Lecture 3-5 Flashcards
What do absolute dates come from?
Radiometric dating of igneous rocks
What does relative dating come from?
Relationships between rocks
What are the three types of rocks?
Igneous, sedimentary, metamorphic
What is an igneous rock?
Fire formed
Formed through the cooling and solidification of magma lava
We can date most accurately igneous rocks (absolute dates)
What is a sedimentary rock?
Formed from chemical precipitates or fragments of earlier formed rocks
Sedimentary rocks
What is a metamorphic rock?
Formed by application of heat and pressure to either igneous or sedimentary rocks
Transformed into other rocks tell us mostly about the relative order in which events occurred
How do we know how old a rock is?
Radiometric dating
The radioactive decay of an isotope is a natural clock
Once magma/lava, it’s radiometric clock begings
Some isotopes decay over time –> we can measure the decay for carbon dating purposes
When the number of parent atoms decrease, the number of daughter atoms increase
What is an isotope?
All atoms of a precise mass for a given element
Different elements have different masses (isotopes)
E.g. carbon 12 (stable), carbon 13 (stable), carbon 14 (unstable)
Unstable isotopes undergo decay (half life = rates of decay)
Why does radiocarbon dating not always work
We can measure the amount of carbon 14 to determine how old something is. This is called radiocarbon dating. But this only works for things that are up to 50,000 years old.
Most rocks are way older than 50,000 years old
What is a solution to radiocarbon dating limitations?
Measuring the decay of other elements found within a rock to determine an absolute age
Minerals found within rocks contain trace amounts of unstable isotopes
What is a parent isotope?
The starting isotope during radioactive decay
What is a daughter isotope?
The new element produced as a result of radioactive decay
Explain isotopic dating of rocks
Determines the ratio of parent to daughter atoms
This is assuming that when a rock forms it contains an unstable isotope and none of the daughter isotope
Also assuming that over geologic time the rock remains a closed system (no parent or daughter enters or leaves the rock)
That rock can be accurately dated by determining th eratio of parent to daughter atoms
What is relative dating?
Inferring the sequences in which older to younger events (recorded in rocks) occurred
In sedimentary rocks, the older rocks are the layers at the bottom. The younger rocks are the layers above them.
What is the principle of superposition?
Sedimentary rock is produced from the gradual accumulation of sediment on the surface. Therefore, newer sediment is continually deposited on top of previously deposted or older sediment.
How can fossils contribute to relative dating
Different organisms throughout history have left their remains as fossils in sedimentary rocks
Geologists can study the order in which fossils appeared/disappeared through time and rocks
Fossils can help to match rocks of the same age, even when you find those rocks a large distance apart
Rocks in different places can be put into separate time sequences. Fossils in some of the rocks can be correlated to help combine these sequences into longer ones.
Where are fossils most abundant?
In marine sedimentary rocks. They are generally not found in igneous or metamorphic rocks.
Why is using fossils not always clear-cut?
A large river may dump a large amount of sediment into the sea. But rocky stretches of coast may see very little sediment accumulation
Far offshore, in the deep-sea, sediment accumulation is much slower
On beaches, a powerful storm can remove meters of sediments in a single event
So we cannot use the thickness of sedimentary layers to estimate how much time any layer represents.
How did Earth form and approximately how long ago?
4.54 billion years ago out of a solar nebula (a swirling cloud made up of bits and pieces left over from old stars that have exploded.
Earth formed when the force of gravity pulled swirling gas and dust in to become the third planet from the Sun.
What does the Goldilocks Zone refer to?
It refers to the habitable zone around a star where the temperature is just right (not too hot and not too cold) for liquid water to exist on a planet.
Based on the amount of energy (i.e. heat) the planet receives from the sun.
What was Earth like during the Hadean Eon?
Initally molten
Constantly bombarded by asteroids and comets
Formation of the moon
How was the moon formed?
A small planet collided with the Earth and most of its mass joined the Earth. However, a small mass was ejected and went into orbit around the Earth; this become the Moon.
What impact did the late heavy bombardment on Earth have?
Smaller bodies resembling present-day meteros and comets bombarded the Earth, heating it.
This heating was also increase by gravitational contraction.
This lead to partial/total melting of Earth, creating a magma ocean. The iron-rich fraction of this liquid was heavier and it settles to Earth’s center (creating its core).
This melting drove off any H, He-rich primordial atmosphere
What was the Earth composed of 4.4 Ga (4.4 billion years) ago?
A solid iron core
An outer core of liquid iron
A partly molten mantle (a siliceous, SiO2-rich, magma)
Perhaps a “thin” skin of solid rock (the earliest crust) at its surface (like the stuff floating on the top of soup)
When did the Earth cool enough that rocks and oceans began to form?
The end of the Hadean
Steam in the atmosphere cooled down and fell as rain on the Earth to create oceans
FIrst continents began to form (rocks in early oceans)
What was the earliest piece of the planet’s crust composed of?
Zircon crystals from Western Australia have been dated to 4.4 billion years making it the oldest rocks on earth.
- This suggests that within the first 100-200 million years of our planet, there was enough cooling to form a crust.
