The Science of Geology • Geology - the science that pursues an understanding of planet Earth: – Physical geology - examines the materials composing Earth and seeks to understand the many processes that operate beneath and upon its surface – Historical geology - seeks an understanding of the origin of Earth and its development through time The Science of Geology • Many important relationships exist between people and the natural environment • Problems and issues addressed by geology include: – Natural hazards – Mineral resources – World population growth – Environmental issues The Nature of Scientific Inquiry How or why things happen is explained using a: – Hypothesis: a tentative (or untested) explanation – Theory: a well-tested and widely accepted view that the scientific community agrees best explains certain observable facts The Nature of Scientific Inquiry • Science assumes the natural world is consistent and predictable • Goal of science is to discover patterns in nature and use the knowledge to make predictions • Scientists collect data through observation and measurements • A hypothesis is: – A generalized statement designed to EXPLAIN a set of scientific observations. – The best hypothesis is one that explains ALL of the existing observations. • A hypothesis must be: – Able to predict (or deduce) future observations - i.e. be testable. – Falsifiable - i.e. there must be the possibility it could be proven WRONG. Hypothesis 1 Steps in Investigation What Was Discovered Observation Gasoline in groundwater Questions from observation From leak in buried tank? Hypotheses (Proposed explanations) From tank Not from tank Make predictions for each explanation: leak should be found in tank Collect data to test predictions No leak in tank; wrong kind of gasoline in tank Conclusion Contamination is from elsewhere Theory • Repeated verification of a hypothesis may result in the formation of a THEORY • A theory is: – “A well tested and widely accepted view that scientists agree best explain certain observational facts.” – Like the hypotheses from which it grew, it must also be testable and falsifiable! – Therefore, all theories are considered provisional. – Nonetheless, theories are the end points of science! Earth As a System • Earth is a dynamic planet with many interacting parts or spheres • Earth System Science: – Aims to study Earth as a system composed of numerous interacting parts or subsystems – Employs an interdisciplinary approach to solve global environmental problems • Testing a hypothesis: – Collect additional data that would be predicted (deduced) on the basis of the hypothesis. – If the data are inconsistent with the prediction, hypothesis MUST be modified or abandoned. – If the data are consistent with the prediction, they support the hypothesis Hypothesis The Nature of Scientific Inquiry • Scientific method involves: – Gathering facts through observations – Formulation of hypotheses and theories • There is no fixed path that scientists follow that leads to scientific knowledge Earth As a System • What is a system? – Any size group of interacting parts that form a complex whole • Examples of systems: – Cooling system in cars, nervous system in animals • Most natural systems driven by sources of energy that move matter and/or energy from one place to another The five interacting spheres of Earth Systems 2 Earth As a System Ocean is an open system: Energy and matter flow into and out of the system • Open system (e.g. leaf): – Energy and matter flow into and out of system • Closed system: – Energy moves freely in and out – Matter does not enter or leave system Earth is a closed system with respect to matter: Although energy enters and leaves earth freely, virtually no matter is exchanged between earth and the universe Sun: External Heat Engine Although Earth is a closed system, the "spheres" that comprise it are open and interact with one another. Basic Rock Types Earth’s Internal Heat Engine Basic Rock Types • Sedimentary rocks: • Igneous rocks: – Formed from cooling and solidification of magma (molten rock) – Examples include granite and basalt – Sediments are derived from weathering of preexisting rock – Sediments accumulate as layers at Earth’s surface – Buried sediments are converted into rocks – Examples include sandstone and limestone 3 Basic Rock Types • Metamorphic rocks: – Formed by “changing” preexisting igneous, sedimentary, or other metamorphic rocks – Driving forces are increased heat and pressure – Examples include gneiss and marble High