Geology - Introduction to Physical Geology - Quick Guides | Geology

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Study GuideGeology–IntroducƟon to Physical Geology1. The Earth’s ExteriorThe outside of the Earth is always changing. This happens because ofexternal forcessuch asclimate and rainfall. These forces slowly shape the land over time.1.1 Weathering: Breaking Rocks ApartRocks on the Earth’s surface do not stay the same forever.Freezing and thawing, along withrunning water, cause rocks to crack and break down. This process is calledweathering. Weatheringturns solid rock into very small pieces.1.2 Erosion: Moving the PiecesOnce rocks are broken into tiny particles, they do not stay in one place.Water, ice, and windcarrythese particles away. This movement is known aserosion. The tiny rock pieces being moved arecalledsediment.1.3 How Landscapes ChangeOver long periods of time, erosion can greatly change the shape of the land. Tallmountains slowlywear down into hills. Flowing water cuts into the land to formvalleys and canyons. Sediment alsohelps createsoil, which is important for plants and life on Earth.1.4 What Happens to SedimentLarge amounts of sediment are deposited in new places by water, wind, or ice. Some of this sedimentmay be eroded again later. Other times, the sediment stays in place, becomes compacted, andhardens over time. When this happens, it formssedimentary rock.In this way, weathering and erosion work together to constantly reshape the Earth’s surface.

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Study Guide2. Geologic TimeThe Earth has a very long history—much longer than human history. Scientists who study the Earth,calledgeoscientists, estimate that our planet is about4.5 billion years old. To understand such ahuge time span, they divide Earth’s history into stages based on major changes.2.1 The Early EarthWhen the Earth first formed, its surface was extremely hot. As thecrust slowly cooled, earlygeologic activity was dominated byvolcanoes. These volcanic processes helped build thecontinental crustand released gases that formed aprimitive atmosphere.2.2 The First LifeLife began very early in Earth’s history. Simplebacterial life formshave been found in rocks that arebillions of years old. These tiny organisms lived mainly in the oceans and were much simpler thanmost life we see today.2.3 Life Becomes More ComplexMuch later, around600 million years ago, more complex ocean life began to appear. Creatures suchastrilobiteslived in the seas and are often found as fossils in ancient rocks.2.4 The Age of DinosaursFrom about245 million to 66 million years ago,dinosaurs and other reptilesdominated theEarth. They lived on land, in the air, and in the oceans, and they flourished all over the world duringthis long period.2.5 Humans: A Very Recent ArrivalCompared to Earth’s long history,humans are very new. Human beings have existed for only aboutthelast 3 million years. This is less thanone-thousandth of Earth’s age, showing just how smallour place is in geologic time.

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Study GuideUnderstanding geologic time helps us see how slowly the Earth changes—and how recent humanhistory really is.3.The Earth TodayEven though Earth is almost5 billion years old, it is far from quiet or finished. Our planet is still veryactive, and its landscapes are constantly changing. Mountains rise and wear down, rivers shift theirpaths, and coastlines slowly reshape over time.Over the years, scientists have explored, studied, and collected samples from most of the rocks foundon the continents. By examining these rocks, they can learn how Earth has changed and continues tochange.3.1 Understanding Earth Through the PresentOne key idea that helps scientists understand Earth is calleduniformitarianism, often explained as“the present is the key to the past.”This means that the natural processes we see happening todayalso occurred in the past in similar ways.When scientists look at very old rocks, they often find textures and patterns that are still formingtoday. These features come from familiar processes such as:•Volcanic eruptions•Earthquakes•Hot springs•Wind and weathering•Flowing rivers•Sediment buildup•ErosionBecause these processes are still active, scientists can study them now and use that knowledge tounderstand ancient rocks.

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