Ratio of O isotopes within this zircon crystal indicates that it likely formed in a cool, wet process at the Earth’s surface
- This suggests that parts of Earth may have been covered with liquid water
What are the defining characteristics of the Archean?
Archean – meaning “beginning, origin”
Liquid water was prevalent
Emergence of life on Earth (first evidence)
Onset of plate tectonics
What is a plate tectonic?
All of the crust on Earth moves around like floating slabs of rock (about 96km thick) on top of hot, mushy rock in the mantle, the hotter part of the Earth between us and the Earth’s iron core.
The Earth is chewing itself up, melting itself down, and making itself anew (a way that the Earth recycles itself)
Made up of rigid cool lithosphere (the rocky crust of the ocean floor and continents, down to the upper mantel)
What is the theory of Continental Drift?
Alfred Wegener in 1915
Idea that the Earth’s continents have moved over geologic time relative to each other, thus apearing to have “drifted” across the ocean bed
Explained why look-alike animal and plant fossils, and rock formations, are found on different continents (fossil evidence)
What are alternative evidence to Continental Drift for fossils on different continents ?
The species evolved independently on separate continents contradicting Darwin’s theory of evolution
They swam to the other continents in breeding pairs to establish a second pop.
What is the geological evidence for Continental Drift?
Rocks of the same age across the ocean
What was the theory of continental drift replaced by?
Plate tectonic theory
What are the three main layers of the Earth?
Core
- rich in iron
- solid inner core
- liquid outer core (source of Earth’s magnetic field)
Mantle: very hot rocks
- rich in silicon and oxygen (SiO2)
- becomes hot enough to become ductile and weak, behaves “plastically”
Crust: cooler, stiffer rocks
- rich in silicon and oxygen (SiO2)
What are the two different kinds of crust on Earth?
Oceanic crust:
- thin (7 km)
- dense (sinks under continental crust) –> nk
- young
Continental Crust
- thick (10-70 km)
- buoyant: less dense than oceanic crust
- mostly old
Density differences due to chemical differences; when it interacts with continental crust it tends to sink
- Continental crust has more aluminum, silicon, less magnesium, iron
- Oceanic crust has more magnesium, iron, less aluminum and silicon
What does the movement of plate tectonics do?
Earth’s crust is divided into 12 major plates which move in various directions
Plate motion causes them to collid, pull apart or scrape against each other
Each type of interaction cuases a characteristic set of Earth structures or tectonic features (e.g. volcanoes, mountains, ocean trenches)
The word tectonic refers to the deformaiton of the crust as a consequences of plate interaction
What causes plates to move?
Stiff tectonic plates are carried along the top of the mantles as if they were on giant conveyor belts
Convections causes the movement
What are plate boundaries?
Zones of contact between plates. They do not correspond to those of the continents.
What are the three types of plate boundaries?
Divergent: moving away from one another
Convergent: moving toward one another
Transform: moving up against one another
Divergent boundaries
The space created can be filled with new crustl material sourced from molten magma that forms below (the magma comes up in space created by divergent boundaries)
Divergent boundaries form within continents to produce rifts in continental crust. When the rift opens wide enough, it will form the thin rocky floor of a new ocean.
Some rifts failed to open, and created long-lasting valleys where major rivers run
Iceland being slowly torn apart is an example of continental rifting
What are mid-oceanic ridges?
Underwater mountain systems
Most active divergent plate boundaries are between oceanic plates. As plates move apart, small amounts of magma rise to the seafloor and add new crust (seafloor spreading – ocean is growing wider)
Where is the oldest sea floor found?
Near the continents
The age of seafloor increases relative to oceanic rifts
What are black smokers, what do they do, and where are they found?
Also known as hydrothermal vents, they are sea-floor hot springs. Molten hot magma heats up the water and pushes it up through the vent.
Entirely different ecosystem of organisms that we never knew about.
We discovered organisms at these vents get their energy from the vent itself and not from sunlight (chemotrophs) –> beginning of life on the planet?
Convergent boundaries
Refer to plates colliding. Typically the denser plate is subducted underneath the less dense plate. The plate being forced under is eventually melted and destroyed.
3 types of convergent boundaries:
- continent-continent collision
- continent-oceanic collision
- ocean-ocean collision
Explain continent-continent collision
Continental crust meets continental crust
Both continental crusts are too buoyant (light) to subduct so a continent-continent collision occurs, creating especially large mountain ranges
Explain continent-oceanic collision
Continental crust meets oceanic crust
The denser oceanic plate is subducted, often forming a mountain range on the continent
When do subduction zones occur?
Occur when one of both of the tectonic plates are composed of oceanic crust. Leads to oceanic crust being recycled.
Explain ocean-ocean collision
Ocean crust meets oceanic crust
Two oceanic plates collid; one oceanic plate is eventually subducted under the other
Formation of really deep trenches such as the Mariana Trench which is 11 km deep
Transform boundaries
Plates slide past one another
As plates rub against each other, huge stresses are set up that can cause portions of the rock to break, resulting in earthquakes.
Places where these breaks occur are called faults
Lithosphere is neither created nor destroyed
How fast do plates move?
Plate motions are on the order of a few centimeters per year
Along their edges, most paltes either converge of diverge
