Humans subdivide time into useable units such as our calendar year, months, weeks, and days; geologists also subdivide time. They have created a tool for measuring geologic time, breaking it

into useable, understandable segments. For the purposes of geology, the “calendar” is the geologic time scale.

One way to distinguish and define each segment of time is by the occurrence of major geologic events and the appearance (and disappearance) of significant life-forms, starting with

the formation of Earth’s crust followed by the appearance of ever-changing forms of life on Earth.

The geologic time scale grew out of necessity: organizing the immensity of geologic time and correlating geologic events on a worldwide scale.

No one person or expert committee proposed the geologic time scale used today. It grew by trial and error through the efforts of numerous geologists

working independently. Today the recognition of formal subdivisions of geologic time is determined by international committees.

MYA= million years ago

Epoch=time period defined by evidence in rock layers—typically last more than three million years.

Period=the basic unit of the geologic time scale; during these spans of time specific systems of rocks were formed-Just as eons are subdivided into eras, eras are subdivided into units of time called periods.

Era-a very long span of geologic; in formal usage, the second longest portions of geological time.

Eon- are the longest portions of geologic time (eras are the second-longest). Four eons are recognized:

the Phanerozoic Eon (dating from the present back to the beginning of the Cambrian Period), the Proterozoic Eon, the Archean Eon, the Hadean Eon. 

Sometimes you can tell how old rocks are by the rocks around them. The law of Superposition, says that unless the layers are disturbed or turned over, the layers at the bottom

are always older than the layers on top.  Using the idea of the oldest layers being on the bottom helps scientists do "relative" dating.  They don't know the exact age,

but they know that one rock is older than another. Sometimes a volcanic rock will cut through other layers.  This is called an igneous intrusion. The Law of Crosscutting Relationships

says that the rock cutting through another layer is younger than the layer being cut through.  This can also happen with a fault cutting through layers.  Sometimes you know the age

of rocks because of fossils in them.  Index Fossils- are only found in a specific section of the geologic time scale. Two different rocks with the same index fossils in them were formed

around the same time period. Sometimes we know how old they are because we use radiometric dating, which is figuring out how long radioactive rocks have been decaying into new

elements......thats a bit of chemistry again!

Remember there are three main categories of rocks.  

IGNEOUS           SEDIMENTARY           METAMORPHIC

Under each of these categories are a variety of rocks.  This chart shows a few of them.

There is even a ROCK CYCLE

a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous.

Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the

atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as

they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time.

Watch this video for more information on the rock cycle: https://www.youtube.com/watch?v=Vp_S3BDiR-I

Structure of the Earth and Plate Tectonics:

The Earth is made up of many layers.  The inner core is solid but the outer core is liquid. 

Tectonic plates float on top of the asthenosphere, and below the Lithosphere. They usually move just a few centimeters a year.

Over million of years, though, that's a lot of movement.  Scientists believe that the continents used to be one supercontinent they call Pangea.

Over 200,000 million years they broke apart and moved to their current positions.  Fossils and shapes of the continents tend to support this idea. 

Fossils of the same species in the same layers exist on more than one continent, and the shapes of the continents fit together like a puzzle. 

The plates don't always move away from each other though.  Why do they move?  The Asthenosphere is hotter and less dense than the plates, a bit like clay. 

They float on top and sometimes dig down into the Asthenosphere. The edge of two plates may be considered:

Convergent- two plates are moving together- running into each other- one plate may move under another and push up mountains

Divergent- two plates are moving away from each other- they tend to form valleys and ridges

Transform- two plates slide past each other- pressure builds up and then the plates slip- causing an earthquake. 

Weathering and Erosion:

Water is a powerful force that shapes the Earth.  When water breaks rocks apart it is called weathering. When water carries away rock and soil, its called erosion.   

Mechanical Weathering- rain dislodges soil particles and loose rock. Water carries away billions of tons of soil and rock, sediment, every year. 

Faster moving, or larger rivers, erode the landscape faster.  Chemical weathering is when rain mixed with rock or pollution turns acidic and weathers the rock faster. 

Limestone is very vulnerable to this. Coastal Erosion- is when waves break up rock.  Weathering and erosion by water is incredibly powerful.

Wind and Ice also cause weathering and erosion.   Here you see a picture of the Great Sand Dunes in Colorado.  The dunes were formed by the right combinations of wind, water, and sediment. Creeks and streams brought in large amounts of sediment and sand into the valley. Wind then blew the sand toward the bend in the Sangre de Cristo Mountains, where opposing storm winds helped squeeze the sand into the tall dunes you see today. The sand grains were too heavy for the wind to take over the mountain so they are dropped at the foot of the mountain forming huge sand dunes.