7th Grade Science Notes

6th Grade Science Notes
The 4 layers of the Earth:
The earth was formed from a condensing ball of gas, dust, and debris leftover from the
nebula that formed our sun.
The earth has 4 chemically and physically different layers:
- the crust
- the mantle
- the outer core
- the inner core
The crust:
The continents and ocean floor that cover the earth’s surface make up the crust.
the crust is very thin (5-100 km) and brittle. It is thinnest under the ocean and
thickest under mountain ranges.
Oceanic crust is made of basalt, a very dense, volcanic rock.
Continental crust is made of granite, a less dense volcanic rock.
The mantle:
The mantle is 2800 km thick and is divided into 2 sections, the lithosphere and the
The lithosphere is composed of the bottom of the crust and the rigid part of the upper
mantle. The lithosphere is broken up into “lithospheric plates” that allow the continents
and ocean floors to float on the liquid portion of the mantle.
The asthenosphere is a semi-solid layer of the mantle composed of molten rock and
minerals such as magnesium, iron, silicon, and oxygen. The molten rock
shows“plasticity” which means it behaves as either a solid or a liquid depending on
heat and pressure.
Convection currents in the asthenosphere allow the lithosphere above it to move.
The outer core:
The outer core is located about 2900 km beneath the crust and is 2200 km thick.
It is composed of liquid iron (Fe) and nickel (Ni) at about 40000 F.
The inner core:
The inner core is only 1250 km thick and is a solid ball of Fe and Ni. The temperature
and pressure are so intense ( about 90000 F and 3 million atmospheres) that the atoms
cannot move but can only vibrate in place.
The solid core actually spins within the liquid, outer core and this generates a magnetic
The earth’s magnetic field has a north and south pole and extends throughout the entire
planet and even out into space.
The earth’s magnetic poles are actually different from the geographic poles which are at
the axis of the planet. A compass points only to the magnetic north pole of the planet.
How do we know?
Geologists have learned to develop theories based on indirect evidence. Observations
from earthquakes, volcanoes, fossils, drilling and mining, sonar and photos from space
are all types of indirect evidence that, when put together, can lead to a theory about the
inside of the earth.
Seismic waves from earthquakes change their speed and direction as they pass
through solids or liquids. By mapping seismograph patterns from all around the globe,
the consistency of the different layers of the earth was determined.
The other terrestrial planets and the moon do not have a magnetic field like the earth
does. Only certain elements (such as iron) can be magnetized. However, the iron
atoms must be spinning to produce a magnetic field.
The magnetic field of the earth has shifted and even flipped many times. This means
that the iron at the core must be shifting too within it’s liquid, outer layer.
The examination of thousands of meteorites indicates that most of the rocky material in
the solar system is composed of iron and nickel so the terrestrial planets, like Earth,
must have formed from the same material. Radioactive dating of meteorites and moon
rocks also show that they were all formed at the same time as the earth was.