Essentials of Geology 3 rd Edition Chapter 4 Norton Media Library
Up from the Inferno: Magma and Igneous Rocks Prepared by: Ronald Parker , Senior Geologist Fronterra Geosciences Houston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, Neuquén www.fronterrageosciences.com
Extrusive igneous rocks – Cool quickly at the surface.
Lava – Cooled liquid.
Pyroclastic debris – Cooled fragments.
Many types of igneous rocks.
All oceanic crust.
Most continental crust.
Sources of Heat
Sources of heat to the early Earth:
Planetesimal and meteorite accretion.
Decay of radioactive minerals.
Tidal pull of the Sun and Moon.
The crust does NOT float on a
sea of molten rock.
Magma forms in special settings that melt existing rocks.
Partial melting occurs in the crust and upper mantle.
Melting is caused by…
Geothermal gradient – The Earth is hot inside.
Crustal temperature (T) averages 25°C / km of depth.
At the base of the lithosphere T ~ 1280°C.
The geothermal gradient varies from place to place.
Base of the crust is hot enough to melt mantle rock.
Due to high pressure, the rock does not melt.
A drop in pressure initiates “decompressional melting.”
Pressure drops when hot rock
rises to shallower depths.
Addition of Volatiles
Volatiles lower the melting T of a hot rock.
Common volatiles include water and carbon dioxide.
Subduction introduces water to the mantle, melting rock.
Rising magma carries mantle heat with it.
This raises the T in nearby crustal rock, which then melts.
What is Magma Made of?
Magmas have three components (solid, liquid and gas).
Solid – Solidified mineral crystals are borne by the melt.
Liquid – The melt itself is comprised of mobile ions.
Dominantly Si and O; lesser Al, Ca, Fe, Mg, Na, and K.
Other ions present to a lesser extent.
Different mixes of elements yield different magmas.
What is Magma Made of?
Gas – Magmas contain abundant dissolved volatile gas.
Dry magma – Scarce volatiles.
Wet magma – To 15% volatiles.
Water vapor (H 2 O)
Carbon dioxide (CO 2 )
Sulfur dioxide (SO 2 )
Nitrogen (N 2 )
Hydrogen (H 2 )
There are four major magma types based on silica (SiO 2 ) percentage.
Felsic (feldspar and silica) 66 to 76% SiO 2 .
Intermediate 52 to 66% SiO 2 .
Mafic (Mg and Fe-rich) 45 to 52% SiO 2 .
Ultramafic 38 to 45% SiO 2 .
Composition controls magma density, T, and viscosity.
The most important factor is silica (SiO 2 ) content.
Silica-rich magmas are thick and viscous.
Silica-poor magmas are thin and “runny.”
These characteristics govern eruptive style.
Type Density Temperature Viscosity Felsic Very low Very low (600 to 850 ° C) Very High: Explosive eruptions. Intermediate Low Low High: Explosive eruptions. Mafic High High Low: thin, hot runny eruptions. Ultramafic Very high Very high (up to 1300 ° C) Very low.
Why are there different magma compositions?
Magmas vary chemically due to…
Initial source rock compositions.
The source of the melt dictates the initial composition.
Mantle source – Ultramafic and mafic magmas.
Crustal source – Mafic, intermediate, and felsic magmas.
Upon melting, rocks rarely dissolve completely.
Instead, only a portion of the rock melts.
Silica-rich minerals melt first.
Silica-poor minerals melt last.
Partial melting, then, yields a silica-rich magma.
Removing a partial melt from its source creates…
Magma melts the country rock it passes through.
Blocks of country rock (xenoliths) fall into magma.
Assimilation of these rocks alters magma composition.
Different magmas may blend in a magma chamber.
The result combines the characteristics of the two magmas.
Often magma mixing is incomplete, resulting in blobs of one rock type suspended within the other.
Magma doesn’t stay put; it tends to rise upward.
Magma may move upward in the crust.
Magma may breach the surface – a volcano.
This transfers mass from deep to shallow parts of Earth.
A crucial process in the Earth System.
Provides the raw material for soil, atmosphere, and ocean.