Lecture 4 Igneous Rocks

• Three Main Rock Types • Formation of Igneous Rocks Where does magma come from?

The nature of volcanic eruptions Volcanic products Intrusive rock bodies • Igneous rock textures • Igneous rock classification • Engineering considerations of igneous rocks

• Three Main Rock Types

• Rocks are divided into three main types depending on their origin: • Igneous rocks are cooled from a molten state.

• Sedimentary rocks are deposited in a fluid medium (usually water).

• Metamorphic rocks are formed from preexisting rocks by heat and pressure.

• Formation of Igneous Rocks

• magma: igneous rocks form from the cooling of molten (or partially molten) rock materials called magma , which consists liquid, dissolved gas, and crystals.

• lava: magma that reaches Earth's surface • extrusive or volcanic: igneous rocks that form when molten rock solidifies at the surface • intrusive or plutonic: igneous rocks that form at depth.

W. W. Norton

Fig. 6.01b

J. D. Griggs/U.S. Geological Survey

• Where does magma come from?

• The Earth's crust and mantle are composed of solid, not molten, rock. So what is the source of magma that produces igneous activity? Magma is generated in the lower crust and upper mantle (at depths of 50 to 200 km also) by (1) raising the

temperature

, (2) reducing the

pressure

, (3) adding

water

. Thus, magma forms in distinctive tectonic settings, generally related to plate boundaries.

• The temperature increases with depth (on average by 20-30 degrees centigrade per kilometer in the upper crust known as the

geothermal gradient

). At convergent plate boundaries as the oceanic crust descends into the mantle, it is heated and dehydrates. The fluid reduces the melting temperature to cause melting.

• At divergent boundaries as hot mantle rock ascends, it moves to zones of lower pressure to trigger melting even without additional heat.

Melting of rocks from the heat of rising magma and decompression W. W. Norton

The Earth’s geotherm and melting curve for mantle rock (peridotite). A rock rises up from A to B will start to melt (known as decompression melting).

W. W. Norton

Melting by addition of water and volatiles.

W. W. Norton

The addition of water and volatiles decreases the melting temperature of rocks. (W.W. Norton)

• At divergent boundaries, hot mantle rock ascends from the ashenosphere and moves to zones of lower pressure as the overlying lithosphere splits and moves apart. This reduced pressure triggers melting even without additional heat.

• At convergent boundaries, as the oceanic crust descends into the mantle, it is heated and dehydrates. The fluid reduces the melting temperature to cause melting.

Explosive Mt. St. Helens eruption 1980. ( Photo by Austin Post of USGS )

• Why some eruptions are explosive and some are "quiet"?

• The primary factors are viscosity and dissolved gas content . The viscosity depends on temperature and silica content. The lower the temperature or the higher the silica content, the greater the viscosity. Very fluid basaltic (low silica) magmas allow expanding gases to migrate easily out of the vent, making the eruptions less violent.

• Viscosity describes resistance to shear during the motion of a fluid.

• Volcanic products

• Lava flows: pahoehoe flows, and aa flows.

• pahoehoe: smooth, ropy surface, from low viscosity flows • aa: rough, jagged blocks, from high viscosity flows.

• Lava tubes: lava conduits • Pyroclastic materials: ash, dust, pumice, bombs.

(Left) pahoehoe flows. (Right) aa flows.

• Corrections for the first paragraph of West p.38: The correct one should be:

“... Pahoehoe forms ropy features on the surface but aa forms a rough jagged blocky surface. Pahoehoe shows a relatively smooth surface. This illustrates that pahoehoe is the more fluid of the two lavas and it yields thinner individual flows.”

View of an active lava tube as seen through the collapsed roof. (Photo by Jeffrey B. Judd, USGS).

•

Intrusive rock bodies

:

Igneous intrusions are masses of rock formed when magma cools beneath the surface (generally called

plutons

). They are classified according to their sizes, shapes, and relationships to the rock they have invaded (known as

the country rock

). Important intrusive rock bodies include: •

batholiths

: large exposures (over 100 km^2) of intrusive rock. Batholiths typically form in the deeper zones of mountain belts and are exposed after considerable uplift and erosion, e.g. Sierra Nevada batholith.

•

stocks

: small plutons with an exposure area of less than 10 km^2.

•

dikes

: narrow, tabular intrusive bodies. They are quite common. All dikes are discordant, cutting across preexisting structure (such as bedding planes). Dikes are related to the fractures caused by magma intrusions on the country rocks.

•

sills

: tabular intrusive bodies formed when magma is injected along the bedding planes of layered rocks.

•

laccoliths

: lens-shaped intrusive bodies between sedimentary beds that formed when the injection of magma arches up the overlying strata.

Basic igneous structures. A) relationship between volcanic and intrusive igneous activities. B) intrusive igneous structures. C) A stock and batholith are exposed after uplifting and erosion.

Fig. 6.11de

Paul Hoffmann

•

Igneous rock texture

• The texture of an igneous rock describes the overall size, shape, and arrangement of of its constituents. Texture reveals a great deal about the environment in which the igneous rock formed. The most important factor contributing to the texture of igneous rocks is

the rate at which magma cools

.

•

Types of igneous rock texture

•

glassy texture

: no distinct grains, conchoidal fracture, produced by rapid cooling.

•

aphanitic (fine-grained) texture

: grains not discernible with the naked eye, relatively rapid cooling at the surface (Greek: phaneros -- visible).

•

phaneritic (coarse-grained) texture

: grains discernible with naked eye, slow cooling below the surface.

•

porphyritic texture

: grains of two distinct sizes, representing two stage cooling •

pyroclastic texture

: composed of rock fragments ejected by volcanic eruptions.

•

pegmatitic texture:

composed of unusually large crystals (> 1cm). Pegmatites are formed at the last cooling stage of magmatic liquid; crystals are able to grow in the fluid (water) rich final melt.

(UL) Aphanitic, (UR) phaneritic, (LL) porphyritic, (LR) glassy

The uniformity of the granite pluton makes carving possible. (S. Marshak)

•

Igneous rock classification

• Igneous rocks are classified on the basis of texture and mineral composition . The major kinds of igneous rocks are granite, diorite, gabbro, rhyolite, andesite, and basalt.

• A general relationship between color and mineral composition can be used: Dark color generally indicates ferromagnesians; light color generally indicates high silica content.

The classification of common igneous rocks is based on texture and composition. The print size of the rock names is proportional to their relative abundance at the Earth’s surface.

Granite at the pavement of the Alma Mater Statue.

•

Engineering considerations of igneous rocks

• An engineering siting must consider: 1) rock types and distribution; 2) type of alteration after formation (tectonic fracturing, weathering).

• Unaltered intrusive igneous rocks are generally suitable for most engineering projects because of the tight interlocking network of mineral crystals.

• Problems for extrusive rocks: The water-bearing capacity is much greater than intrusive rocks, making them unsuitable for reservoir or tunnel construction. Extrusive rocks with pyroclastic materials are much weaker.