These notes include Interlude C

Interlude C: The Rock Cycle

• The components of a rock can later be rearranged or moved elsewhere to form a new rock in the same class or a different one.

• Rock cycle: the succession of events that results in the transformation of Earth materials from one rock type to another.

  • Forming igneous rock requires heating and melting.

  • Forming sedimentary rocks requires erosion, transportation, and deposition.

  • Forming metamorphic rocks requires burial and/or heating.

Rock Cycle Paths

Paths Reflect Geologic History

• Different environments have different types of rocks.

• Rocks may go through the full cycle or transform into the same rock again.

A Case Study of the Rock Cycle

Rates of Transfer

• Rocks do not go through the cycle at the same rate.

• There are rocks of many different ages on the surface of Earth.

• Oceanic crust has igneous rocks (basalt and gabbro), topped with sediment.

What Drives the Rock Cycle?

• Earth’s internal heat and gravitational field drives plate movements and generates plume-associated hot spots.

• Plate interactions results in the uplift of mountain ranges, exposing rock to weathering and erosion

• Plate interactions also generate the geologic settings in which pre-existing rock melts and produces magma, metamorphism occurs, and sedimentary basins develop.

• Wind, rain, ice, and currents all contribute to the rock cycle, causing erosion and weathering.

Cycles of the Earth System

• A cycle is a series of interrelated events or steps that occur in succession and can be repeated.

• A region that contains a volume of material is called a reservoir.

• Two categories of cycles:

  • Geochemical cycles involve primarily physical components of the Earth System.

  • Biogeochemical cycles involve both physical and living components.

  • The rock cycle can be either depending on the conditions.

Chapter 7

7.1: Introduction

• James Hutton is the father of geology

• Outcrops: bedrock that is visible on the surface.

• Metamorphic rock: forms when a pre-existing rock (protolith) undergoes a solid-state change in response to the modification of its environment.

• Protolith: the original rock that has undergone metamorphism.

• Metamorphism: the process by which one kind of rock forms into a different type of rock.

• Solid state means that a metamorphic rock does not form by solidification of magma.

• Change means the rock contains new minerals and/or a new texture (arrangement of mineral grains) that are not like the protolith.

• Modification of environment can be several things,

  • Change in temperature or pressure

  • An application of stress (a directed compression, stretching, or shearing)

  • Exposure to hydrothermal fluids (solutions of very hot water)

7.2: Consequences and Causes of Metamorphism

What Is a Metamorphic Rock?

• Metamorphic minerals: new minerals that grow in place within solid rock during metamorphism.

  • A new group of minerals produced by metamorphism can form a metamorphic mineral assemblage.

• Metamorphic textures: arrangement of their mineral grains.

• Metamorphic foliation: parallel alignment of inequant grains or by the presence of alternating light-colored and dark-colored laters.

Processes That Take Place During Metamorphism

• Metamorphism is a very slow process.

• Recrystallization: changes the shape and size of grains without changing the identity of the mineral making up the grains.

• Phase change: transforms one mineral into another (a polymorph).

  • Same composition but different crystal structure.

  • Involve rearrangement of atoms.

• Metamorphic reaction: (aka neocrystallization) results in growth of new minerals that differ from those of the protolith.

  • Chemical reactions digest minerals of the protolith and makes new minerals grow.

• Pressure solution: the surface of one mineral grain pushes against the surface of another, under conditions where a water film separates the grains.

• Plastic deformation: occurs when a rock becomes warm enough to behave like soft plastic, allowing minerals to change shape without breaking.

Metamorphism Due to Heating

• Mineral grain warms, stretches and bends chemical bonds that locked atoms into place.

• Atoms detach, shift, and form new bonds with other atoms.

• Solid-state diffusion: atoms either rearrange within grains or migrate into and out of grains.

• Diffusion can result in recrystallization or neocrystallization.

Metamorphism Due to Pressure

• Material collapses inward.

• Crystals structures are more open when close to the surface, but under pressure the atoms may pack together and cause denser minerals to form.

• Occurs during phase changes or during neocrystallization.

Changing Both Pressure and Temperature

• As depth increases, the original mineral assemblage in a rock becomes unstable, and a new mineral assemblage becomes stable.

Compression, Shear, and Development of Preferred Orientation

• Differential stress: the push or pull in one direction differs in magnitude from the push or pull in another direction.

• Two different types of differential stress:

  • Normal stress pushes or pulls perpendicular to a surface.

    • Push is compression (flattens materials, perpendicular).

    • Pull is tension. (stretches materials, parallel).

  • Shear stress: moves one part of a material sideways relative to another part.

• Compression and shear can cause cracking or breaking.

• Preferred orientation: The parallelism of inequant grains in a metamorphic rock.

  • Inequant grains have different dimensions in different directions.

  • Equant grains have roughly the same dimensions in all directions.

