Rocks that change when exposed to intense heat and or pressure are called metamorphic rocks.
Factors influencing the nature of metamorphic rocks
Mineral Composition of pre-existing rock - Mineral composition determines how different climatic changes affect the rock. For instance, at high temperatures, some minerals undergo chemical changes and become different minerals while others only change their shape.
Temperature and Pressure – They determine the kind of transformation that emerges from a rock. For instance, under very high temperatures, shale, a sedimentary rock changes to slate. With even higher temperatures and pressure slate changes to schist rock. Pressure also influences the texture of a rock. As Pressure increases the minerals get compressed and become thicker.
Delegate your assignment to our experts and they will do the rest.
Water-Water as a solvent is essential for fastening and increasing the metamorphic process
by dissolving and moving ions within minerals and moving materials within the crust.
Time- The process of atoms diffusing in the minerals causing metamorphosis is very slow.
This could take millions of years for a slight change to occur under climatic changes.
Formation of foliation in metamorphic rocks
Foliation refers to the recurrence of mineral layers in rocks because of exposure to direct pressure and heat. This compresses the minerals in the rock, causing a texture change and alignment reflecting the direction that pressure was applied. For instance, Mica and Chlorite minerals which flatten to the side of the pressure.
Texture and mineral content in metamorphic rocks
Metamorphic textures are either foliated or non-foliated. Foliated textures form slaty, phyllite schistose, and gneissic rocks. They have a complex composition with many different types of minerals. There is a preferred alignment of crystals within the rocks caused by differentiated stress and elongated mineral shapes.
Non-Foliated metamorphic rocks include hornfels, marble, quartzite, and novaculite. They do not have a preferred mineral orientation for reasons that they either, lack differentiated stress or the minerals have equal shape.
Settings for metamorphic rocks formation and links between plate tectonics and metamorphism
Plate tectonics control pressure and temperature in the crust. Metamorphosis occurs in three different tectonic settings. Divergent plate boundaries are found near mid-ocean ridges, subduction zones where the cold oceanic crust subdues and collision zones where continents collide.
Important processes of regional metamorphism
Regional metamorphism-It occurs over large areas generated by massive geological processes such as mountain building and subduction plates. It requires a huge amount of pressure and temperatures vary depending on depth. It creates foliated rocks such as gneiss and schist.
Important processes of contact metamorphism and metasomatism
Contact metamorphism occurs over smaller areas ranging from 1-10 km. It occurs when pre-existing rocks come into contact with magma. Their temperatures rise and fluid from magma infiltrate the rocks. Mostly, it happens from high temperatures and varying pressures. It creates non-foliated metamorphic rocks.
Metasomatism- This is when metamorphic rocks undergo significant changes in the chemical composition during recrystallization. This can be brought about by the chemical active fluids mainly s2o and CO2 which add or subtract materials during the recrystallization of rocks in the solid-state.
Relative and absolute age-dating techniques
Relative dating informs about the order in which events occurred but not how long ago they happened. Absolute dating gives the numerical figure of how old something happened. For instance, radiometric dating which calculates the age of rocks and minerals that contain certain radioactive elements.
History of geological time scale and the relationship between eons, eras, periods, and epochs
Scientists use geological time scale to record various stages on earth’s history. Eons are the largest group of time ranging from a half-billion to nearly 2 billion years long. The earliest Eon is called Hadeon. It was followed by Archean Eon (4- 2.5 billion years), Phanerozoic Eon (541 million years- present). Eons are made up or divided into Eras. The first era of the phanerozoic Eon is the Paleozoic era (541 million years ago). All Eons before it are collectively known as Precambrian. The Mesozoic era also known as the age of reptiles began (252-66 million years ago). The Cenozoic era (66million years ago – present.) It marks the rise of mammals. The period is one of the subdivisions of geological time. Epoch is longer than an age and shorter than a period. The current period is Quaternary and Holocene Epoch. (Berry, 1987).
Types of Unconformities
Unconformities are major breaks in sedimentation. Erosion, deposition, and tectonic activities are factors that cause Unconformities. (Alden, 2017)
Nonconformity -It represents a contact or surface between metamorphic or igneous rocks and a sequence of sedimentary rocks.
Angular Unconformity – Occurs when there are two different sequences of sedimentary rock. When the beds beneath erosion surface are folded or tilted creating a difference in the angle of the younger and older rocks.
Disconformity- The lower and upper series of bed dips at the same angle and direction and the contact plane is an erosional surface.
Applications and limitations of isotopic techniques for geological dating.
Isotope decay of potassium -40 to argon – 40 determines the decay rate of over a period of 1.3 billion years.
These techniques cannot be used with sedimentary rocks. This is because sedimentary rocks consist of rocks that had daughter isotopes when forming but isotopic dating assumes that there was no daughter isotope in the formation of the rocks.
References
Alden, A. 2017. Unconformity types and Examples. ThoughtCo Retrieved from:
https://www/Thoughtco.com/unconformity-types-and-examples-4123229
Berry, W. 1987. Growth of a prehistoric time scale: based on organic evolution. Revised edition, Blackwell Scientific Publications, Palo Alto, 202p.
