IGNEOUS ROCKS and IGNEOUS ACTIVITY

 

Introduction:

Both temperature and pressure increase with increasing depth and it is the rate of increase that is important.

The geothermal gradient measures the rate at which temperature increases within the Earth.

Pressure increases at a rate of about 333 bars per kilometer in the crust. A bar is about one atmosphere. Therefore the pressure gradient is about one-third of a kilobar (1000 bars) per kilometer.

 

Partial Melting:

Most rocks are mixtures of minerals and each mineral has its own set of physical characteristics.

Quartz melts at about 1725°C at one atmosphere total pressure; here, melting is defined as the temperature at which solid and liquid of the same composition are in equilibrium.

In general we must specify the pressure in order to state a unique melting point.

If Quartz is mixed with Alkali Feldspar in some proportion (e.g. 70% feldspar and 30% quartz) melting occurs but not in the same way that the melting of a pure compound occurs. In general, there is no single temperature at which any mixture of minerals goes from solid to liquid. Rather, there is a range of temperatures at which liquid and solid are present. This is the interval of partial melting or partial crystallization.

The amount of liquid decreases as the temperature drops until all of the liquid is used up in producing solids.

Here partial melting is initiated at about 700 degrees Centigrade and completed by 1000°C.

Cooling is the reverse. This mixture would be 100% liquid until a temperature of about 1000°C. Crystallization begins and the amount of solids increase and the amount of liquid decreases as the temperature cools. At about 700° all of the liquid is gone.

The concept of partial melting plays a crucial role in igneous processes.

Essentially all magmas are formed by partial melting which did not reach the temperature at which all of the parent material was molten.

In general, liquids tend to be less dense than the solids that crystallize from them. In a mixture of crystals and liquids the liquid (less dense) will attempt to migrate upwards whereas the crystals may sink.

 

Initiation of Melting

Melting temperatures rise with increasing pressure:

When magma reaches the surface it is called lava. Magmas that cool at the surface of the Earth are extrusive whereas those that cool within the Earth are intrusive.

 

Plate Tectonics

Review the relationships between plate boundaries and igneous activity.

 

Classification of Igneous Rocks

Why do we classify things?

Two properties of igneous rocks that we will focus on are texture and mineralogy.

Texture refers to the size, shape and arrangement of the grains in the rock.

Minerals in igneous rocks have an interlocking texture.

Minerals crystallize and compete for space.

Rapid cooling leads to fine-grained aphanitic rocks — extrusive.

Slower cooling in an intrusive mass (lower temperature contrast with surroundings) should lead to a phaneritic texture.

Mineralogy - Remember that the most abundant mineral groups in the crust are the plagioclase and alkali feldspars. Norman Bowen (about 1915) proposed the following sequences of crystallization of silicates from a magma.

Bowen's Reaction Series Here

 

On the discontinuous, left side of Bowen’s reaction series are minerals rich in iron and magnesium:

high temperature

low temperature

 

On the continuous side of Bowen’s reaction series:

high temperature

low temperature

 

On the bottom portion of Bowen’s reaction series the following minerals crystallize:

high temperature

low temperature

 

Generalize from the feldspars:

high temperature

low temperature

 

Classification scheme for igneous rocks using texture and mineralogy.

Temperature estimated by the feldspar(s) present

Cooling rate (and thus extrusive vs intrusive) estimated from the texture

   Alkali Feldspar  Sodium Plagioclase  Calcium Plagioclase
 Phaneritic  Granite  Diorite  Gabbro
 Aphanitic  Rhyolite  Andesite  Basalt
 Silica Content  >65 %  53-65%  45-52%

 

Granite is a coarse grained igneous rock which contains abundant alkali feldspar. Granites also contain quartz. This is a low-temperature assemblage.

Rhyolite is the mineralogical equivalent of granite but it formed as a result of rapid cooling giving the rock the fine-grained texture.

Think about the analogous relationships between Diorite and Andesite and Gabbro and Basalt.

Remember the steps on the west side of the Memorial Library, around the Library Mall fountain and the base of the Lincoln statue - what name would you give these rocks?

If the rock is a granite but with a porphyritic texture it would be a granite porphyry. It if is a rhyolite but with a porphyritic texture it would be a rhyolite porphyry.

Viscosity is a measure of "resistance to flow".

A liquid with high viscosity flows with difficulty. In general, as the temperature of the liquid increases the viscosity of the liquid decreases and the liquid flows more easily. Magmas with lower silica contents also flow more readily - these effects are both in the same direction for igneous melts and make basalts much more fluid than rhyolites.

 

Shapes of Igneous Bodies

 

Mechanics of Batholith Emplacement

 

Source of Heat to Partially Melt Solid Rock

Radioactive elements (U235, U238, Th 232 and K40) decay giving off heat. Each decay gives off a very small amount of heat but over long time periods, this heat can result in temperature increases sufficient to initiate partial melting.

 

Extrusive Igneous Activity and Features

Volcanoes

Features of extrusive igneous rocks:

 

Magma-forming Environments