Transcript Hello Eighth Graders! In an attempt to remind you of what

Hello Eighth Graders! In an attempt to remind you of what you may
encounter on this year’s MCAS that you haven’t seen since grade six
science, I’ve put together the following slides on geology.
Try to answer the following questions related to the last slide.
• Do you know which of the earth’s layers contains the most molten
rock and do you know why the answer is a bit unexpected?
• Do you know why the earth hasn’t completely cooled to the point
where there is no molten rock?
• Do you know how thick the crust is compared to the remainder of
the earth?
Answers are found on the next slide.
Answers
• Interestingly, the outer core is mostly molten even though the inner
core is likely to be quite a bit hotter. The added pressure on the
inner core results in it remaining solid… a very hot solid!
• Our moon and planet Mercury, as well, are now completely solid
but each has a history of molten lava/magma and volcanic
eruptions. Earth as well as other planets are larger so will take
longer to cool. In addition, decay of radioactive elements adds a lot
of heat to the earthly system.
• The earth’s crust ranges from about 5 km to 50 km thick (thickest
on mountainous continental masses and thinnest below oceans
especially near mid ocean ridges) while earth’s diameter is about
12,000 km.
Earthly plates: Questions to follow
See if you can answer the following questions.
• What do the red dots on the diagram represent and what is
important about their location?
• Notice two red dots near the center of the Pacific Plate called the
‘Hawaiian Hot Spot’. What is unusual about it’s location?
• Find the Mid-Atlantic Ridge. It is know as a ‘spreading center’…a
place where a crack in the mid-ocean floor that occasionally widens
exists. The average rate of widening is about an inch per year. At
that rate how long will it take the Atlantic ocean to widen by one
mile?
• Where might one find a collision zone and what happens there?
The spreading boundary illustrated below might be
considered a good representation of how the Atlantic
formed.
Answers
• The red dots indicate the locations of active volcanoes. They are
almost always found along tectonic plate boundaries.
• The Hawaiian volcanoes are found in the center of a tectonic plate
which is unusual but not unheard of. Geologists struggle to explain
the reason for such mid-plate ‘hot spots’ but interestingly enough
such a hot spot exists below ‘Old Faithful’ in Yellowstone Park, WY!
• 1 inch/year X 12 inches/foot X 5280 feet/mile = 63,360 years
But even that much motion wouldn’t be noticeable on a map…the
earth changes very slowly but has been around long enough for
huge changes to take place!
• A Collision zone is found on the west coast of S. America which has
given rise to the Andes Mt. range. Do you know what plates are
colliding there?
I’d bet that many know what place this is…but do you
know the name of the river responsible?
The Colorado River is responsible for the creation of the
Grand Canyon. Well, partially responsible.
• What type of rock makes up the walls of the Grand Canyon and
what do I mean when I say that the Colorado river is “partially
responsible”?
• How old is the Grand Canyon?
Answers
• The walls of the Grand Canyon are 99% sedimentary rock (or
metamorphic rock that was previously sedimentary) like the
sandstone pictured in the previous slide. I say ‘partially’
responsible for two reasons. First, the Colorado river couldn’t have
eroded so deeply unless tectonic forces were forcing the plateau to
rise meaning that part of the world was located near a plate
collision. Secondly, the sedimentary rock had to be formed
previously.
• The age of the Grand Canyon is a tough question. The Colorado
River has probably been eroding the canyon for several million
years or so. But, the rocks at the bottom of the canyon are over a
billion years old, in some cases, so the answer depends on your
point of view. More info about the G C found via link below.
• http://www.nps.gov/grca/naturescience/geologicformations.htm
Triassic period: Mesozoic era: Can geologists find
evidence of the following in the Grand Canyon?
Probably not! Just like the dirt supporting the houses
below is soon to be eroded away, much of the dirt/rock
that was, long ago, part of the Grand Canyon rock
formations, was eroded away. So as a result evidence
from the Age of dinosaurs is mostly missing from GC rocks!
Take a look at a modern day event of rapid erosion.
Some of you may know that the mudslide below is
responsible for the destruction of hundreds of homes
and 41 deaths. This occurred in Oso, Washington a
month or so ago.
Of course, water is not the only erosive agent. Glaciers
move a lot of rock and sediment, too. In fact, most of
the rocks you find in your back yard were moved by
glaciers!
Finally, our last topic! Topography/Maps
You may remember that each of the curvy lines are called contour lines
but do you remember what contour interval means? Can you
determine where a river might flow?
Of course, contour interval is the change in vertical
elevation between consecutive contour lines. And
since contours make a ‘V’ shape upstream the river
would be flowing downhill towards the west. See how
much information you can glean from the map below:
Topography wrap-up!
• I notice that the contour interval on the map is 40 feet. Every fifth
contour was labeled 200 feet higher/lower and 200/5 = 40 (it must
be feet since meters would mean these mountains were as high as
the Himalayas!).
• There are very high mountains ( some nearly 10,000 feet tall on the
southwest portion of the map) that trend from northwest to
southeast and that border a valley in between that trends in the
same direction. There are also some lakes or ponds and
rivers/streams that are found in the valley (notice the contour lines
near the rivers!)
• I hope MCAS goes well for you. But, more importantly, good luck
@LHS next year!
Mr. Rogers