Magma and Igneous Rocks



Big Island of Hawaii

  • one of the most volcanically active locations on Earth




View looking north



By Martin D. Adamiker - Elevation data: USGS, texture data: OpenStreetMap, rendered by TruFlite, CC BY-SA 3.0,







Igneous rock freezes or solidifies at temperatures between 650oC and 1,100oC depending upon the chemical makeup.



  • Extrusive Igneous Rock: lava that solidifies above ground
    • pyroclastic debris: fragments of blasted lava ranging in size from very small to large chunks.
      • volcanic ash: very small pyroclastic debris
  • Intrusive Igneous Rock: magma that solidifies below ground
    • magma







Why Does Magma Form & What is it Made Of?



Earth's interior source of heat:

  • compression of mass
  • sinking of iron to form core
  • impact of meteorites
  • decay of radioactive elements


The Earth remains hot inside because of decay of radioactive elements.

Even though there is a lot of heat in the Earth, most of the crust and mantle remain solid because of immense pressures.


  • Magma forms only in special places, where conditions trigger melting of pre-existing solid rock:
    • decompression
    • addition of volatiles
    • heat transfer






Decompression melting

  • takes place where mantle rock rises slowly
  • as rock moves up, its pressure becomes less
  • temperatures remain nearly unchanged because rock is such a good insulation






Melting Due to Addition of Volatiles:

  • when volatiles mix with hot mantle rock magma can form
  • volatiles are substances that evaporate relatively easily such as:
    • water
    • carbon dioxide
  • when volatiles mix with hot, dry rock, they cause chemical bonds to break so that the rock begins to melt
  • this is called flux melting







Melting Due to Heat Transfer

  • when very hot magma from the mantle rises into the crust, the heat can melt the crust.



  • Flux melting occurs above subducting plates.
  • Decompression melting occurs at mid-ocean ridges and rifts.
  • Melt composition is controlled by partial melting and assimilation.







The Major Types of Melt

  • All molten rock (magma or lava) contain silica (SiO2)
  • they also contain varying proportions of other elements:
    • aluminum (Al)
    • calcium (Ca)
    • sodium (Na)
    • potassium (K)
    • iron (Fe)
    • magnesium (Mg)
  • because molten rock is liquid, its molecules do not lie in an orderly, crystalline lattice but rather in clusters or chains that can move around


Dry melts

  • do not contain volatiles


Wet melts

  • include up to 15% dissolved volatiles including:
    • water (H2O)
    • carbon dioxide (CO2)
    • nitrogen (N2)
    • hydrogen (H2)
    • sulfur dioxide (SO2)
  • these volatiles come out of the earth at volcanoes as gas
  • water is usually half of the gas erupting at a volcano

*** Molten rock contains not only the molecules that make up the minerals in solid rock but also the molecules that become water and air ***




Molten rocks differ from one another in terms of the proportions of chemicals they contain:

Magma type              Weight percent of silica

   Felsic                                 66 -76%
   Intermediate                       52 - 66%
   Mafic                                 45 - 52%
   Ultramafic                          38 - 45%


  • Geologists distinguish four major compositional types depending on the proportion of silica (SiO2) relative to magnesium oxide (MgO) and iron oxide (FeO or Fe2O3) in the melt


  • Mafic melts contain a relatively high proportion of magnesium and iron oxide compared to silica,
    • ma in mafic stands for magnesium and -fic comes from the Latin word for iron


  • Ultramafic melts have an even higher proportion of magnesium and iron oxide, relative to silica


  • Felsic melts have a fairly high proportion of silica compared to magnesium and iron oxide



Why does the composition of melts vary?

Source rock composition varies

  • partial melting:
    • silica melts first so partial melting favors a felsic magma which is different from the parent rock
    • for example, partial melting of an ultramafic rock produces a mafic magma
  • Assimilation:
    • magma that sits underground before solidifying completely, may incorporate chemicals dissolved from wall rocks or from blocks that detached from the wall and sank into the magma






Movement & Solidification of Molten Rock


Why does magma rise?

  • molten rock is less dense
  • the weight of overlying rock creates pressure at depth that squeezes the magma upward


What Controls Speed of Flow?

