Introduction to Geology

Lecture Notes Overview

• The lecture notes are to be used with associated reading as the textbook may convey information differently.
• "Grey" information is supplementary and not necessary to memorize, such as the Earth's radius.

Introduction to Geology

• Geology studies the Earth, astronomical objects, their rocks, and how they change over time.
• Geology is referred to as Geoscience and Earth Science.
• Geology subdivides the Earth or solar system into specific aspects, such as volcanology or sedimentology, and integrates mathematics and other sciences.
• Geologists find deposits of rare earth minerals for technology like electric cars, smartphones, computers and clean energy.
• Geology helps to understand processes on other rocky planets and moons via analogs like lava, wind-blown dunes, impact craters, and ice.
• Earth has been radically different allowing inferences on other possible worlds; for example Pale-orange dot = microorganism induced toxic chemical haze; Snowball Earth = An ice planet with either full ice sheets to the equator or a slushy equatorial opening; Super continent Earth = a single north-south, dual hemisphere continent creating large ocean basins; Glacial Earth -> a dryer, colder, earth with large ice sheets

Principle of Uniformitarianism

• A principle devised by James Hutton that states natural processes operate the same in the past as now.
• The laws of nature are uniform across space and time.
• "The present is the key to the past" means modern geologic processes help understand ancient rocks.

Key Concepts: Time

• Geologists study when events happened, their order (cause and effect), and the speed of events.
• Geologists build timescales because rocks and processes don't come with an age.
• Relative dating uses relative ages, putting events in sequence.
• Absolute dating uses definitive ages or dates.
• Geologists divide geological time into Eons, Eras, Periods, Epochs, and Ages, from largest to smallest.
• Eons represent hundreds of millions to billions of years.
• Eras represent tens to hundreds of millions of years.
• Periods represent millions to tens of millions of years.
• Epochs represent thousands to millions of years.
• Ages represent thousands to millions of years.
• Unconformity = a gap caused by erosion or non-deposition.
• Law of superposition = in sediments, bottom layers are older than top layers.
• Original horizontality = sediments deposit on horizontal surfaces.
• Cross-cutting relationships = things cutting across are younger than what they cut across.
• Included fragments = rock A with fragments of rock B means rock B formed first.

Key Concepts: Space

• Geologists study spatial relationships like land and ocean distribution through Earth's history.
• Earth's equatorial radius: 6,378 km or 3,963.105 miles.
• Earth's polar (meridional) radius: 6,357 km or 3,950.057 miles.
• Earth's equatorial circumference: 40,075.017 km or 24,901.461 miles.
• Earth's polar (meridional) circumference: 40,007.863 km or 24,859.734 miles.
• Earth's surface area: 510,072,000 km².
• Land makes up 29.2% (148,940,000 km²) of the Earth's surface.
• Water makes up 70.8% (361,132,000 km²) of the Earth's surface.

Key Concepts: Systems

• A system consists of interactive objects and processes and earth system science views the entire planet as a combination of systems that interact with each other via complex relationships.
• The (A) CLIMATE SYSTEM is composed of Cryosphere, Atmosphere, Biosphere, and Hydrosphere.
• The (B) GEODYNAMO SYSTEM is composed of Inner core and Outer core.
• The (C) PLATE TECTONIC SYSTEM is composed of Lithosphere and Asthenosphere.
• Deep mantle is the mostly solid bulk of Earth's interior.

Earth's Structure

• The Earth's structure includes the crust, mantle, outer core, and inner core.
• The crust is split into oceanic and continental crust.
• Oceanic crust is 10km thick, high density igneous - basalt.
• Continental crust is 50km thick, low density igneous + sedimentary rocks.
• The crust is composed of silicon, oxygen, aluminium, iron, magnesium, and others.
• The Mohorovičić discontinuity (Moho) separates the crust and the mantle.
• The mantle extends down to 2,900km below the surface.
• The mantle's upper part is hot and flexible.
• The mantle is made of peridotite.
• The mantle is composed of high density silica, iron, and magnesium.
• The outer core is liquid and mostly composed of liquid iron and nickel.
• The inner core is solid and iron.

Lithosphere and Asthenosphere

• Lithosphere is the outer cool, rigid, and brittle layer of the Earth.
• The Lithosphere comprises the crust and part of the upper mantle.
• Lithosphere's base has a temperature of 1300° C.
• The Lithosphere's thickness is 100 km average, varying from 1-2 km to 300 km.
• A segment of the lithosphere is a Lithospheric Plate with earthquake activity along its margins and in certain situations.
• The Asthenosphere"Layer of the Earth immediately below the lithosphere; this layer is hotter and weaker than the lithosphere above and is capable of plastic flow when stress is applied. The top of the asthenosphere is indicated when a temperature of 1300° centigrade is reached.
• Below the Lithosphere and within the Mantle is the ASTHENOSPHERE (100 - 350 km) behaving like a soft vicious fluid (plastically)

Accretion & Terrane

• Accretion adds material to a tectonic plate at a subduction zone, frequently on the edge of existing continental landmasses. The added material may be sediment, volcanic arcs, seamounts, oceanic crust or other igneous features.
• Terrane is crust fragment formed on a tectonic plate (or broken off from it) and accreted or sutured to crust lying on another plate.

