Geoscience Exam 2

How do metamorphic rocks form?

As a result of changes to a protolith rock of some kind of composition, of pressure and temperatures

What kinds of rocks can protoliths be?

Igneous or sedimentary

What are the two main kinds of metamorphism?

Regional and Contact

What is regional metamorphism?

Usually heating in the upper plate of subduction zones

What is contact metamorphism?

Local heating of wall rocks from intruding magma

How do minerals react in metamorphism?

Solid-state transformations, not melting and recyrstallization

What forces drive metamorphic reactions?

Increase/decrease in temperatures and pressure

How can you classify metamorphism?

Based on presence of foliation, environment of formation (contact vs regional), bulk composition, index minerals, and facies

What are index minerals?

set of characteristic minerals found in peltic (meta-mudstone) rocks

What are facies?

sets of mineral assemblages repeatable in space and time, characteristic of a given pressure and temperature

What is a migmatite?

A mineral that has the upper end of metamorphism and partial melting of metamorphic rock

Is the age of the Earth universal?

No, it is recognized to be vast by different cultures for over 2,000 years

Why is Hutton important to Earth's history?

Hutton introduced Uniformitarianism concept in 1785, 'the present is the key to the past', Earth must evolve only due to observable processes at present rates

How can you figure out relative age of sediments?

Law of Superposition, lateral continuity, cross-cutting relationships, and characteristic fossil assemblages

What is the law of superposition?

Proposed by Niels Stensen (Nicholas Steno), the idea that oldest rocks are at the bottom of layers and youngest are at the top

What is lateral continuity?

Layers of sediment initially extend laterally in all directions, in other words, they are laterally continuous. Steno's law.

What are unconformities?

Breaks in sedimentary depositional record

What are the three types of unconformities?

angular unconformity, disconformity, nonconformity

What is an angular unconformity?

Tilted rocks with straight rocks deposited on top

What is a disconformity?

Rocks below and above are straight but there is an obvious break in time (much younger fossils above than below)

What is a nonconformity?

Sedimentary rocks being deposited over NON-sedimentary rocks (like sandstone over granite)

What is the Great Unconformity?

Clear break in time across the United States, Cambrian sandstone over pre-Cambrian metamorphic rocks

How can you determine absolute geological time?

decay of radiogenic isotopes in minerals, the parent-daughter isotope at known isotope-specific decay rate (half-life)

How can you determine a parent-daughter isotope's age?

Measure parent and daughter isotope concentrations and do the math

What are common dating systems?

K40 and C14

What does a mineral's age mean?

The age it cooled, the time since cooling below some decay-system specific temperature or date of exhumation to surface (not crystallization)

What is deformation?

Changes in rock geometry since solidification or deposition

What is stress?

Force per unit area in a given direction, often varies by direction

What is isotropic stress?

Stress that is the same in all directions, aka pressure

What are the three different kinds of stresses?

Tensional/extension, compressional, and shear

What is tensional/extension stress?

Pulling in two different directions, material often gets thinner on the top and faults appear, how continents rift apart

What is compressional stress?

Squeezing the rock together, shortens and thickens the material (material above gets thicker but individual layers get shorter)

What is shear stress?

Pulling the top in one direction and pulling the bottom in the other so that surfaces slide past one another (angular distortion)

What is a strain?

A rock's response to stress

What are beds?

Layers of sediment

What are folds and how do they vary?

Bending of rock layers due to tectonic forces, vary due to layer thickness, spacing, and strength

What are the 3 types of folds?

Anticline, Syncline, monocline

What is anticline fold?

when both limbs dip away from each other, looks like the top of the letter A, often tilted or asymmetrical

What is syncline fold?

Two beds tilt down towards each other, looks like the bottom of the letter S

What are synform and antiform folds?

When you don't know if rocks are the right way up, more common for folds in metamorphic rock

What are faults and what are the three kinds?

Discontinuities with visible offset, normal, reverse, and strike-slip

What are normal faults?

Where the upper block moves down and lower block moves up

What is a reverse fault?

Where the upper block moves up and lower block moves down

What are strike-slip faults?

where two blocks of rocks slide past each other but don't move up or down

What is the difference between right and left lateral strike-slip faults?

Rocks on the other side of the fault move to the right or moves to the left

What is the difference between the Hanging Wall and Foot Wall?

The hanging wall is the block above tilted surface and the block below tilted surface is the foot wall

Where does brittle/localized deformation predominate?

Upper few km of the crust

Where does ductile/distributed deformation predominate?

Middle and lower crust

What is the Hadean era?

The first 600 Ma of Earth's history

What happens to the Earth in 4.6 Ga to 4.0 Ga?

Transition from molten-liquid to solid Earth and onset of earliest form of plate tectonics

What happens to Earth at 4,500 Ma?

Planetary accretion, molten iron settles to Earth's core, molten crust begins to solidify, and first atmosphere was very different from today (lots of N2, CO2, and H2S)

What happens to Earth's atmosphere at about 4,500 Ma?

Collision of Earth with Mars-sized planet makes debris field that coalesces into the moon (first atmosphere removed and new atmosphere closer to today's atmosphere begins to develop)

What are zircon grains and what is their importance?