  • viscosity depends upon:
    • temperature: hotter melt is less viscous
    • volatile content: more gasses make it less viscous
    • silica content: more silica means more viscous


  • a very hot mafic lava has relatively low viscosity and can flow to form thin sheets
  • a cool felsic lava has relatively high viscosity and clumps up into a bulbous mound


Mafic lava has relatively low viscosity. It can erupt in
fountains, move long distances, and form thin lava flows





Felsic to intermediate lava is very viscous. When it erupts, it
may form a mound-like lava dome around the volcano's vent






Transforming Melt into Rock

Three factors control the cooling time of magma:

  • the depth of the intrusion: deep in the crust, wall rock is hotter and so melt cools more slowly
  • the shape and size of the magma body: the greater the surface area of the melt the faster it will cool. An intrusion shaped like a pancake will cool faster than one shape like a melon.
  • the presence of circulating groundwater: carries away heat like a car radiator








Fractional Crystallization

  • molten rock contains many different compounds
  • during freezing of molten rock, crystals of many different minerals may form
  • the minerals may not grow at the same speed
  • early-formed crystals tend to be relatively mafic (high in Mg and FeO)
    • this removes magnesium and iron from the magma
  • the remaining magma become more felsic (higher in SiO2 relative to Mg and FeO)
  • as different crystals grow, the melt composition changes progressively



  • if an originally mafic magma freezes before much crystallization has occurred, a mafic igneous rock forms
  • freezing of magma left after much crystallization has occurred yields a felsic igneous rock








Bowen's Reaction Series

  • Norman L. Bowen performed a series of laboratory experiments in the early 1920s designed to determine the sequence in which silicate minerals crystallize from a melt









Where Do Melts Cool?

Extrusive igneous settings

  • different volcanoes extrude molten rock in different ways
  • some volcanoes erupt streams of low-viscosity lava that flood down the flanks and the cover broad swaths of the countryside
  • when this lava freezes it forms a relatively thin lava flow
  • such flows may cool in days to months


  • some volcanoes erupt viscous masses of lava that pile into rubbly domes
  • other volcanoes erupt explosively
    • they send clouds of volcanic ash and debris soaring upward
    • and/or avalanches of ash down the sides of the volcano


  • the type of eruption depends largely on:
    • the magma's composition
    • volatile content





A stack of over 50 thin lava flows, capped by
debris, visible from inside Mt. Vesuvius, Italy




Two types of ash eruptions




Thick layers of ash deposited by explosive
eruptions in New Mexico about 1.14 mya






Intrusive igneous settings

Types of intrusions based on their shape

  • tabular or sheet intrusions: roughly planar and have a fairly uniform thickness
    • most are centimeters to tens of meters thick
    • tens of meters to tens of kilometers long
      • dikes: a tabular intrusion that cuts across pre-existing layering
      • sills: are tabular intrusions that inject between layers
      • laccolith: starts as a sill but then blisters up
  • plutons: blob-shaped intrusions that range in size from tens of meters across to tens of kilometers across
  • batholith: numerous plutons in a region
    • can be hundreds of kilometers long and wide









Large sills of basalt intruded sandstone beds in Antarctica








a) Plutons form when volcanoes of magma cool slowly at depth. Molten rock that reaches the surface erupts as lava

b) Erosion exposes plutons. This example, from the Mojave Desert shows the top of a pluton

c) A composite of many plutons is a batholith. As erosion progresses, dikes, sills and laccoliths are exposed



See a) above





During the Mesozoic (248 to 65 mya), subduction
produced a huge volcanic arc. In the crust, beneath
the arc, large granite batholiths formed. They are
now exposed by erosion.






The Sierra Nevada of California provide exposures of one of these
Mesozoic batholiths. The huge granite cliffs are popular with climbers.











Making room for igneous dikes and sills.


a) The crust stretches sideways during the intrusion of igneous dikes.






b) The Earth's surface rises to make room for sills.








How Do You Describe Igneous Rocks?


Color & Texture

  • describing color might not be easy, because some igneous rocks contain many visible mineral grains, each with a different color.
  • texture specifies whether the rock consists of:
    • interlocking crystals
    • stuck-together fragments
    • solid glass
  • texture definitions:
    • crystalline texture:
      • crystals grow around each other where they come in contact.
      • later-formed grains fill irregular spaces among earlier-formed grains
      • phaneritic: coarse-grained rocks with crystals large enough to see with the naked eye
      • aphaneritic: fine-grained crystals
      • porphyritic rocks have larger crystals (called phenocrysts) surrounded by a mass of fine crystals (called groundmass)
    • fragmental texture: formed from pyroclastic debris and consists of:
      • chunks and/or shards that are packed together, welded together or cemented together after they have solidified
    • glassy texture: rocks made of a solid mass of glass or tiny crystals surrounded by glass.
      • typically fracture conchodially (fractures are curved, like a clam shell)




  • grain size reflects cooling rate





Classifying Igneous Rocks

Types of Crystalline Igneous Rocks: based on silica content

  • ultramafic
  • mafic
  • intermediate
  • felsic





























  • as a rough guide, the color of igneous rocks indicates its composition:
    • mafic rock tends to be black or dark gray
    • intermediate rocks tend to be lighter gray or greenish gray
    • felsic rocks tend to be light tan to pink or maroon





Types of Glassy Igneous Rocks:

Rapidly cooling lava freezes while it still contains a high concentration of gas bubbles. These gas bubbles remain as open holes known a vesicles.

