141_lecture3

Earth’s Interior
MANTLE - Middle thin layer Density = medium Composition is silicon and oxygen-based but also includes iron and magnesium. Consistency is plastic. Contains two parts, Upper and Lower Mantle. CORE - Inner layer Density = high Composition is primarily iron and nickel. Contains two parts Inner core is solid. Outer core is liquid.
Other Important Geologic Principles
Superposition - The scientific law stating that in any unaltered sequence of rock strata, each stratum is younger than the one beneath it and older than the one above it, so that the youngest stratum will be at the top of the sequence and the oldest at the bottom.
The Earth’s Earliest History
Beginning of the Earth was extremely violent. Grew by planetesimal impact. Became very hot, heated to the melting point of iron. Innermost rocks began to become compressed, so more heat. Radiogenic heat was added due to radioactive fission. Earth underwent differentiation into layers.
Contains parts of the upper mantle and all of the crust. Called the LITHOSPHERE (rock layer).
Lithosphere is a strong layer, but brittle. Represents the outer approximately 100 km of the Earth. Thicker where continents exist, thinner under oceans. Below the lithosphere resides the ASTHENOSPHERE (weak layer).
However Meteorites are 35 % iron, while Earth’s surface rocks only 6 %.
Early Differentiation of the Earth
Where did the iron go?
As Earth was still accreting, temperature rose above melting point of iron. Iron liquified. Because of higher density, iron sank into the proto-Earth’s center due to gravity. Lighter elements rose to the surface. Originally, Earth was homogeneous. Due to heat and melting, Earth materials separated forming concentric zones of differing density.
Thus, Differentiation.
Differentiation and the Earth’s Interior
Earth’s Interior
Three Principal Layers Each has different Composition and density (mass/volume). CRUST - Outermost layer Density = low Composition is silicon and oxygen-based minerals and rocks. Crust is extremely thin. Consistency is rocky. Composed of two general types. Continental crust Oceanic crust
The outermost sublayer is the most active geologically. Large scale geological processes occur, including earthquakes, volcanoes, mountain building and the creation of ocean basins.
Early Differentiation of the Earth
What was the Earth’s early composition?
Need to consider meteorites that have struck the earth to get an idea of composition.
Principle of Original Horizontality - The scientific law stating that sediments settling out of a fluid (air and water) are deposited horizontally or nearly horizontally in layers that lie parallel or nearly parallel to the Earth’s surface.
Most are iron and nickel. Some contain chondrules. Small rocky bodies within the meteorites that may represent matter condensing from the original solar nebula. Earth’s composition should be similar to these meteorites.
Photo by Kevin Hefferan Badlands National Park
Principle of Cross-Cutting Relationships - The scientific law that states an igneous intrusion or geologic structure is always younger than the rock that surrounds it.
So… Where do we begin our study of
The Earth ?
The Creation of the Solar System
Begin with the “Big Bang” approximately 12 billion years ago. Space expanded rapidly and then began to contract. As temperatures cooled, Hydrogen and Helium gases formed. Denser pockets of gas condensed further due to gravity. Accumulations became galaxies. Began to rotate to form discshaped clouds. Center collapsed to form the Sun. As heat increased in the Sun, particles were blown away as “solar wind”. Particles collided and accreted becoming planetesimals.
Overlying the lithosphere is the ATMOSPHERE. Composed of gases released during volcanic eruptions and from plant respiration.
Subdivisions of the Earth’s Interior
Within these three principal layers are subdivisions. Crust consists of OCEANIC CRUST (brown) CONTINENTAL CRUST (green). Oceanic crust is thin (8-10 km), dense, and found below ocean basins (blue). Continental crust is thicker (20-70 km), has low density and forms the bulk of continents. The crust rides on the very upper most portion of the mantle.
So how did we get to here?
As larger and larger particles collided, larger planetesimals were formed. Some of these continued to collide and the largest became the planets, while the smaller ones may have become moons. Intense solar radiation heated the closest planets causing the lighter elements to be vaporized and blown out into space. This concentrated the heavier elements like iron and nickel on the inner planets and the lighter elements on the outer planets.