Rock Deformation, Magmatism, and Seismic Activity

Posted on Jan 15, 2025 in Geology

Deformation of Rocks

  • Compressive stresses: Convergent forces that usually occur in subduction zones and collision areas.
  • Tensional stresses: Divergent forces that act on the rocks of rifts.
  • Shear stresses: Transform forces that occur at transform boundaries.

How Do Rocks Respond to Stress?

Rocks respond in an elastic way (they accumulate elastic energy). If the stress continues near the surface, the rocks break and release the elastic energy (they are rigid), causing earthquakes and fault joints.

Factors affecting the rock response: Pressure, temperature, types of rocks, water content, and type of stress.

Magmatism

Magma is molten rock that forms underground in the deep crust or upper mantle.

There are three factors controlling the formation of magma:

  • Temperature: An increase in temperature can melt the rocks.
  • Pressure: A decrease in pressure reduces the rocks’ melting point.
  • Water content and amount of other volatile substances: A high amount of liquids and gases makes the rock easier to melt.

Formation of Magmatic Rocks

Slow: Magma can solidify slowly underground if it doesn’t find a way out to the Earth’s surface. In this case, a plutonic rock is formed. As magma solidifies slowly, minerals have time to form large visible crystals. Examples: granite and pegmatite

Fast: Sometimes, magma gets to the surface through cracks in the crust and solidifies quickly. If this happens, only microcrystals can form. Sometimes no crystals at all form: we say the material is vitreous. Examples: basalt, obsidian, and pumice

Material Expelled by Volcanoes

  • Solid: Pyroclasts – big (bomb blocks), medium (lapilli), and fine (ash)
  • Liquid: Lava
    • Low viscosity: It can flow long distances before it solidifies; gases escape easily. Formed by felsic materials.
    • High viscosity: It solidifies near the crater; sometimes it can block the vent; gases don’t escape easily. More violent eruption. Formed by mafic materials.
  • Lahars are volcanic mudflows or debris flow normally produced when, during a volcanic eruption, large masses of snow or ice melt at the top of the volcano.
  • Pyroclastic flows are clouds of incandescent pieces of rock which run downslope at huge speed.
  • Lateral explosions can occur when the main vent is blocked by solidified magma.
  • Gases, such as CO2, CO, SO2, SO3, H2S, NOx.

Plutonic Activity

Plutonic activity is the rise of magma through the lithosphere, and its accumulation, cooling, and solidification inside the crust to produce plutonic rocks or plutons.

Volcanic Activity

Volcanic activity occurs when magma rises up and emerges at the surface through an opening in the crust called a volcano.

Seismicity

What is an earthquake? An earthquake is the trembling of the Earth’s crust, caused by a sudden release of elastic energy when rocks which have been subjected to tectonic stress break or move quickly.

The focus is the point in the crust, below the surface, where the earthquakes originate. The point where energy is released.

The epicenter is the point of the surface which is directly above the focus. The one that first receives the waves and where the earthquake has its highest magnitude.

Scales

  • The Richter Scale: It measures the magnitude of the earthquake, which is a physical measurement of the amount of energy released by the earthquake. 0-10
  • The Mercalli Scale: It measures the intensity of the earthquake, which is a human measurement of the damage caused by the earthquake. I-XII

Metamorphic Rocks

Metamorphic rocks occur with an increase in temperature and pressure and infiltration of water.

Changes: Mineral composition, crystal size, and crystal arrangement (when crystals arrange themselves in a perpendicular direction to the compressional stress).

Metamorphic rocks can be:

  • Foliated: Examples: schist, gneiss
  • Non-foliated: Examples: quartzite and marble