  • obsidian
    • solid, felsic glass
    • tends to be black or brown
    • breaks conchoidally with sharp edges
      • used for
        • arrowheads
        • scrapers
        • knife blades
        • surgical scalpels



  • tachylite
    • rare vesicle-free mass consisting of more than 80% mafic glass
  • pumice
    • felsic volcanic rock that consists of tiny vesicles that are surrounded by thin glass
    • can look like a sponge
    • some specimens can float on water
    • formed from quickly cooling, volatile rich frothy lava that resembles beer foam






  • scoria
    • mafic volcanic rock with lots of vesicles
    • bubbles are generally bigger than in pumice
    • usually darker





Types of Pyroclastic Igneous Rocks

  • When volcanoes erupt explosively, they spew out pyroclastic debris:
    • clots or droplets of lava
    • glass shards
    • larger fragments of pumice
    • broken up chunks of recently formed igneous rock
  • pyroclastic rock can be distinguished by grain size:
    • tuff: mostly volcanic ash with some larger fragments of pumice












Plate Tectonic Context of Igneous Activity


The conditions that lead to melting and to igneous activity can develop in four geologic settings:

  • at hot spots
  • along volcanic arcs bordering oceanic trenches
  • along mid-ocean ridges
  • within continental rifts




Products of Hot Spots

  • hot spot igneous activity includes rising plumes of hot mantle rock, not magma
  • this solid rock is hot enough to be soft and to flow plastically a few centimeters a year
  • when the hot rock reaches the base of the lithosphere, decompression causes partial melting.
  • this process generates mafic magma
  • at oceanic hot spots, much of the mafic magma erupts at the surface as basalt
  • some heat is transferred to the continental crust, which partially melts, producing felsic magmas



Products of Subduction

  • some minerals in oceanic crust contain volatile compounds (mostly water).
  • at shallow depths, these volatiles are bonded to other elements in the mineral crystals
  • when subduction carries crust down into the hot asthenosphere, the crust warms up, and at a depth of about 150 km, it becomes so hot the volatiles separate and diffuse into the overlying rock
  • addition of theses volatiles cause flux melting of rock



Forming Igneous Rocks at Mid-Ocean Ridges

  • the entire oceanic crust is a 7-10 km thick layer of basalt (extrusive)  and gabbro (same composition as basalt but intrusive and large-grained)
  • as sea floor spreading takes place:
    • oceanic lithosphere plates drift away from the ridge
    • hot asthenosphere rises and undergoes partial melting due to compression
    • this mafic magma will produce gabbro if it cools below the surface and basalt if it extrudes as lava.


Forming Igneous Rock at Rifts

  • rifts are places where horizontal stretching of continental lithosphere takes place
  • as a result of this stretching, the lithosphere also thins vertically
  • as this process takes place, the asthenosphere undergoes decompression
  • partial melting due to decompression produces basaltic magma which rises into the crust



Large Igneous Provinces (LIPs)

  • in many places on Earth, particularly large quantities of mafic magma have erupted or intruded
  • places where these regions occur include:
    • margins of continents
    • interior of oceanic plates
    • interiors of continents
  • the largest of these is the Ontong Java oceanic Plateau of the western Pacific
  • it covers 5,000,000 square km and has a volume of about 50,000,000 km3
  • such provinces also occur on land


  • Mafic LIPs may form when a mantle plume first reaches the base of the lithosphere
  • more partial melting can occur in a mantle plume because of higher temperatures
  • a very large quantity of very hot mafic magma forms
  • the mafic magma has low viscosity so that it can flow tens to hundreds of kilometers across the landscape forming a vast sheet of basalt
  • this process can be repeated hundreds of times, yielding thick stacks of thin, broad lava flows know as flood basalts







Flood basalts form the layers exposed in Palouse Canyon, Washington


Palouse Canyon, Washington










The Palisades (Hudson River)

  • The cliffs stretch north from Jersey City about 20 miles (32 km) to near Nyack, New York.
  • They rise nearly vertically from near the edge of the river, and are about 300 feet high at Weehawken, increasing gradually to 540 feet high near their northern terminus.
  • formed about 200 million years ago
  • The molten material cooled and solidified before reaching the surface.


Palisades near Hudson Heights neighborhood in Manhattan