1sci 1a24 - earth sci fall

solar system formation

4.5-4.6 billion years ago, a swirling cloud of interstellar gas and dust collapsed

solar nebula

a spinning, swirling disk of material

accretion

the coming together and cohesion of matter under the influence of gravitation to form larger bodies

formation of planetary discs

1000s of years

growth of planetesimals (~1 km)

unknown

growth of larger planetesimals (1000 km)

~100,000 years

formation of larger planets

millions of years

formation of the solar system

estimated ~50 million years

meteorites

• fragments of asteroids

• sample of solar system material

• many come from the asteroid belt between mars and jupiter

chondrules

• small particles in meteorites solidified from molten droplets

• rich in iron and silicon

iron meteorites

• Fe, Ni

• formed in the cores of asteroids

• widmanstatten pattern

stony-iron meteorites

• formed between core and mantle of asteroids

chondrites

• rocky/stony meteorites

• most abundant

carbonaceous chondrites

• contain ~5% carbon compounds (amino acids)

• building blocks of DNA

uniform age

4.53 - 4.58 billion years

earth’s core radius

3500 km

earth’s inner core

solid

earth’s outer core

liquid

earth’s core

Ni, Fe (nickel, iron)

earth’s mantle radius

radius: 2500 km

earth’s mantle state of matter

solid but can flow

earth’s mantle

oxygen, silicon, iron, magnesium

oceanic crust

• thin — 5 to 12 km

• basaltic

• density: 3.0 g/cm³

continental crust

• thicker, up to 75 km

• granitic

• density: 2.7 g/cm³

lithosphere

the crust and uppermost solid mantle, which are rigid

mohorovicic discontinuity (moho)

boundary between the crust and mantle

athenosphere

underlying mantle is soft and flows

differentiated planet

• a celestial body that has undergone differentiation

• venus, mercury, earth, mars

3 types of evidence to determine earth’s internal structure and composition

• drilling

• seismic data

• earth materials

the deepest hole drilled on earth

12 km

seismic reflection

return some of the energy of seismic waves after reflection from a rock boundary

seismic refraction

• the bending of seismic waves as they pass from one material to another

• seismic waves bend to lower velocity material

secondary waves

• shear waves

• can only travel through solids

primary waves

• compression waves

• can travel through liquids/solids

lithoprobe

multidisciplinary project investigating structure and composition of canadian shield

volcanoes

• molten magma forms lavas (parts of the mantle coming up)

• composition can provide information about earth’s composition

diamonds

• high pressure form of crystalline carbon

• form at > 1500°C, > 55 kilobars

• depths of > 150 km below earth’s crust (in the mantle)

• found in kimberlite pipes (volcano like vents) on oldest parts of continents

• most are ~3.3 Ga

lunar highlands (surface of the moon)

cratered topography, impact breccias (80% of surface)

maria (surface of the moon)

smooth flat-lying plains composed of basalt (16% of surface)

carbon 14

• a way of dating geological materials

• can date bone < 20,000 years old

isotopic dating

determining the age of a rock through analysis of its radioactive elements

radioactive decay process

• protons/neutrons leave nucleus

• when protons/neutrons are lost, atom becomes a different element (daughter)

alpha decay

atom loses electrons

beta decay

atom loses neutron

half life

amount of time required to reduce amount of radioactive isotope by half

238U half-life

• 4.5 billion years

• produces 206Pb

40K half life

• 1.25 billion years

• produced 40Ar

the best rocks for dating geologic events

• igneous

• metamorphic

• those commonly found in rock-forming minerals

• those with a unique daughter product

zircon

• mineral ZrSiO4

• common in igneous rocks

• hard, resistant to erosion

• contain trace amounts of uranium and thorium

• commonly dated by uranium-lead and fission track techniques

• oldest material dated on earth is a zircon crystal (4404 ± 8 Ma)

two main sources of energy on earth

• internal (geothermal) heat

• external (solar) heat

plate tectonic theory

earth’s surface is divided into a series of large plates that move around allowing continents to change position

alfred wegener (1912)

• published ideas on continental drift and proposed pangea, an ancient supercontinent

• suggested continents moved by plowing through sea floor

supercontinent

the assembly of most (if not all) of earth’s continental blocks or cratons to form a single large landmass

laurasia

modern day north america and asia

gondawana

modern southern continents

glacial deposits of gondawana

distribution suggests southern continents formed single landmass in the pastm

mid-ocean ridges

a seafloor mountain system formed by plate tectonics

hess (1962)

• introduced the idea of sea floor spreading

• sea floor created at mid ocean ridges reabsorbed into mantle at trenches

geomagnetism

• earth’s magnetic field is a vector quantity and is constantly changing

how much did the north magnetic pole move between 1831 and 2001?

