Name: ____________________________________________
Date: ________________
th
8 Grade Science
Mr. Vorstadt
Chapter 6 “Forces that Change Earth’s Surface”
NO person has ever been to me center of the Earth but scientist do have a good idea of what they would
probably find. Like any detective, scientists must conduct investigations and interpret the evidence. These
investigations are very diverse.
Of course, the most direct way to fin out about
the interior of the Earth would be to retrieve rock
from deep underground. That’s been tried. The
Russians have drilled the world’s deepest bore
hole to a depth of about 16 kilometers in the
Russian Arctic.
Deep drilling in many places has shown US that
the sedimentary rocks that cover most of die
Earth are underlain by metamorphic and igneous
rock. The continent contains a core of granites
rocks. Below the continental granites, as well as
the oceanic sediments is a darker more dense
igneous rock; basalt.
Seismic measurements have shown that earthquake waves travel faster just below a boundary known as the
Mohorovicic discontinuity. It was named after the Yugoslavian scientist who discovered it. This interface is
often called the "Moho". The speed of seismic waves changes with changes in rock type. For this reason.
many geologists believe that the Moho is the region where basalt changes to a more dense, high basaltic rock
known as eclogite. The depth to the Moho is about 40 km under the continents, but only about 10 km under
the oceans. The layer above the Moho is known as the Crust, and the layer below this interface is Earth
mantle.
A government project to drill through the Earth’s Crust and into the mantle was started in the 1950’s. Drilling
took place at sea because the Earth’s crust is the thinnest in the ocean basins. The project was dropped after
about 10 years when the cost of the project escalated to about five times the original projection. Although we
learned a great deal about deep-ocean drilling and the structure of the crust, the dream of bringing up an actual
rock from the mantle was never realized.
1. What is the name of the boundary at the bottom of the crust? _____________________________________
2. Unlike the oceans the continents contain what kind of rock? ______________________________________
3. The layer under the Earth’s Crust is the. . . ______________________________________
4. Where is Earth’s crust the thickest? ______________________________________
5. How does granite appear different from basalt? ______________________________________
We have also obtained information about the center of the Earth from the study of meteorites. Most scientists
think that the Earth and the solar system originated from a great cloud of dust and other particles drawn
together by gravity. If this is true, the composition of the Earth should be similar to the composition of
meteorites because of their common origin.
To interpret evidence from out studies of meteorites we must
make assumption. If the Earth was formed from meteorites,
the average composition of the Earth should be similar
to the average composition of meteorites. In fact, the crust
and the mantle (at least what we think is in the mantle) are
similar in composition to the stony meteorites. But these layers
contain relatively little iron. The table to the right shows that
many meteorites are rich in iron. If meteorites represent the
composition of the Earth, where is Earth’s missing iron?
TERRESTRIAL METEORITES
Class
Iron
Stony-Iron
Stony
Totals:
Fall %
5
1.5
93.5
100%
Find %
67
7.5
25.5
100%
6. Why do scientists Study objects from outer space to investigate Earth's interior? ______________________
________________________________________________________________________________________
7. Which layer of the Earth has a composition most like the stony meteorites? __________________________
8. Why do you think that iron meteorites, which fall less frequently, are the kind most often discovered?
________________________________________________________________________________________
The next step is deduction. If iron is common in meteorites, but not in the crust or the mantle it is likely that
the Earth's core is rich in iron. Being very dense, iron would be drawn by gravity toward the center of the
Earth. Additionally calculation of the mass of the Earth show that the average density of the Earth is much
greater than the density of rocks found in the crust and the mantle. All of these considerations lead to the
interpretation that the Earth’s core is made of a high density material; mostly iron.
We also get important information about the Earth's
core from seismic records. Consider an Earthquake at
point X on the diagram to the left energy waves would
radiate outward through the Earth.
From X to both A and A’, P-waves and S-waves
would be recorded. However, at station C, on the
opposite side of the Earth only P-waves arrive.
Because S-waves will not penetrate a liquid, we infer
that the center of the Earth is a liquid, because liquids
stop S-waves.
The zone around the Earth between A and B is called
the shadow zone. Refraction (bending) of earthquake
waves at the edge of the outer core means that no
direct seismic waves are detected in the shadow zone.
The shadow zone encircles the Earth in a ring shaped
like a doughnut.
Now, back to the lab. As the depth within the Earth increases, so does the pressure. In fact. the only way to
produce these pressures in the lab is to squeeze minerals between the flat faces of diamonds. increasing the
pressure until the diamonds shatters. When tiny bits of iron are squeezed among diamond faces and high
temperature. We therefore think that the liquid iron outer core surrounds a solid iron inner core.
Investigations of the interior of the Earth have brought together a wide variety of disciplines from geology,
astronomy, physics, and chemistry. As these studies converge, we have constructed a consistent model of the
interior of our planet.
9. Why is there an earthquake zone? _____
_________________________________
10. Which kind of seismic wave cannot
travel through a liquid?
___________________________________
11. According to this graph, how deep
would you have to dig to reach the
Earth’s outer core? ________________
12. What is the average density of the
Earth? ________________________
13. Why isn’t the Earth’s crust shown in
the graph?_______________________
________________________________
14. Why does the density change quickly at
2900 km? ______________________
15. What is the temperature at a depth of
2000 km? _______________________
16. What is the pressure at a depth of
5000 km? _______________________
17. What is the density of the mantle?
___________________________________
18. In layer do we find Iron and Nickel?
___________________________________
19. What is the actual temperature of the
earth at a depth of 2900 km? ________
20. At what layer is the pressure the
greatest? _______________________