Geologic Province

• Geologic province is a spatial entity with common geologic attributes: Shield, Orogen, Basin, Platform, Large igneous province (LIP), and Extended crust

Isostasy

• Isostasy describes the physical, chemical, and mechanical differences between the mantle and crust.
• All regions of Earth aren't balanced in isostatic equilibrium.
• Isostatic equilibrium depends on the density and thickness of the crust, and on the dynamic forces at work in the mantle
• The term isostasy originates from the Greek: isos "equal" and stasis "stand still"
• John Pratt studied the Himalaya Mountains and hypothesized that floating blocks of Earth's crust have different densities, but they all sink to the same compensation level within the mantle.
• George Airy hypothesized that floating blocks of Earth's crust have the same density but different thicknesses and the continental blocks are higher because they are thicker.

Rock Cycle

• The Rock cycle is "an idealized cycle of processes undergone by rocks in the earth's crust, involving igneous intrusion, uplift, erosion, transportation, deposition as sedimentary rock, metamorphism, and further melting and igneous intrusion"
• Sedimentary rocks form from sediments that have undergone transport (e.g., river, ice, wind, etc.) + deposition; burial + lithification (compaction + cementation). Subtypes include clastic, organic, and chemical
• Metamorphic rocks form through textual and/or chemical change. Subtypes: Foliated and non-foliated
• Igneous rocks form through crystalisation, or solidification through mineral growth; Subtypes: Intrusive and Extrusive
• Sediments produced by the physical and chemical breakdown of rocks into smaller fragments and removal from their original location are transported and come to rest in the solid state
• Pre-existing rocks, such as sedimentary, igneous, or metamorphic rocks may change via metamorphosis (meta- means change and -morphos means form or shape).
• If the temperature and pressure conditions melt the rocks to create magma and lava, the rock cycle begins anew

How Do Rocks Melt?

• The outer core is liquid and everything else is solid but behaving plastically hence it needs to melt before becoming an igneous rock.

• The temperature of the Earth rises with depth caused by heat left over from the Earth's formation and ongoing radioactive decay.

• The rate at which temperature increases with depth is called the geothermal gradient.

• The solidus is the conversion between liquid and solid. It increases with pressure. There are 3 ways of melting

• Heat-induced melting occurs when applying heat to a rock.

• Flux melting occurs when volatiles (water etc.) are added to the mantle lowering the melting point and occurs at subduction zones.

• Decompression melting occurs when pressure is reduced allowing the melting point to lower and occurs at the oceanic ridges.

• Minerals melt at different temperatures, and some melt, and some remain solid

• Minerals with lower melting points turn into liquid magma, those with higher melting points remain as solid crystals

Plate Tectonics

• Plate tectonics is the scientific theory that the outer layer of the Earth is divided into the Asthenosphere and Lithosphere, with the latter comprising several large tectonic plates, which have been slowly moving.
• Three types of plate boundary: Convergent plate boundary, Divergent plate boundary, and Transform plate boundary
• Convergent Boundary: Plates move toward each other resulting in the subduction or collision, depending on the density of the involved plates.
• Divergent Boundary: Lithospheric plates move away from each other and occurs in continental rift zones and mid-ocean ridges.
• Transform Boundary: Lithospheric Plates slide past each other in the horizontal plane. Dextral, also known as right-lateral, movement describes the opposing plate moving to the right and Sinistral, also known as left lateral, movement describes the opposing plate moving to the left
• Collision zones where plates converge occurs during the closing of an ocean basin. Subduction is not possible between the equally buoyant plates and plates slam together.
• Oceanic-continental subduction occurs when an oceanic plate dives below a continental plate and has a trench, a mantle wedge and a frequent volcanic arc.
• Oceanic-oceanic subduction zone involves older plate subduction because it is colder and denser and volcanism causes chain of volcanism forms an island arc if they grow out of the water

Plate Tectonics: Active or Passive Margins

• Active plate margin = convergent boundary with an oceanic plate crashing into a continental plate
• Passive plate margin = a single tectonic plate transitions from oceanic crust to continental crust and seafloor will ultimately encounter passive plate margins, while seafloor created at the East Pacific Rise will likely encounter a subduction zone

How Do Plates Move?

