Lecture 3 - Magma and Igneous Rocks

Magma

• Extremely hot liquid and semi-liquid rock located under Earth’s surface

• Besides molten rock, magma may also contain suspended crystals and gas bubbles

• Igneous rocks are formed from cooled magma

• Magma composition is important as it determines how far the reaction process reaches (based on available silica) and what kind of rock it will harden into

• Can be classified based on its composition. It can be Mafic, Intermediate, or Felsic

  

• SiO2 (Silica) increases

  • MAFIC magma (45-52%) to INTERMEDIATE magma (55-65%) to FELSIC magma (65-75%)

• MgO, FeO, CaO (Magnesium Oxide, Ferrous Oxide, Calcium Oxide) decreases

  • MAFIC magma (25%) to FELSIC magma (5%)

• Felsic Magma

  • Have high silica content (>63%)

  • High silica content made it very viscous which causes explosive volcanic eruptions

  • Can erupt at low temperatures (800°C)

  • Felsic rocks (made from cooled felsic magma) form continental rocks

  • They are light in color

• Mafic Magma

  • Have low silica content (45-52°) and high ferromagnesian content

  • Low viscosity

  • Can erupt at high temperatures (1100°C)

  • Mafic rocks compose the dark ocean floor

• Intermediate Magma

  • Have silica content (52-63%) and relatively high ferromagnesian content

  • Can erupt at temperatures (850 to 1100°C)

• Ultramafic Magma

  • Have low silica content (<45%) and very high ferromagnesian content

  • Can erupt at high temperatures (1600°C)

  • Highly mobile liquid in eruption

  • Rare and very old; mantle has cooled to form ultramafic magma

Igneous Rocks - formed by cooling and crystallization of magma (molten rock)

• Intrusive Igneous Rocks

  • forms within crust/below ground surface;

  • takes centuries - millions of years to form;

  • also called Plutonic rocks

  • can be found in great mountain ranges

• Extrusive Igneous Rocks

  • forms above ground surface;

  • takes seconds to years to form

  • also called Volcanic rocks

  • can be found near volcanoes and ocean floor

Bowen’s Reaction Series

• Shows how minerals are formed as magma cools and becomes igneous rocks

• The term REACTION is very important as you will see later on that the first minerals to form may react with other materials in the magma, resulting in new minerals in the rock.

Norman Levi Bowen worked on research involving cooling processes of magma. He worked mostly with basaltic magmas.

1. He will melt the rock to magma

2. Said magma will be cooled SLOWLY to a specific temperature X. The slow cooling facilitates the mineral formation during said process.

3. After reaching temperature X, the rest of the magma is cooled QUICKLY (also known as QUENCHING). Quenching does not allow minerals to form anymore.

4. The resulting rock therefore only shows minerals that can form up until temp X.

5. He then identified which minerals form as you slowly lower your temperature by doing more experiments at different temperatures X1, X2, etc.

• Has 2 branches — Discontinuous and Continuous branch. The progress of each branch depends on availability of certain components of the magma.

• Discontinuous Branch:

  • starts with OLIVINE

  • Olivine is (typically) made up of Iron, Magnesium, and Silica

  • If there is still silica in the magma, Olivine reacts with the silica to form Pyroxene as the temperature drops (slowly)

  • As temperature further goes down (slowly) and silica is available, the process happens but this time with Pyroxene to Amphibole. Amphibole to Biotite may also happen under the right circumstances.

  • is called as such because you transform some/all of your previously formed minerals to new minerals.

