Unit 2 Lesson 1 (without questions): Geologic Change Over Time

Geologic Change Over Time

How do we learn about Earth's history?

The idea that erosion has occurred the same way throughout Earth's history is an example of uniformitarianism.

• What does this term mean?

Uniformitarianism

States that geologic processes that happened in the past can be explained by current geologic processes.

How do organisms become preserved as fossils?

• FOSSILS are the trace (or remains) of an organism that lived long ago.

• Most commonly found in sedimentary rock.

Trapped in Amber

• Organisms get stuck in soft sticky tree sap that hardens and gets buried and preserved.

• Some of the best fossils are formed this way.

Trapped in Asphalt

• Asphalt ("tar") wells up to Earth's surface in certain spots.

• Organisms get caught in the thick, sticky goo.

Buried In Rock

• Most of the time organisms die and are quickly decomposed or eaten by other organisms.

• Getting buried slows decay, and the hard parts of the organism eventually become part of the rock.

• "Bone" is no longer present, but the minerals remain.

Frozen

• Freezing temperatures slow down decay, so organisms that get frozen may get preserved by the ice.

• Ötzi, also called the Iceman, is the natural mummy of a man who lived between 3400 and 3100 BCE, discovered in September 1991 in the Ötztal Alps (hence the nickname "Ötzi") on the border between Austria and Italy.

Petrification

• Happens when an organism's tissues are replaced by minerals.

Trace Fossils

• A fossilized structure that formed in sedimentary rock by animal activity.

• A "trace" or sign of the organism that it left behind, not the organism itself.

• Tracks, burrows, trails, or waste

What can fossils tell us?

The FOSSIL RECORD contains all of the fossils found on Earth.

It shows PART of the history of life on Earth.

Fossils also tell us about environmental changes over time.

• Fish fossils indicate there was once water where there is now dry land.

• Fossils of trees and dinosaurs have been found in Antarctica.

• Palm fronds indicate a tropical environment.

The Atacama desert is the driest place on Earth (currently).

How does sedimentary rock show Earth's history?

• Rock and mineral fragments move from one place to another by erosion.

• Eventually, sediment is deposited in LAYERS.

• New layers cover older layers, which become compacted.

• Dissolved minerals, such as calcite and quartz, separate from water that passes through the sediment, forming a natural cement.

• Deposition --> Compaction --> Cementation --> Sedimentary Rock

Scientists use different characteristics to classify sedimentary rock.

• This provides evidence of the environment the rock formed in.

• Also tells us what it's made of.

Composition

• Shows the source of the sediment that makes up the rock.

• Sandstone = sand grains get cemented together.

• Limestone = remains of dead plants and animals

• Coal = partially decomposed plant material buried beneath sediment.

How is oil made?

Texture

• Can show how the material was carried and deposited, providing clues about the environment at the time.

• Layers can differ from one another; kind of material, color, and size of sediments.

Special Features

• "Ripple Marks" record motion of wind or water waves over sediment.

• "Mud Cracks" form when fine-grained sediments covered by water are suddenly exposed to air and dry out quickly.

Studying sedimentary rocks on Mars

What do earth's surface features tell us?

• Earth's surface is ALWAYS changing.

• TECTONIC PLATES are divisions of the Earth's surface (the lithosphere).

• They move, causing the continents to change position over time.

Earth's Tectonic Plates

At one time, the continents formed a single land mass called PANGEA.

Pangea broke apart about 200 million years ago.

• Movement hasn't stopped.

Earth in 200 million years

Evidence of Pangea

• Distribution of rock types, mountains, and fossils.

• E.g. Appalachian and Atlas Mountains

Landforms Change Over Time

• When plates pull apart, magma can be released in volcanic eruptions.

• When plates collide, volcanoes and mountains can be created.

• When plates grind past one another, breaks in Earth's surface form, causing earthquakes.

Weathering and erosion break down landforms.

• For example high, jagged mountains become more rounded over time.

• This can provide clues about the geologic history of the mountains.

What can other materials tell us about Earth's climate history?

• The CLIMATE of an area describes the weather conditions in the area over a long period of time.

• Mainly temperature and precipitation averages

Trees

• When trees grow a new layer of wood is added each year/season.

• They can tell the age, as well as how the environment was when it grew. ("Dendrochronology")

• Thick ring = lots of water/sunlight/warmth

• Thin ring = poor growing conditions

Multiple Choice

Which side of this tree do you think was getting the most sun?

• Top

• Bottom

• Left Side

• Right Side

Sea-Floor Sediments

• Long cylinders of ocean sediment are collected and analyzed.

• Fossils of organisms, chemical composition of sediments, and thickness of layers all tell a story of what Earth was like at the time the layer was forming.

Clastic Sediments

Seafloor sediments are "clastic sediments", which are particles that have been eroded and broken down from other rocks and combined with a lot of organic material—skeletons, shells and other remains from creatures that either lived in the water column of the ocean or on the seafloor.

Sediment core samples in storage.

Ice

As new snow falls, layers are created for each snow event.

As the layers build they compress on the layers below, eventually compacting into ice layers (like when you make a snow ball).

Gas bubbles, organisms/traces, chemicals, pollen, dust, volcanic ash, etc... can all be trapped in these layers.

• 24 hours of sunlight during the summer, and warmer temperatures, contrasted with 24 hours of darkness and colder temperatures change the texture/color of the ice.

• The "summer ice" is lighter in color than the clear, dark "winter ice".

Gas bubbles provide samples of atmospheric conditions.

National Ice Core Laboratory in Denver, CO