They are the oldest mineral grains (4.2 Ga) in younger metasandtones, only direct evidence to rock record

What is evident from 4,300 Ma to 3,900 Ma?

First water develops, evidence from detrital mineral grains

What happened to Earth and the Moon from 4,100 to 3,800 Ma?

The Great Bombardment, meteor bombardment of Earth and Moon, evidence from lunar landings

How did continents function in the Hadean Period?

They started out very small and grew with time at their margins

When is the Archean era and what major things happen during it?

4.0 Ga to 2.5 Ga, much of the continental crust develops

What are the small suite of rock types in the Archean?

Gneiss, granite, greenstone, graywacke, chert and/or limestone (small continental masses separated by subduction zones)

How old are the oldest known rock and what is it called?

4.0 Ga, Acasta Gneiss

How old is the oldest sedimentary rock and what is it called?

3.7 Ga, Graywacke

What happened to life in 3.4 Ga?

Blue-green algae mats develop (stromatolites), prokaryotes, no nucleus

What happened to microplates by 2.7 Ga?

Archean microplates mostly joined into cratons

When was the Proterozoic eon?

2.5 Ga to 0.541 Ga

What and when was the Hudson Orogeny?

The Hudson Orogeny was the evolution of North America's craton, in 1.9-1.6 Ga

What were the distinctive igneous rocks of Proterozoic eon and what did they indicate?

Anorthosites, Rapiwiki, and granites, indicated that lower crust and upper mantle were working differently than they are today

What happened to the atmosphere in 2.4 Ga?

Banded Iron Formation indicates first free oxygen in atmosphere

What happened to the Earth in 2.2 Ga?

1st snowball earth, Gowganda tillites, full extent of ice cover unclear

What happened to life in >1.0 Ga?

First eukaryotic algae (have nucleus) and possibly bacteria

When was the Neoproterozoic?

1,000 Ma to 541 Ma

What two supercontinents formed in the Neoproterozoic eon?

Rodinia (1,000 Ma) and Pannotia (600 Ma)

What happened to the Earth in 730-620 Ma?

A second snowball earth, most likely ice-covered for 10-20 Ma at least twice, glacial deposits common world-wide in rocks from this age

What happened in 635-541 Ma?

Ediacaran, earliest multicellular complex life

What are the controls on explosiveness of magma?

bulk composition, volatile content, and gas content

What effect does bulk composition have on magma viscosity?

mafic, low viscosity, felsic, high-viscosity

What effect does water content have on magma viscosity?

It reduces viscosity

What effect does gas content have on explosiveness of magma?

Higher gas content causes more expansion near the surface, causing it to be more explosive

What are the two different kinds of volcanoes?

Shield (basaltic, low-relief but large) and stratovolcanoes (classic volcano morphology, seep slopes, smaller)

What is a caldera and where can one be found?

a crater collapsed into its underlying magma chamber, like in Crater Lake, Yellowstone

What are cinder cones and fissures?

A simple volcano built from blobs of lava ejected from a single vent, result of basaltic magma with a lot of gas, very distinctive features and fissure is magma reaching the surface along long fractures and is not point source

What are tephra?

Volcanic ejecta, classified by size, range from bombs (large) to pumice (medium) to ash (small), can travel 100s of km

What are lahars?

volcano-driven mudslides

What are pyroclastic flows?

hot ash hugging around, like Pompeii

How do volcanic explosions today compare to in the past?

Largest known eruptions in historic record are puny in comparison to some known in the geologic record

When is the Paleozoic eon?

541-252 Ma

What happens to Pannotia at the beginning of the Cambrian period?

Pannotia breaks into 4 pieces: Laurentia, Gondwana, Siberia, Baltica

What is notable about Gondwana?

Many fragments rift off it, like Avalonia, and collide with other cratons

What happened to Laurentia from the Cambrian to Devonian?

Much of North America (Laurentia) is on/near the equator, covered mostly by warm shallow seas

What happens in the Carboniferous/Permian period?

many continents reassemble into Pangea, the supercontinent, and multiple major mountain-building events happen globally

What are the 3 mountain building, Paleozoic events in North America?

Volcanic arc addition (Taconic-Ordovician), micro-continent (Avalonia-Devonian), and Northwest Africa, Appalachian/Alleghenian/Hercynian (Carboniferous-Permian)

What is notable about Pangea?

In the Permian eon, was a giant supercontinent that was mostly desert

What happens at the end of the Permian era?

End-Permian Extinction, largest fossil record, Siberian Traps volcanism most likely cause

What is the Cambrian Explosion?

explosion of genera of complet life with shelly hard parts at the beginning of the Cambrian Period

What is notable about life in the Cambrain?

First shelly fossils

What is notable about life in the Ordovician?

First vertebraes

What is notable about life in the Silurian?

Age of fishes

What is notable about life in the Devonian?

First amphibians, significant life on land

What is notable about life in the Carboniferous?

Known for coal, trees, insects, first reptiles

When was the Mesozoic?

252 Ma to 66 Ma

When was the Cenozoic?

66 Ma to present

When was the Triassic?

252-201 Ma