Rock Formation and Metamorphism: An In-Depth Look

Posted on Dec 14, 2024 in Geology

1. The Cycle of Rocks

Internal processes lead to the formation of new rocks and the formation of relief. External processes cause wear, sediments are deposited in the basins, and new rocks are formed. The geological cycle describes the geological processes that contribute to the generation of rocks, acting upon each other in a succession of events, ordered or not.

Milestones:

  • Orogeny: Mountain range formation, relief, and inner strength from within to the exterior.
  • Gliptogenesis: Destruction of geological relief due to external agents.
  • Lithogenesis: Formation of new rocks.

According to the latest formation process that has acted on the material, resulting rocks are considered:

  • Endogenous when the processes have their origin inside the Earth’s crust.
  • Exogenous when the processes are located on the surface.

Internal Processes:

  • Igneous or magmatic rocks: Melting and solidification.
  • Metamorphic rocks: Chemical-mineralogical and structural transformation in a solid-state of preexisting rocks.

External Processes:

  • Sedimentary rocks: Erosion, transport, and deposition of preexisting rocks.

2. Endogenous Rocks

Inside the Earth, there are two mechanisms generating rocks:

  • Solidification based on a mass of molten rock that forms magmatic rocks.
  • Transformation of rocks, never melting, due to the increase of pressure conditions and/or temperature that causes metamorphic rocks.

3. Magmatic or Igneous Rocks

These come from the solidification of magma (a mixture of rocky material mixed with solid minerals and rock fragments and a gaseous phase). Magmas are formed by the melting of rocks located at the base of the upper crust or upper mantle.

The solidification of magma depends on its chemical composition, temperature, gas content, and the relations between the magma and the environment, controlling the cooling rate.

Characteristics of Magma:

  • Chemical composition dominated by oxygen and silicon, forming silicate combined with aluminum, calcium, sodium, potassium, iron, magnesium, manganese, etc. 99% of the minerals that form magmatic rocks are from the silicate group.
  • Temperature between 700 and 1200°C, depending on their chemical composition: if the percentage of Si and O is relatively low, temperatures are higher (1000 to 1200°C); the more abundant these elements are, the lower the temperatures (700-900°C).
  • The most abundant gas is water vapor. You can find nitrogen, CO, CO2, hydrogen sulfide, sulfur dioxide, hydrogen, and halogen gases such as hydrochloric and hydrofluoric acid.
  • The chemical composition, temperature, and the presence of gases are the factors that control the density and viscosity of magma.
  • Raises the melting temperature.
  • The pressure prevents the merger.
  • SiO2 = + + = viscosity – density
  • – SiO2 = – + = viscosity density

Rocks as the total SiO2 + 66% of

Percentage Type Rock Examples Color

Acidic Rocks Quartz Clear

52 – 66% Intermediate Rocks Dark

45 – 52% Basic Rocks Basalts

45% Ultrabasic Rocks Pyroxenes and amphiboles, inosilicate

The temperature at which minerals begin to melt first is called the solidus point. The temperature until all minerals are melted is called the liquidus point.

The Ascent of Magma and Differentiation

When materials are melted in a melting zone of the Earth, they are called primary magma. These are promoted because they are less dense than the rocks around them. This rise is melting the surrounding rocks or through microcracks. So, there is an intrusion, which is surrounded by encasing materials.

Encasing rocks are found around the magma. The minerals crystallize from magma not simultaneously, but according to their solidification temperature. The first are those with less silicon (nesosilicates) and finally those with more silicon (phyllosilicates or tectosilicates).

Magmatic differentiation: The process of crystallization of magma in which there are phenomena that are changing the composition, thus forming different types of magma (magma side).

  • Fractional crystallization: Obtained from a parent magma, others can form that are more acidic, as the first crystallizing minerals are neosilicates (olivine), with low silicon content. If these minerals are deposited on the bottom and are isolated, the rest of the magma is more acidic because it has more silicon.
  • Magmatic Assimilation: In some of the rocks, melting encasing materials, new elements enter the magma chamber.
  • Blends with different composition magmas can also be produced.

Place of Emplacement

Much of the magma solidifies before entering the Earth’s surface, so intrusive rocks are formed. When doing so deep, rocks are considered plutonic. At more moderate depths within microcracks or fractures, they are hypabyssal. When the magma reaches the surface causing volcanic phenomena, we derive the effusive or volcanic rocks.

Cooling Speed Location Magma Structure that you form Rocks that form

Slow Plutons Plutonic

Moderately fast Dikes Hypabyssal

Fast Volcanoes or seams, casting lava Volcanic Microcrystalline

Very fast Volcanoes (pyroclasts) Volcanic glass

Textures

It is the absolute and relative size and mineral components and spatial relationships between them.