The Role of Hydrothermal Fluids

• Hydrothermal fluids: solutions of very hot water. Hot water, steam, supercritical fluid.

• They react chemically with rock by dissolving, transporting or providing ions.

• They can provide water molecules that become a part of the minerals.

• Metasomatism: the process by which a rock’s overall chemical composition changes during metamorphism because of the reactions with hot water that bring in or remove elements.

7.3: Types of Metamorphic Rocks

• Metamorphic rocks have two fundamental classes: foliated rocks and non-foliated rocks.

Foliated Metamorphic Rocks

• Foliation: the parallel surfaces or layers that can occur in a metamorphic rock.

• Stripes or streak appearance in outcrop.

• Gives ability to split into thin sheets.

• A rock has foliation either because its minerals have a preferred orientation or because different minerals concentrate in different layers so the rock displays alternating dark-colored and light-colored layers.

• Can be distinguished from one another according to their composition, grain size, and nature of their foliation.

• Most common types:

  • Slate - finest grain foliated rock, formed by shale or mudstone, low pressures and temperatures, thin sheets.

  • Phyllite - fine grained, white mica, silky sheen. Forms under temperature high enough to cause neocrystallization.

  • Schist - medium to coarse grained, contains type of foliation called schistosity, higher temperature.

  • Metaconglomerate - a metamorphic rock cause by metamorphism of a conglomerate, typically contains flattened pebbles and cobbles.

  • Gneiss - a compositionally banded rock typically composed of alternating dark and light colored layers.

  • Migmatite: Under high temperatures in the presence of water, gneiss may begin to melt, producing felsic magma and leaving behind solid, relatively mafic metamorphic rock.

Nonfoliated Metamorphic Rocks

• Nonfoliated: a metamorphic rock that contain minerals that recrystallized or grew during metamorphism, but the rock overall has no foliation.

• Did not undergo compression or shear.

• Common types

  • Hornfels

  • Quartzite

  • Marble

Defining Metamorphic Intensity

• Metamorphic grade: a representation of the intensity of metamorphism, meaning the amount or degree of metamorphic change.

• Depends on temperature and pressure at which metamorphism takes place.

• Temperature places dominate role in recrystallization and neocrystallization.

• Rocks forming under low temperatures are low grade rocks.

• Rock forms at high temperatures are high grade rocks.

• Rocks formed in between those temperatures are intermediate grade rocks.

• As grade increases, both types of crystallization produces coarser grains and new mineral assemblages.

• Metamorphic zone: the region between two metamorphic isograds, typically named after an index mineral found within the region.

7.4: Where Does Metamorphism Occur?

Thermal or Contact Metamorphism

• Metamorphic aureole: aka contact aureole, the region around a pluton stretching long distances, in which heat transferred from the pluton in the country rock and metamorphosed the country rock.

• Thermal Metamorphism: It occurs due to high temperatures without pressure changes, altering rock mineralogy.

• Contact Metamorphism: It results from rocks coming into contact with magma, leading to changes in mineralogy and texture.

Burial Metamorphism

• Burial metamorphism: Metamorphism due only to the consequences of very deep burial 

Dynamic Metamorphism

• Dynamic metamorphism: This process occurs only as a consequence of shearing under metamorphic conditions, without requiring a change in temperature or pressure.

• At greater depths, rock becomes warm enough to behave like soft plastic when shear takes place along the fault. During this process, the minerals in the rock recrystallize.

Dynamothermal or Regional Metamorphism

• Dynamothermal metamorphism: Metamorphosis that involves heat, pressure, and shearing.

• Regional metamorphism: Metamorphism of a broad region, due to burial during an orogeny.

Hydrothermal Metamorphism at Mid-Ocean Ridges

• Hot magma rises beneath the axis of mid-ocean ridges, cold seawater sinks into oceanic crust through cracks along the ridge and heats up and transforms into hydrothermal fluid.

• The fluid then rises through the crust near the ridge and causes hydrothermal metamorphism of seafloor basalt.

• Hydrothermal metamorphism: The process by which very hot water passes through the crust and causes metamorphism of rock .

Metamorphism in Subduction Zones

• The plate tectonics theory explains the movement of Earth's lithosphere plates, which float on the semi-fluid asthenosphere. It describes how plates interact at boundaries, leading to processes like earthquakes and volcanic activity.

Shock Metamorphism

• Sock waves generate heat that can melting or vaporization of rock in that area.

• Shock metamorphism: The changes that can occur in a rock, due to the passage of a shockwave, generally resulting from a meteorite impact.

Where Do You Find Metamorphic Rocks?

• Exhumation: The process (involving uplift and erosion) that returns deeply buried rocks to the surface.

• Weathering, landslides, river flow, and glacial flow strip away rock at the surface and exposes rock that was once below the surface.

• Mountain ridges contain metamorphic rock.