1. © Sierra College Astronomy Department Terrestrial Geology Basics

2. © Sierra College Astronomy Department 2 Terrestrial Geology Basics Earth’s Structure & Composition The Interior of the Earth (overall density = 5.5 g/cm 3 ) Earth’s interior is determined by analyzing travel times of two types of waves generated by earthquakes. Earth’s interior is made up of three layers: –Crust is the thin (<100 km) outermost layer of the Earth and has a density of 2.5–3 g/cm 3. –Mantle is the thick (2,900 km), solid layer between the crust and the Earth’s core. Density of the mantle is 3–9 g/cm 3. The crust “floats” on top of the mantle. –Core is the central part of the Earth, composed of a solid inner core and a liquid outer core. Density of the core ranges from 9–13 g/cm 3 and is probably composed of iron and nickel. Increasing density trend is called differentiation - sinking of denser materials toward the center of planets or other objects.

3. © Sierra College Astronomy Department 3 Terrestrial Geology Basics Earth’s Structure & Composition Layering by Strength Most of the Earth is not molten and most of the lava from volcanoes rises upward from a narrow region of the mantle which is partially molten. The shape of a planet is determined by the strength and fluidity of the inside as well as the strength of gravity –Large worlds (> 500 km diameter) are round –Small worlds are irregular in shape The crust and the top part of the mantle is relatively cool region of rock called the lithosphere that floats on the rest of the mantle.

4. © Sierra College Astronomy Department 4 Terrestrial Geology Basics Causes of Geological Activity Geological Activity describes how much ongoing change occurs on the surface of a solar system body Interior heat is the primary driver for geological activity But how do interior heat up and cool off?

5. © Sierra College Astronomy Department 5 Terrestrial Geology Basics Causes of Geological Activity How planets heat up Heat of accretion –Energy brought from afar from colliding planetesimals – potential energy converted into kinetic energy Heat of differentiation –As the planet redistributes its mass and denser material sinks towards the core gravitational potential energy is converted to thermal energy via friction Heat from radioactive decay –Decay from radioactive materials heats up the interior as some of the nuclear decay energy ( E = mc 2 ) gets transferred to thermal energy Note: the first of these two tend to happen early in a planet’s history while the last (radioactive decay) happens throughout the history of the planet, but is strongest at the beginning of the formation of the planet. Radioactive decay likely contributes several times more energy over the life of the planet than does accretion and differentiation.

6. © Sierra College Astronomy Department 6 How Interiors Cool Off: Conduction –Transfers occurs between atoms –Examples: metal rod in fire, Earth’s core and lithosphere Convection –Warmer (less dense) air rises and carries energy into cooler (denser) regions –Requires large temperature gradient –Examples: Lava lamp, Earth atmosphere and mantle, Sun’s outer layers Radiation –Photons directly transfer energy –Less efficient in high density situations –Photons take ~ 200,000 years to get of Sun. –Examples: Heat lamp, Earth’s surface, Sun’s interior Terrestrial Geology Basics Transfer of Energy

7. © Sierra College Astronomy Department 7 How the Earth moves energy from the core to the surface: Convection is the most important process in the Earth’s deep interior –The ongoing process of transferring heat upward creates convection cells –Ongoing mantle convection goes at the rate of 1 cm/year: It would take about 100 million years to move the mantle from the base to the top At the lithosphere, conduction is probably the most important process Terrestrial Geology Basics Transfer of Energy

8. © Sierra College Astronomy Department 8 A small object cools more quickly than a large object So size is the most important factor in planetary cooling This can be seen in the terrestrial worlds: –Earth and Venus: still very active. –Mars: Activity in the past, but mostly dead now. –Moon and Mercury have been dead for 3 billion years or so. Terrestrial Geology Basics Planetary Size

9. © Sierra College Astronomy Department 9 Terrestrial Geology Basics Earth’s Magnetosphere Earth’s Magnetic Field A magnetic field is a region of space where magnetic forces can be detected. The region around a planet is called a magnetosphere Earth’s magnetic poles are not located at its poles of rotation. The location of the magnetic poles changes with time. Dynamo effect is the model that explains the Earth’s and other planets’ magnetic fields as due to currents within a liquid iron core and a rapidly spinning planet.

10. © Sierra College Astronomy Department 10 Terrestrial Geology Basics Earth’s Magnetosphere The Van Allen belts are doughnut-shaped regions composed of charged particles (protons & electrons) emitted by the Sun & captured by the magnetic field of the Earth. Auroras result from disturbances in the Earth’s magnetic field that cause some of the particles to follow the magnetic field lines down to the atmosphere, where their collisions with atoms of the air cause it to glow.

11. © Sierra College Astronomy Department 11 Terrestrial Geology Basics Shaping the Earth There are 4 processes which shape the virtually all features on Earth 1. Impact Cratering Bowl shaped from asteroids or meteors Bowl shaped from asteroids or meteors 2. Volcanism Eruption of lava from planet’s interior Eruption of lava from planet’s interior 3. Tectonics Disruption of planet’s surface by internal forces Disruption of planet’s surface by internal forces 4. Erosion Wearing down or building of geological features by wind, water, ice etc… Wearing down or building of geological features by wind, water, ice etc…

12. © Sierra College Astronomy Department 12 Terrestrial Geology Basics Impact Cratering As a general rule the craters made by meteors are 10 times bigger than the impactor and 10- 20% as deep as the crater is wide. Most impacts happened very early in the history of the solar system The most prominent impact crater on Earth is Meteor Crater near Winslow, Arizona (only 50,000 years ago). Many of the craters on the Earth have been wiped out by erosion processes –Not true for Moon and Mercury

13. © Sierra College Astronomy Department 13 Terrestrial Geology Basics Volcanism Volcanism occurs when underground molten rock finds it way through the lithosphere. This is due for 3 reasons: –Molten rock is generally less dense than solid rock –Most of the Earth’s interior is not molten and it requires a chamber of molten rock to be squeezed up the surface –Molten rock often has gas inside of it, leading to dramatic eruption and to outgassing The most common gasses released are water vapor, carbon dioxide, nitrogen, and sulfur gasses (H 2 S or SO 2 )

14. © Sierra College Astronomy Department 14 Terrestrial Geology Basics Plate Tectonics Plate Tectonics Alfred Wegener is credited with first developing the idea of continental drift - the gradual motion of the continents relative to one another. Rift zone is a place where tectonic plates are being pushed apart, normally by molten material being forced up out of the mantle. Subduction Zone is where two plates are forced together.

15. © Sierra College Astronomy Department 15 Terrestrial Geology Basics Erosion The surface of the Earth is changed by erosion, the processes that break down or transport rock through the action of ice, liquid, or gas –Valleys shaped by glaciers –Canyons carved by rivers –Shifting of sand dunes by the air Erosion can pile up sediments into layers called sedimentary rocks (Ex. Grand Canyon) The Earth has the most erosion of any terrestrial planet

16. © Sierra College Astronomy Department 16 The number of craters in a given region can tell one the age of the planet/moon since the last major change on surface –Does not necessarily indicate formation age Erosion from wind, water, and lava will wipe out craters in a given region –This led to determining the development of different parts of the planet/moon Terrestrial Geology Basics Terrestrial Geology Basics Age of surfaces

17. © Sierra College Astronomy Department 17 The End