lectures 2-5 Flashcards
How do we keep time?
- Absolute dates comes from the radiometric dating of igneous rocks.
- Relative dating, from relationships between rocks.
What are the 3 types of rocks?
- Igneous: formed through the cooling and solidification of magma or lava.
- we can date most accuratly igneous rocks. - Sedimentary: formed from chemical precipitates or fragments of ealier fofrmed rocks.
- tells us mostly about the relative order in which events occured - layers. - Metamorphic: formed by application of heat and pressure to either igneous or sedimentary rocks.
How do we know how old a rock is?
- Radiometric dating: the radioactive decay of an isotope = natural clock.
- Radiocarbon dating: we can measure the amount of carbon 14 to determine its age.
- We can measure the decay of other elements found within rock to determine an absolute age.
- ratio of parent to daughter atoms.
- more daughter atoms overtime.
How do we determine age in sedimentary rocks?
Sedimentary rock is produced from the gradual accumulation of sediment on the surface. New sediment is continually depositd on top of previously deposited or older sediment.
The older rocks are the rocks at the bottom, the younger ones at the top.
How do fossils help with relative dating?
Fossils are left in sedimentary rocks. Fossils can help to match rocks of the same age, even if the rocks are found at a large distance apart.
Where are fossils most abundant?
Fossils are most abundant in marine sedimentary rocks.
They are generally not foudn in igneous or metamorphic rocks.
Why are fossils not always reliable?
Different levels of sediment accumulation depending on the fossil location affects the relative dating of a fossil.
We cannot use the thickness of sedimentary layers to estimate how much time any layer represents.
How did the Earth form?
Earth formed when the force of gravity pulled swirling gas and dust in to become the third planet from the Sun.
Earth fromed out of a solar nebula: a swirling cloud made up of bits and pieces left over from old stars that have exploded.
Habitable zone: temperature is just right and liquid water can exist.
What was Earth like during the Hadean Eon?
- Initially molten.
- Constantly bombarded by asteroids and comets - heated the earth.
- Formation of the moon.
- Not a liveable time on the planet.
- Partial to total melting of Earth ensued, creating a magma ocean - iron-rich fraction of this liquid was heavier and it created the Earth’s core.
What occured at the end of the Hadean eon?
The Earth cooled enough that rocks and oceans began to form.
- Steam in the atmosphere cooled down and fell as rain on the Earth to create oceans.
- First continents begin to form.
What is the earliest piece of the planet’s crust?
Zircon crystals from Western Australia dated back to 4.4 billion years ago.
- There was enough cooling to form a crust.
- Ratio of oxygen isotopes in the crustal indicates that it was formed by a cool, wet process at the earth’s surface.
- Parts of Earth may have been covered with liquid water.
Describe the Archean eon.
- Liquid water was prevalent.
- Emergence of life on earth.
- Onset of plate tectonics.
What are plate tectonics?
The earth’s crust moves around like floating slabs or rock on top of hot, mushy rock in the mantle - the hotter part of the Earth between us and the Earth’s iron core.
What is continental drift? What is the evidence?
Idea that the Earth’s contients have moved over geologic time relative to each other - appears that they have drifted across the ocean.
Rocks of the same age acrosss the ocean.
The theory of continental drift was replaced by the movement of plate tectonics.
What are the 3 main layers of the earth?
- Core.
- inner core (solid), outer core (liquid, magnetic field), rich in iron. - Mantle (hot rocks).
- rich in SiO2.
- becomes hot enough to become ductile and weak, behaves plastically. - Crust (cooler, stiffer rocks).
- rich in SiO2.
What are plate tectonics made of?
Plates are made of rigid, cool lithosphere.
Litosphere: the rocky crust of the ocean floor and continents, down to the upper mantle.
What are the 2 different kinds of crust on earth?
- Ocean crust.
- thin, dense (sinks under continental crust), young. - Continental crust.
- thick, buoyant, old.
Chemical differences lead to a different in density.
What causes plates to move?
Convection currents in the mantle drive plate tectonics.
- intense heat of the core causes hot liquid rock in the mantle to move.
- warm material rises, cools, sinks down - restarts.
- this cycle creates a motion and pushes the floating plates.
What are plate boundaries? What are the 3 types?
Zones of contact between plates (middle of a continent or ocean). Do not correspond to continents.
- Divergent
- Convergent
- Transform
Explain divergent boundaries.
Plates moving aparts.
Forms within continents to produce rifts in the continental crust. When rifts open wide enough = formation of the thin rocky floor of a new ocean.
Failure of open riftS: valleys for major rivers.
Where are the most active divergent plate boundaries?
Between oceanic plated = mid-oceanic ridges (under water mountain systems).
As plates move apart, small amounts of magma rise to the seafloor and add new crust.
Forms most extensive chain of mountains on earth (underwater).
Where is the mid-atlantic ridge?
Located along the floor of the Atlantic Ocean.
Longest mountain range in the world..
Explain convergent boundaries.
Plates colliding.
The denser plate is subducted underneath the less dense plated. The plate being forced under is eventually melted and destroyed.
What are the 3 types of convergent boundaries?
- Continent-continent collision.
- Continent-oceanic collision.
- Ocean-ocean collision.
Depends on the type of plates colliding because of the different densities.
Explain continent-continent collision.
Continental crust meet continental crust.
Both continental crusts are too buoyant (light) to subduct so a continent-cotinent collision occurs - creates 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.
Explain oceanic-oceanic collision.
Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. Leads to oceanic crust being recycled - formation of moutnains.
