content
Describe the influence on Temperature on magma melting
The heat responsible for melting rock and generating magma comes from the Earth’s core. This heat is conducted towards the Earth’s surface through the solid parts of the mantle and crust.
Convection cells in the mantle distribute heat from the lower mantle towards the upper mantle and crust (through partially melted or semi-solid layers).
Describe the influence on Pressure on magma melting
The melting point of a mineral increases with
increasing pressure. Both temperature and pressure increase with depth in the Earth’s crust. For this reason, a rock that melts at a given temperature at the Earth’s surface, will require a higher temperature to melt deep in the crust.
Describe the influence on Water Content on magma melting
The presence of gas, especially water vapour,
significantly lowers a mineral’s melting
temperature. This occurs because water under high pressure helps break the silica bonds in the
mineral.
State 4 ways magma can evolve.
Partial Melting (Bowen’s Reaction Series)
Differentiation
Magma contamination (Assimilation)
Magma mixing
Describe the process of Partial Melting
If pre-existing rock is melted, the minerals will melt in the reverse order in which their created, following Bowen’s Reaction Series. Partial melting of ultramafic rock in the mantle, generally produces mafic (basaltic
rich) magma.
Describe the process of differentation
This is the process by which ferro-magnesian minerals separate from a magma leaving behind a more silica-rich
magma.
This process takes place in magma through crystal settling, which is the downward movement of minerals that are denser.
These settle (more dense) at the bottom of the magma chamber to form cumulates.
Describe the process of assimilation
Very hot magma could melt some of the surrounding country rock and assimilate the newly molten material into the magma.
If the magma is mafic, and melts continental country rock, the magma becomes
diluted with silica and becomes intermediate and cooler.
Describe the process of magma mixing
Two magma types merge in the crust to produce an intermediate magma. Mixing equal amount of mafic magma and silicic magma would result in an intermediate magma producing diorite if it solidifies beneath the surface, or erupt on the surface to solidify as
andesite.
What are enclaves?
Incomplete magma mixing can lead
to the formation of areas within a
pluton with a different composition
to the rest of the body.
Outline the origin of magma at ocean ridges (divergent boundaries)
Mafic magma is created by decompression and partial melting of the asthenosphere at divergent plate boundaries. Localised mantle plumes rise beneath ridge, pressure decreases
and melting begins, thus yielding magma.
The presence of water also lowers the
melting temperature
What type of rock is the asthenosphere made of and what magma does it create?
ultramafic rock (peridotite)
mafic-rich magma
Outline the origin of magma at subduction Zones
When the descending oceanic plate reaches
the depth at which the temperature is high
enough to initiate partial melting, the water in the oceanic crust also enhances melting and magma formation by lowering
the melting point of the rock