• Continuous Branch:

  • Plagioclase Feldspar can range from pure anorthite (Ca(Al2Si2O8)) to pure albite (Na(AlSi3O8))

  • Plagioclase Feldspar also forms around the same point as Pyroxene

  • Plagioclase Feldspar goes from calcium-rich to sodium-rich form as the temperature drops and as long as sodium is available in the magma

  • If there are still Silica left at ~750-800°C, the remaining magma will form Potassium Feldspar, Muscovite Mica, and Quartz

  

Igneous Rocks

• The mineral composition for different Igneous rocks (ultramafic to felsic)

• Shown also are the names of the typical rocks depending on their composition (ultramafic to felsic) and where they form (intrusive vs extrusive)

  

• Mafic magma typically cools into Gabbro (intrusive form) or Basalt (extrusive form)

• Intermediate magma usually cools into Diorite (intrusive form) or Andesite (extrusive form)

• Felsic magma commonly cools into Granite (intrusive form) or Rhyolite (extrusive form)

• Magma classification is also known by other names based on the extrusive rock commonly formed:

  • Mafic: Basaltic

  • Intermediate: Andesitic

  • Felsic: Rhyolitic

• Mafic VS Intermediate VS Felsic

  • Mafic rocks tend to be darker then felsic rocks due to ferromagnesian silicates (which are generally darker then other minerals)

  • Mafic rocks can have 50-100% ferromagnesian silicates while felsic rocks can have anywhere from 1-20%

• Intrusive VS Extrusive

  • Intrusive rocks tend to be “rougher”. This is because they cool slower under the ground, the crystals have time to form/”grow”.

  • Extrusive rocks cool very quickly, being exposed to above ground temperatures, the crystals can’t grow as much and the rock attains a much smoother surface.

Intrusive Bodies

• Intrusive rocks solidifies under the earth. Magma from the upper mantle generally enters the crust. When no volcanic activity arise, the magma solidifies into intrusive rocks

• Magma is usually less dense than the rock around it such that magma slowly moves up towards the surface

• Plutons can be classified based on its mass, shape, or orientation

• Plutons that are large and irregularly shaped can be a STOCK or BATHOLITH. The difference is based on the exposed area at the surface (when these are eventually uplifted and exposed)

• BATHOLITHS have exposed area at the surface of more than 100km² while STOCKS are anything with less than that

  

a) STOCKS

b) SILL - a pluton that is sheet-like and is parallel to the existing layering of the underground materials

c) DYKE - similar to sill, but cuts across the existing layers of underground materials

d) LACCOLITH - a sill that has expanded and pushed the rock above it

e) PIPE - a “cylindrical” conduit (may have any cross-sectional shape) that serves to allow movement of magma from one point to another

f) PIPE or DYKE - but given the view, it is not possible to say if its shape is sheet-like or “cylindrical”

Engineering Properties of Some Igneous Rocks

Plutonic Rocks

• Unjointed and un-weathered plutonic rocks can handle high in-situ stresses and is typically the ideal bedrock

• Joints and faults are not uncommon in outcrops and can be traced in explorations

• Sheet joints are common in granitic rock formations

Excavations on Plutonic Rocks

• Open joint surfaces may pose a problem of sliding in excavations due to lack of interlocking

• Deep cuts in rocks should be continually supported in the process by rock bolts or anchoring

• For fresh rocks (no jointing or weathering), blasting may be a problem in excavation due to difficulty in blasting (blow up)

Foundations on Plutonic Rocks

• Fresh rocks offer high bearing capacity

• Footings founded on rocks with different weathering grades can experience differential settlement

• Driving piles in rocky weathering zones can be difficult

Weathered Plutonic Rocks

• The degree of weathering dictates the overall properties of a granitic rock

• Fresh rocks tend to hold high in-situ stresses and works well in underground works and dams

• Weathered plutonic rocks allows groundwater flow and easier excavatability

Volcanic Rocks

• Volcanic eruptions may cause different depositions of rocks and are seldom homogenous

• Intact volcanic rocks is hard and dense and high in strength

• Some volcanic rocks contains holes if unfilled with mineral matter during cooling

• Exploration should identify parts of the rock mass with low strength and high permeability

Stability in Volcanic Rocks

• Ledges of volcanic rocks are usually underlain by pyroclastic sediments or nonvolcanic sediments

Permeability of Volcanic Rocks

• Columnar joints and rock pores provides high permeability in volcanic rocks

Construction Materials

• Extrusive rocks are used as aggregates in concrete, rock fill for dams, railroad ballast, and highway base course