1100 km

nmp since 1970

• accelerated and now moving at more than 40 km per year

• will reach siberia in 50 years OR may change course OR slow down

magnetic field reversal

periodically earth’s magnetic field flips, north magnetic pole changes to south polar position (reversed polarity)

magnetic north present position (normal polarity)

only in the last 780,000 years

magnetic field reversed polarity intervals

intervals from 100,000 to tens of millions of years

reversed polarity documentation

over the past 330 million years

polarity change average

over the last 65 million years, changes occur every 300,000 years

earth’s magnetic field weakening

over the past 2000 years

before reversal

• field weakens, is erratic

• additional N and S poles appear at the core

earth’s spinning core

• causes reversals to occur

• model of circulating metallic fluids in earth’s core

• produces magnetic reversal after 35,000 years of simulated time

how long it takes for a reversal to occur

approximately 5000 years (ranging from 1000 to 8000 years), but probably variable rates due to periods of very rapid change

what happens in a reversal

• N end of compass would point S

• weaker field would reduce protection of earth from solar wind, solar flares, solar storms, mass ejections of plasma and gas

• fucks up our technology

how rocks record magnetic field

magnetic materials (ie. magnetite) are ‘fixed’ in the rock when it solidifies or lithifies

paleomagnetism

magnetic field that is fixed in the rock can be measured with a magnetometer

anomalies

changes in magnetic properties

paleomagneism

magnetic reversals documented for the past 330 million years

last reversal time

780,000 years ago

age of the sea floors on earth

200 million years (the oldest is 340 million years)

north american plate

moving west - southwest, ~2.3 cm/year

pacific plate

moving northwest, ~7 - 11 cm/year

divergent boundaries

• when plates move away from each other

• in ocean or on a continent

• create new ocean basins, new oceanic crust

• involves thinning, rifting, extension

• associaed with volcanism (basaltic lavas) and shallow focus earthquakes

convergent boundaries

• when two plates move toward each other

• creates mineral deposits (associated with rich natural resources)

convergent: oceanic + continental

• oceanic + continental: the oceanic plate subducts (goes down into the mantle) because it is denser

convergent: oceanic + oceanic

• most dense plate descends (subducts)

• form deep trenches

• island arc volcanoes

convergent: continental + continental

• continental crust does not subduct (too buoyant)

• not as dense, will keep forming mountains continuously

• plates crumple and form mountain ranges (called obduction)

convergent margins

associated with shallow, intermediate, or deep earthquakes

hazard assessment

where areas are subject to more earthquakes

andesitic magma, explosive volcanoes

caused by partial melting (convergent)

ring of fire

• rimmed with convergent margins

• volcanoes and earthquakes common

how deep is the deepest ocean trench?

10.9 km

how high is the highest mountain (a.s.l.)?

8.8 km

transform boundaries

• two plates slide horizontally past each other

• associated with shallow-focus earthquakes

• often link sections of divergent or convergent margins

what does north america consist of?

many small micro-continents (provinces) joined together

joins/sutures

areas of weakness in the crust — potential sites for future earthquakes

what does hamilton lie close to?

• old suture zone in shield

• CMBBZ (central metasedimary belt-boundary zone)

north american plate movement

• moving westward (2.7cm/year)

• generates stresses

modified-mercalli intensity (MMI) scale

ten stage scale from I to X that indicates the severity of ground shaking

earthquake

motion or trembling of the ground caused by sudden displacement of rock

where do most earthquakes occur?

• at plate margins (divergent, convergent, transform)

• along faults (breaks in rock)

• can be in surface or buried

planar fracture or discontinuity in a volume of rock

where there has been significant displacement as a result of rock-mass movements

________ acting on rocks cause ________

stresses, strain