• Plate and mantle are a 'coupled system' with the plates helping to drive mantle convection patterns.
• The lithosphere participates in convective overturn because its negative buoyancy is the principal driver of its subduction back into the mantle
• Newly formed oceanic lithosphere is less dense than the asthenosphere but becomes denser with age.
• Cooling, thickening, and getting denser causes it to sink into the mantle at subduction zones
• The newly formed plate slide sideways off these high areas pushing the plate in front of them. This is ridge push.

History of Plate Tectonics

• Continental drift – Alfred Wegner proposed continents move or drift relative to each other over geologic time based on coastlines, rock formations, fossils, and climate on different continents matching.
• Seafloor spreading - Harry Hess proposes magma could come up at ridges that lead to creation of new seafloor and spreading, which proposes trenches are where seafloor is destroyed and recycled. This is based upon seafloor mapping and magnetics(Vines-Matthews-Morley hypothesis (magnetic anomalies occur perpendicular to ridges with the magnetic reversals creating bands of normal and reversed polarity either side)
• Igneous rocks can record the magnetic field of the Earth from which an indication of where magnetic north was and the rocks relative position can be inferred. Not motion of Earth's pole, but of the continent relative to pole or the Trace of the paleo-apparent poles through time for a given location = apparent polar wander

Wilson Cycle

• Opening and closing of ocean basins, and subduction and divergence of tectonic plates during the assembly and disassembly of supercontinents
• Phases can be quickly described as the opening and closing of ocean basins

Rocks & Minerals

• Petrography is the study of rocks their mineral composition, texture, structure, and how they form. Monomineralic rock is made of a single mineral.
• Minerals have to be naturally occurring, inorganic, and solid
• Essential minerals have to be present in a rock for the rock to be given it's name and varietal and accessory minerals are additional
• Minerals form from crystallizing magma, precipitating from solution/gas, from metamorphism, weathering, or biological precipitation
• Minerals are classified by the anion or the anion group they contain

Minerals: Bowen's Reaction Series

• Bowen's reaction series describes the order at which minerals crystallise from a magma.
• Discontinuous branch: silica tetrahedron will link together ("polymerize") to form chains, sheets and frameworks( different minerals in progression along Discontinuous Branch
• Discontinuous series reaction occur at fixed temperature at constant pressure wherein early-formed mineral is converted to a more stable crystal or net-transfer reactions involve the breakdown of preexisting mineral phases and corresponding nucleation and growth of new phases.(Nucleation is the process in which a crystal begins to grow from one or more points, or nuclei.)
• Continuous reaction series "in which plagioclase is continuously reacting with liquid to form a more albitic phase as temperature decreases."
• Describes the order at which minerals crystallise from a magma

Lava and Magma

• Lava is extrusive, magma is intrusive. Magma is composed of melts (Si, O, Al, K, Fe and Mg), solids (crystals), and volatiles (water vapor, carbon dioxide, sulphur, and chlorine) Why do we have different rocks especially if the mantle has the same rock type? Is to do with the history of the rock
• Bowen's reaction series describes the order at which minerals crystallise from a magma and crystal settling

Igneous Rocks

• The removal and segregation from a melt of mineral precipitates

• Magmatic differentiation occurs when minerals undergo bulk chemical change during cooling, emplacement, or eruption

• Assimilation is where pieces of country rock - often more felsic -are added to the magma. Pieces may melt or remain intact, causing xenoliths

• Magma mixing = blending between a crustal melt and mafic material from the mantle

• Partial melting has minerals with lower melting points turn into liquid magma as others remain solid.

• --Extrusive rocks has magma associated with volcanism

Intrusive Rocks

• Crystallize underground and unless brought to the surface, rocks are fine grained Involves plutons, batholiths, dikes and sills

Rock Composition and Texture

• The composition of Igneous rocks can be roughly divided into four groups: Feisic, Intermediate, Mafic, Ultramafic
• Rocks come in various textures which provide evidence to the history of the rock:Glassy, Aphanitic, Porphyritic,, Phaneritic, and Pegmatitic
• Assumptions:Larger crystals= slower cooling

Volcanic Hazards

• Involves volcanic gases mixing with atmosphere and the eruption of tephra (rocks, gas, ash) or lava flows Magma rises from below into the storage region with some crystals and possible stagnation Partial melting of the mantle can result in the origin of magma
• -Vent is needed for volcanoes to occur

Why Live Near a Volcano?

• Near active volcanoes soil is often rich in mineral deposits
• --Geothermal energy can power systems and involves hospitality as well

Volcanic Eruptions

• ---All eruptions involve magmas reaching the Earth’s surface, but eruptions vary in violence and size and are relatively low energy

• May yield larger fountains and yield localized lava flows and are violent explosive that blast magma and fragmented material into the air. Eruptions are commonly divided in order of increasing degree of explosiveness, or how much is dissolved in the magma

• -Involves viscosity or the decompression rate of magma