Earth Materials: (b) Primary Rocks

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A rock

bullet is a mono- to multi-minerallic aggregate (i.e., made up of one or more kinds of minerals) that
bullet can be either primary or igneous (i.e., plutonic and volcanic rocks) or secondary (i.e., sedimentary and metamorphic rocks).

Rock Cycle" ― a theoretical concept that relates tectonism, erosion, and the various rock forming processes to the common rock types ― plausibly began with the formation of granitic crust and granite, the most common plutonic rock is also what the "Rock Cycle" ends with.

Try the following links for
"Rock Cycle" presentations:

  Plutonic or "Intrusive" Igneous Rocks

Igneous rocks are primary rocks in the rock cycle and form by solidifying from molten condition.


Igneous rocks are of two kinds: plutonic and volcanic.

How do igneous rocks form?

Visit the URL:

for the animation of igneous rocks and for more information on the igneous rocks.


Intrusive or plutonic rocks form from the slow cooling and solidification of magma.


These intrusive bodies can form either near-surface (volcanic necks or plugs, dikes and sills) or be deep-seated (plutons like stocks and batholiths).

The red and pink shaded regions in this geological map of
California are intrusive igneous bodies. This map can be accessed
at the California Geological Survey website
These igneous intrusives form the bulk of the Sierra Nevada Ranges. Contrasted with these large batholiths are such more common though shallow
or near-surface features as dikes and sills.


The intrusive igneous rocks have phaneritic (or coarse grained) to porphyritic textures, compared to the porphyritic to aphinitic textures of extrusive igneous rocks.


Felsic (60-75% silica, with “free” quartz) composition dominates the intrusive or plutonic rocks (e.g., granites and granodiorites), compared to the mafic (with ~50% silica, no “free” quartz) basalts in the case of extrusive or volcanic rocks. Gabbro, the plutonic equivalent of basalt, is mafic and diorite, the plutonic equivalent of andesite, is of intermediate composition.



Granites and granitization complete the “rock cycle” as granite batholiths, the most common plutonics, often form in the core zones of folded mountain belts through the process of dynamothermal metamorphism.


But then, note that the primordial crust too is likely to have been granitic.

Click on this title "Mantle decarbonation and Archean high-Mg magmas" to learn about the nature of Archean crust.

Geological evidence suggests that Earth may have had surface water - and thus conditions to support life - billions of years earlier than previously thought.
                      Scientists reconstructed the portrait of early Earth by reading the tell tale chemical composition of the oldest known terrestrial rock. The 4.4-billion year-old mineral sample suggests that early Earth was not a boiling ocean of magma, but instead was cool enough for water, continents, and conditions that could have supported life. The age of the sample may also undermine accepted current views on how and when the moon was formed. The research is published in this week's issue of the journal Nature.  "This appears to be evidence of the earliest existence of liquid water on our planet," says Margaret Leinen, assistant director of NSF for geosciences. "If water occurred this early in the evolution of earth, it is possible that primitive life, too, occurred at this time."
                      By probing a tiny grain of zircon, a mineral commonly used to determine the geological age of rocks, scientists from the University of Wisconsin-Madison, Colgate University, Curtin University in Australia and the University of Edinburgh in Scotland have found evidence that 4.4 billion years ago, temperatures had cooled to the 100-degree Centigrade range, a discovery that suggests an early Earth far different from the one previously imagined.
                      "This is an astounding thing to find for 4.4 billion years ago," says John Valley, a geologist at UW-Madison. "At that time, the Earth's surface should have been a magma ocean. Conventional wisdom would not have predicted a low-temperature environment. These results may indicate that the Earth cooled faster than anyone thought." Previously, the oldest evidence for liquid water on Earth, a precondition and catalyst for life, was from a rock estimated to be 3.8 billion years old.
                      The accepted view on an infant Earth is that shortly after it first formed 4.5 to 4.6 billion years ago, the planet became little more than a swirling ball of molten metal and rock. Scientists believed it took a long time, perhaps 700 million years, for the Earth to cool to the point that oceans could condense from a thick, Venus-like atmosphere. For 500 million to 600 million years after the Earth was formed, the young planet was subject to intense meteorite bombardment. About 4.45 billion years ago, a Mars-size object is believed to have slammed into the Earth, creating the moon by blasting pieces of the infant planet into space.
                      The new picture of the earliest Earth is based on a single, tiny grain of zircon from western Australia found and dated by Simon Wilde, of the School of Applied Geology at Curtin University of Technology in Perth, Western Australia. Valley worked with William Peck, a geologist at Colgate University, to analyze oxygen isotope ratios, measure rare earth elements, and determine element composition in a grain of zircon that measured little more  than the diameter of two human hairs. Colin Graham's laboratory analyzed the zircon to obtain the oxygen isotope ratios. Graham is a contributor to the paper and geochemist at the University of Edinburgh.
                      "What the oxygen isotopes and rare earth analysis show us is a high oxygen isotope ratio that is not common in other such minerals from the first half of the Earth's history," Peck says. In other words, the chemistry of the mineral and the rock in which it developed could only have formed from material in a low-temperature environment at Earth's surface.
                      "This is the first evidence of crust as old as 4.4 billion years, and indicates the development of continental-type crust during intense meteorite bombardment of the early Earth," Valley says. "It is possible that the water-rock interaction (as represented in the ancient zircon sample) could have occurred during this bombardment, but between cataclysmic events."
                      Scientists have been searching diligently to find samples of the Earth's oldest rocks. Valley and Peck say such ancient samples are extremely rare because rock is constantly recycled or sinks to the hot mantle of the Earth. Over the great spans of geologic time, there is little surface material that has not been recycled and reprocessed in this way.
                      The tiny grain of zirconium silicate or zircon found by Wilde in western Australia was embedded in a larger sample containing fragments of material from many different rocks, Valley says. Zircons dated at 4.3 billion years were reported from the same site a decade ago, but the new-found zircon grain is more than 100 million years older than any other known sample, giving scientists a rare window to the earliest period of the Earth.
                      "This early age restricts theories for the formation of the moon," Valley says. "Perhaps the moon formed earlier than we thought, or by a different process." Another intriguing question is whether or not life may have arisen at that early time. Low temperatures and water are preconditions for life. The earliest known biochemical evidence for life and for a hydrosphere is estimated at 3.85 billion years ago, and the oldest microfossils are 3.5 billion years old.
                      "It may have been that life evolved and was completely extinguished several times" in catastrophic, meteorite-triggered extinction events well before that, Valley says.




