Astronomy 2021A/B Chapter Notes - Chapter 4: Van Allen Radiation Belt, Oceanic Trench, Outer Core

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Week 4
4.1 Geology and Life
- Why is earth so dierent?
oDistance from sun – but moon is same distance and lacks qualities that
make earth habitable
oEarths larger size is important but venus is only a little smaller so that
can’t be all
oCombination of size and distance from the sun lead to earth’s geology
and atmosphere
- How is geology crucial to our existence?
oGeology = the study of earth
oBut scientists extend the meaning to encompass the study of any
world with a solid surface
oAlso used to describe the processes and features that shape worlds –
we can mean the composition of our planet, the volcanoes or other
processes that rework the surface
oOver long time scales, geology and life are deeply intertwine
oEarth’s geology is what makes our planet habitable – ultimately
allowing not only the existence of life but also the long-term evolution
of life into complex forms that include us
oThree aspects of earth’s geology especially important:
1. Volcanism: a volcanic eruption can be a spectacular sight, but
volcanoes are important to our existence on a much deeper
level: volcanic activity releases gases trapped in Earth’s interior,
and these gases were the original source of earth’s atmosphere
and oceans – volcanism releases heat and creates chemical
environments that helped lead to the origin of life on earth
2. Plate tectonics: earth’s surface has been shaped largely by the
movement and recycling of rock between the surface and the
interior (plate tectonics)- process known for rearranging the
continents – most relevant to life in terms of earth’s climate, it is
responsible for the long-term climate stability that has allowed
life to evolve for 4 billion years+
3. Earth’s magnetic 8eld: compass needles point north bc our
planet has a global magnetic 8eld generated deep in it interior –
shifts earth’s atmosphere from the energetic particles of the
solar wind and without this shielding it is likely that a signi8cant
portion of our planet’s atmosphere would by now have ben
striped away
oWe’d like to understand the likelihood of 8nding these factors on other
planets, so must understand how they work and came about
4.2 Reconstructing the History of Earth and Life
- How can we know anything about the long history that preceded human
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oRecorded in rocks and fossils – relics of organisms that lived and died
lone ago – preserve clues we can us to unravel the past
- What can we learn from rocks and fossils?
oMatter found in three basic phases: solid, liquid, gas
oAtoms and molecules of liquids or gases are in constant motion,
remixing so rapidly that we can’t possibly learn much about how they
were arrange din the past
oSolids are dierent – atoms and molecules are locked in place in a solid
so they preserve info about the time when they 8rst became locked
together – the time the object solidi8ed
oWe must look to the rocks and that make up the geological record
when we seek to learn about earth’s history
- Types of rocks:
oIgneous – made from molten rock that cools and solidi8es
oMetamorphic – has been structurally or chemically transformed by high
pressure or heat that was not quit high enough to melt it
oSedimentary – made by gradual compression of sediments such as
sand and silt at the bottoms of seas and swamps
oRocks can change from one type to another – ie. Igneous transformed
by high pressure or heat into a metamorphic rock, both igneous and
metamorphic rock may be eroded into sediments and become part of a
sedimentary rock, sedimentary rock may then be carried deep
underground, were it can melt and re-solidify as igneous rock
- Rock cycle
oBecause a rock can be recycled among the types, a rock’s type doesn’t
necessarily tell u much about its composition
oRocks usually contain a mixture of dierent crystals in close contact –
each individual crystal represents a mineral which is the word we use
to describe a crystal of a particular chemical composition and structure
oGeologists have de8ned 4300+ distinct mineral types but we often
group them by their primary constituents i.e. all minerals that contain
substantial amounts of silicon and oxygen are called silicates; familiar
silicates include quartz and feldspar
oCarbonates, such as limestone, are minerals containing large amounts
of carbon and oxygen
oA rock’s type tells us how it was made while its mineral composition
tells us what it is made of
oRocks type can have mixes of minerals though so geologists use
names to sub classify rocks i.e. two subtypes if igneous rocks form
much of our planet’s crust: Basalt (a dark, dense igneous rock that is
commonly produced by undersea volcanoes and rich in iron and
magnesium-based silicate materials), Granite (lighter in colour and less
dense than basalt, an igneous rock common in mountain ranges, gets
name from its grainy appearance and it is composed largely of quarts
and feldspar minerals)
- Sedimentary strata
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oSedimentary rock is particularly important to our study of earth’s
history for two reasons:
1. Most fossils are found in sedimentary rock
2. Sedimentray rock forms in a way that tends to produce a record
of time
oThe sediments that make sedimentary rock are produced primarily by
erosion on land – wind, water and ice can all help break up solid rock
into small pieces, some smaller than a millimeter across – these small
pieces comprise sediments
oSediments can be carried away by rivers and deposited on @oodplains
or in the oceans
oSediments pile up on the sea@oor over millions of years and the weight
of the upper layers compresses underlying layers into rock
oFossils can be made when remains of living organisms may be buried
in sediments simply because they settle to the bottom of the sea –
some land organisms form fossils when their remains are swept into
bodies of water – in other cases, remains of land organisms may be
buried in place by windblown silt and alter compressed by sediments
deposited on top of them when sea levels rise
oSediments deposited at dierent times tend to look dierent as a result
of changes in the rate of sedimentation, in the composition or grain
size of sediments settling to the bottom, or in the type of organisms
leaving fossils
oSedimentary rock tends to be marked by distinct layers – strata
oWe can view the strata in sedimentary rocks by things such as the
gradual action of river carving through the rock over millions of years
or by a cut made through a mountain to make way for a road
oSedimentary rock builds up gradually over time so at any location the
more deeply buried layers are older – this lets geologists determine the
relative ages of rocks and fossils buried in sediments
oSedimentary strata record most of earth’s history, but no single
location contains a full record – geologists have put together a fairly
detailed geological record by comparing sedimentary strata from many
sites around the world
- Rock analysis
oYou can often tell a rock’s type from its appearance i.e. a piece of
recently solidi8ed lava is obviously igneous, while a rock with an
embedded seashell is probably sedimentary
oScientist can analyze rocks in many ways, three types are particularly
important in reconstructing a rock’s history, and thus our planet’s
1. Mineralogical analysis generally means identifying the minerals
present in a rock
2. Chemical analysis generally means determining the elemental or
molecular composition of a rock or mineral i.e. will tell you the
percentages of a rock that consist of ion, silicon, carbon or other
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