temperatures melt rock to form magma Weathering breaks down igneous rocks into sediment Rocks and the Rock Cycle The Rock Cycle: • The loop that involves the processes by which one rock changes to another • Illustrates the various processes and paths as earth materials change both on the surface and inside the Earth Magma or lava cool and solidify into igneous rocks Sediment is buried and is converted into sedimentary rock 4 Higher heat and pressure convert sedimentary rock into metamorphic rock Melting of metamorphic rock produces magma Our Solar System Today Asteroids Any rock type can be converted to another rock type under appropriate conditions Other objects Uranus Jupiter Sun Mars Venus Mercury Neptune Saturn Earth and Moon Orbit of planets elliptical around Sun 01.08.a1 TERRESTRIAL PLANETS Venus Mercury Earth Mars • Closest to the Sun • Generally small, rocky bodies with densities greater than 3gm/cm3 • Composed mainly of silicates, Fe and Ni • Volcanism mostly basaltic 5 JOVIAN (GIANT) PLANETS Saturn Jupiter Neptune •Each has solid rocky core surrounded by layers of frozen or liquid hydrogen, helium, methane, and ammonia • Multiple moons • Impressive ring systems composed of dust- to boulder-sized particles of mostly ice Uranus Composition of the Solar Nebula Nebular Hypothesis • Solar system began as a giant rotating cloud of gas and dust called the solar nebula: - Nebula composed mostly of hydrogen and helium • Rotating nebula began to contract about 5 billion years ago: - Developed into a flat, disk shape with the proto-Sun (pre-Sun) at the center - Inner planets formed from metallic and rocky substances - Larger outer planets formed from fragments of ices (H2O, CO2, NH3 and CH4) that accumulated around rocky cores Gases and dust start to gravitationally collapse Proto-sun Nebula contracts into rotating disk Small Bodies Aggregated Into Larger Bodies Cooling nebula condenses tiny solid particles Tiny particles coalesce into asteroid-size bodies 6 Larger Bodies Accreted Into Planets The entire solar system formed in a few tens of million of years The Earth 4.5 Billion Years Ago Initial magma ocean was hundreds of km deep • Hot, softened inner (terrestrial) planets separated into layers based on densities of different materials • Heavy metals (mostly iron) sank to the center • Lighter, molten material migrated towards the surface to produce a primitive crust • This chemical separation established the basic divisions of Earth’s interior and surface Magma ocean cooled and crystallized from the bottom-up over millions of years to form a solid mantle capped by a primitive crust of basalt Formation of the Moon (about 4.5 billion years ago) Earth’s major subdivisions: • Atmosphere • Crust • Mantle • Core 7 Earth’s Crust Oceanic Crust: - Ranges from 0 - 10 km thick - Average composition of basalt Continental Crust: - Ranges from 33 - 70 km thick - Average composition close to granite The weak upper asthenosphere consists of partially molten (~2%) peridotite Peridotite Solid peridotite • Lithosphere includes crust and solid upper portion of mantle • Upper mantle composed of an Mg-Fe silicate rock called peridotite • Base of the lithosphere marked by the upper boundary of the asthenosphere Lithospheric Plates Partially molten peridotite (weak layer) • Below the asthenosphere, the mantle is solid down to a depth of ~2,900 km where it meets the outer core • Lower mantle composed of a highdensity, Mg-silicate rock called perovskite • Rigid lithosphere glides over weak asthenosphere: – Lithosphere broken into a series of plates • Surface of the earth envisioned as a mosaic of lithospheric plates 8 Outer Core: - Extends from 2,900 km to 5,100 km depth - Consists mainly of molten (liquid) iron - Source of Earth’s magnetic field Inner Core: - Extends from 5,100 km depth to the center of the Earth - Composed mostly of solid iron and nickel Geologic Time • Geologists now able to assign accurate dates to events in Earth history • Relative dating: – Geologic events and materials are placed in their proper sequence or order of occurrence without knowing their ages • Absolute dating: – Uses radioactive isotopes to assign actual ages to rocks and events Geologic Time The magnitude of geologic time: • Involves vast times – millions to billions of years • Many geological processes are very gradual and only noticeable over time scales of thousands to millions of years Origin of Earth and Solar System Origin of the Earth and Solar System The History of Earth Origin of Earth and beyond 9