Want to take a self-test on igneous rocks and processes?
Try North Dakota State University’s Geoscience website at the URL:


To view the samples of different igneous rocks, try the URL:

bullet For a plutonic rocks exercise
using Bowen's reaction series,
ry the URL:
Volcanism and the "extrusive"
or volcanic rocks

Click on this image to access
 this online edition of Robert Tilling's
 USGS publication "Volcanoes"

Links for terrestrial and extraterrestrial volcanism:

Extraterrestrial Volcanism


Basaltic Volcanism on the Terrestrial Planets


Active Volcanism On Mars


Volcanism on Mars


Volcanism on Venus


Volcanism on the Moon


Jupiter’s Volcanic Moon


Large Igneous Provinces

or LIPs are voluminous emplacements of predominantly mafic extrusive and intrusive rock whose origins lie in processes other than 'normal' seafloor spreading. LIPs include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Click on this map of LIPs, shown below, to read about the research on LIPs.


Notable LIPs on Land

The most notable of continental flood basalts and flood basalt provinces are:

Columbia River Basalts (~15 Ma),


Ethiopian Traps (~38 Na),


Deccan Traps (~65 Ma),


Parana/Serra Geral lavas (~135 Ma),


Karoo/Stromberg, Patagonian
and Ferrar lavas (~180 Ma),


Siberian Traps (~250 Ma), and


Keweenawan Lavas (~1,250 Ma).

Other Kinds of Volcanism      


Spreading Submarine Ridges: Spreading submarine ridges and rises (e.g., Reykjanes/Mid-Atlantic Ridge, East Pacific Rise, hydrothermal vents) and associated volcanic islands (e.g., Iceland).


Volcanism at the Subduction Zones: Convergent plate margins  have volcanism towards the edge of the plate that is being subducted (e.g., Cascade Ranges and the Pacific “Ring of Fire”).



'Hot Spots' and Aseismic Ridges: Intra-plate or “hot-spot” volcanism (e.g., Hawaii-Emperor Seamounts, Yellowstone-Snake River volcanics)


  Explore Volcano World at the URL: Also available on-line,
at is
the treatise: Basaltic Volcanism on the Terrestrial Planets. For extra-terrestrial volcanism, try the URL:
  The sketch below, taken from the URL:

explains the entire cycle of volcanism from Juan de
Fuca Ridge to Juan de Fuca subduction (i.e., the
Filled Trench) and the Cascades.

Volcanic materials/ products and rock classification:


Volcanic Rocks:
Increasing order of silica (SiO2) content, from silica-rich rhyolite at one end and the more common basalt at the other, with the compositionally intermediate andesite denoting crustal contamination of the magma on its passage through the crust.  

Why andesites on Mars, then?


Other products:

Cinder cones/Pyroclastics (Ash, Cinders, Blocks/Bombs, Lahars)

Gases and Gas Clouds: Nuees Ardentes, Toxics, Climate Change

Other materials: Pyroclasts, Volcanic Breccia


Textures of Volcanic Rocks:

Glassy (Obsidian),

Vesicular (Scoria, Pumice),
Aphanitic (Andesite, Basalt, Rhyolite),

Porphyritic (Andesite Porphyry, Basalt Porphyry, and Rhyolite Porphyry).


Visit the USGS volcanoe sites, starting with and its links.
Also try the site:




Volcanic Precursors

bullet Seismicity (Harmonic Tremors?)
bullet Bulging or Uptilting?
bullet Exotic Gas Emanations?

Is Volcanism Predictable?


Browse USGS Case Study from Kilauea and Mount St. Helens volcanoes at the URL:

Volcanism and Climate


for a comparison of the climate effects of
Mt. Pinatubo and El Chichon events.

American Geophysical Union has an interesting
discussion of terrestrial degassing at the URL:

Also try the URL:


Other Links:

bullet WorldWide Volcanism Links
bullet Submarine volcanism home page at the Monterey Bay Aquarius Research Institute
bullet NOAA Ocean Explorer: Submarine Ring of Fire
bullet Flood-Basalt Volcanism
bullet Volcanism and Climate: A Comparison of the Mt. Pinatubo and El Chichon events
bullet USGS Photo Glossary on Volcanic rocks
bullet Minerals, Magmas and Volcanic Rocks
bullet This "USGS Volcano Hazards Program" site leads you to all the information you would need about volcanoes, volcanism and volcanic rocks

Access here the home page of Smithsonian Institution's Global Volcanism Project with links to

bullet Catalog of volcanoes active during the last 10,000 years
bullet Smithsonian reports of current activity since 1968
bullet Links to other volcanology resources on the Web