GEO 101 Lecture Notes - Lecture 2: Convergent Boundary, Subduction, Divergent Boundary
23 Jun 2018
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The Earth's Origin
According to the widely accepted nebular hypothesis, the planets and moons in the
solar system, including Earth, formed from a huge cloud of mostly hydrogen and helium.
Contraction, rotation, and dropping temperatures resulted in the formation of small
particles, the first being nickel and iron. These began to stick together, and after tens of
millions of years of condensation and accretion, the earth was formed about 5 billion
years ago. Although the earth has been cooling ever since and has formed a hard outer
crust, part of the interior is still hot and molten.
The Earth's Structure
The earth can be divided into four concentric zones (Figure ). The innermost is called
the inner coreand is thought to be a solid, spherical mass of iron. Its radius is about
1,216 kilometers (730 miles). The next zone, called the outer core, is believed to be a
layer of molten liquid rich in nickel and iron that is about 2,270 kilometers (1,362 miles)
thick. The outer core is overlain by the mantle, which is a solid yet puttylike rock that
can actually flow. The mantle is about 2,900 kilometers (1,740 miles) thick. The crust,
the outermost zone, is the hardened exterior of the earth and varies in thickness from
about 5 to 50 kilometers (3 30 miles).‐‐
Continental crust is thicker than oceanic crust. The solid lithosphere is composed of
the crust and the upper part of the mantle. The softer, more flexible part of the mantle
underneath the lithosphere is the asthenosphere
As the earth cools, the intense heat being produced in the core creates convection
currents in the mantle that bring hot mantle material up toward the crust, and colder
mantle and crustal rocks sink downward. This heat engine drives plate tectonics, or the
movements of large segments of the earth's crust (plates) that are separated along
deep cracks called faults. The plates move over the asthenosphere, which is softer and
less resistant. The crust breaks into these segments because of the upward movement
of molten material below. The powerful internal tectonic forces squeeze and fold solid
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Document Summary
According to the widely accepted nebular hypothesis, the planets and moons in the solar system, including earth, formed from a huge cloud of mostly hydrogen and helium. Contraction, rotation, and dropping temperatures resulted in the formation of small particles, the first being nickel and iron. These began to stick together, and after tens of millions of years of condensation and accretion, the earth was formed about 5 billion years ago. Although the earth has been cooling ever since and has formed a hard outer crust, part of the interior is still hot and molten. The earth can be divided into four concentric zones (figure ). The innermost is called the inner coreand is thought to be a solid, spherical mass of iron. The next zone, called the outer core, is believed to be a layer of molten liquid rich in nickel and iron that is about 2,270 kilometers (1,362 miles) thick.