  • Granular: Whole rock formed by crystals, visible, of around the same size.
  • Porphyritic: Two different sizes; large crystals formed by slow cooling and other small or glass formed by faster cooling.
  • Phaneritic or macrocrystalline: Crystals visible to the naked eye.
  • Aphanitic: Crystals not visible even with the magnifier. Microcrystalline arrays of crystals when the crystals are visible by light microscopy.
  • Vitreous: When composed of glass, amorphous material.

Crystallization of Magma

The various minerals in consecutive series arrays are called Bowen’s reaction series. Two series are known:

  • Ferromagnesians: Changing the structure of the one and the other is called a discontinuous series.
  • Plagioclase: Their composition varies over time but retains the same structure and is a continuous series.

Classification of Magmatic Rocks

  • Intrusions: Solidify inside the Earth.
  • Plutonic or hypabyssal, depending on whether the volume is large or small solidified.
  • Effusive or volcanic on the exterior.

Most Common Magmatic Rocks

Plutonic

  • Granite/granodiorite (quartz, potassium feldspar, plagioclase, and mica); granular; clear.
  • Syenite (potassium feldspar, plagioclase, and mica); granular; clear.
  • Diorite/Gabbro (ferromagnesian minerals and plagioclase); granular; dark.
  • Peridotite (olivine and pyroxene); granular; dark.

Volcanic

  • Rhyolite (quartz, potassium feldspar, plagioclase, and mica); porphyritic crystals just visible; clear.
  • Trachyte (potassium feldspar, mica, hornblende); porphyritic; dark.
  • Andesite/basalt (plagioclase and ferromagnesian minerals); porphyritic; dark.

Hypabyssal

  • Pegmatite (potassium feldspar, muscovite, and quartz); granular; clear.
  • Aplite (potassium feldspar, plagioclase, and quartz); microscopic granular; clear.
  • Lamprophyre (potassium feldspar, biotite, and plagioclase); granular porphyritic; dark.
  • Granite porphyry (K-feldspar, plagioclase, quartz); porphyritic 50%; clear.
  • Dioritic porphyry (ferromagnesian minerals and plagioclase); porphyritic 50%; dark.

4. The Metamorphic Rocks

These come from the transformation of other rocks in a solid-state. It occurs when the original rock is subjected to physical conditions (temperature and pressure) other than during their formation. The boundary between metamorphic and sedimentary processes is located around 200°C. The boundary between the metamorphic and magmatic processes is given by the beginning of the fusion of rock (gneiss, extreme metamorphism, melting, limit); this fusion process is called anatexis. When there is a fluid that acts causing an ion exchange, chemical changes take place in arrays called metasomatism. Metamorphism is from 10 km towards the interior of the earth.

Variables that control the metamorphic process:

  • Temperature: More movement and mixing of molecules. Causes recrystallization, large size increases, the process is called blastesis.
  • Pressure: Movement (directed) breaks the mineral foliation causes, changing texture, makes it smaller. Confinement (static)
  • Time and presence of water.

Degrees of Metamorphism

  • Lightweight: Fewer layers, very thin crystals, more schistosity.
  • Strong: More layers, the crystals are, cannot present schistosity.

Slate, Phyllite, shale, micaschist, mylonite, Mica, gneiss

Types of Metamorphism

  • Regional metamorphism extends over large areas of hundreds or thousands of square kilometers and is common in the mountains.
  • Local metamorphism is restricted to limited areas.
  • Contact Metamorphism: Elevated temperature. Occurs in encasing rocks, close to magmatic intrusions rising toward colder areas.
  • Cataclastic or dislocation metamorphism: Occurring in areas with intense deformation (faults).
  • Shock Metamorphism: Areas affected by meteorite impacts or nuclear explosions.
  • Thermal Metamorphism (contact) due to high temperatures.
  • Dynamic Metamorphism: Due to high pressures.
  • Dynamothermal metamorphism (regional): The two processes act simultaneously.

Facies: Associations of rocks that occur in a given environment, under conditions of pressure and temperature.

Texture of Metamorphic Rocks

Foliation: Looks like a rock as a large size of minerals.

  • Slate-like
  • Schistosity: Restructuring plans, lamination occurs in many minerals under pressure.
  • Mineral banding
  • Granoblastic: Disorderly disposal of minerals, no foliation.

Most Common Metamorphic Rocks

Name Derives from Texture Type of Metamorphism

Slate/Phyllite Argillaceous Foliated mudstones, slate, very smooth and regular planes, no crystals visible Regional low

Shale/Micaschist Clay mudstones, sandstones Foliated, shaly, irregular surfaces, low-medium crystals visible Regional

Gneiss Mudstones, sandstones, granites Foliated with mineral banding, no planes, well visible crystals Regional medium-high

Marble Limestones Granoblastic, visible crystals Regional or contact

Quartzite Sandstone quartz Granoblastic, little crystals visible Regional or contact

Hornfels Sandstones or mudstones Granoblastic very fine Contact high

Speckled Slate Mudstones, slates, schists Foliations of the original rocks, new crystals visible as spots Contact low grade