Astronomy 83 - Earth

Overview

This lecture covers Earth's internal structure, geological activity, magnetic field, and compares Earth's unique features to other terrestrial planets.

Earth's Internal Structure

• Earth has four main internal layers: crust, mantle, liquid iron outer core, and solid iron inner core.
• The inner core is solid, mainly iron with some nickel and other metals, due to high pressure keeping it solid.
• The outer core is liquid iron and nickel; it surrounds the inner core.
• The mantle is mostly solid but contains some metal and lots of silicates (rocky materials made of silicon and oxygen).
• The crust is Earth's thin, outer rocky layer, mostly silicates but contains some iron.

Layer Composition and Formation

• Iron is heavier than silicates, so during Earth's liquid formation, iron sank to the center and silicates rose to the top.
• This process created Earth's layered structure, with more silicates near the crust and more iron toward the core.

Special Zones and Their Effects

• The top of the mantle, called the asthenosphere, is partly molten and allows for slow material flow.
• Lava from volcanoes originates from the asthenosphere, not the iron core.
• Earth's magnetic field is generated by the liquid iron outer core because liquid metals conduct electricity.

Plate Tectonics and Geological Activity

• Plate tectonics refers to the movement of the rigid plates that make up Earth's crust.
• Plates move several inches per year, mostly due to convection in the underlying asthenosphere.
• Fault lines are found where plates meet, causing earthquakes (e.g., California, Japan, Himalayas).
• Volcanoes often form along plate boundaries, especially around the Ring of Fire in the Pacific.
• Mountain ranges form where plates collide, creating high elevations along plate boundaries.
• Widespread mountain ranges due to plate tectonics are unique to Earth among terrestrial planets.

Earth's Magnetic Field and Aurora

• The Earth's magnetic field acts like an umbrella, protecting us from the solar wind (charged particles from the Sun).
• The magnetic field is weakest at the poles, where some solar wind enters and interacts with the atmosphere.
• This interaction produces auroras (northern and southern lights), seen mainly near the poles.

Key Terms & Definitions

• Crust — Thin, rocky outer layer of Earth.
• Mantle — Thick layer beneath the crust, mostly solid rock with some metal.
• Asthenosphere — The soft, partially molten upper mantle layer that enables plate movement.
• Outer Core — Liquid iron and nickel layer beneath the mantle.
• Inner Core — Solid iron and nickel center of Earth.
• Silicates — Rocks composed mostly of silicon and oxygen.
• Plate Tectonics — Movement of large crustal plates on Earth.
• Fault Line — Boundary between two tectonic plates.
• Magnetic Field — Region around Earth produced by moving liquid iron core.
• Solar Wind — Charged particles emitted by the Sun.
• Aurora — Glowing atmospheric effect caused by solar wind interacting with Earth's magnetic field at the poles.

Action Items / Next Steps

• Review diagrams of Earth's internal layers and major tectonic plates.
• Be prepared to identify and explain the source of Earth's geological and magnetic activity.
• Read course notes on planetary formation and solar wind effects.