Finding the Relative Age of Rocks Fossils the

Finding the Relative Age of Rocks Fossils & the Laws of Superposition and Cross Cutting

Relative and Absolute Ages ¡ ¡ Relative Age The age of a rock compared to the ages of rock layers. Absolute Age The age of a rock given as the number of years since the rock formed.

Position of Rock Layers Its difficult to determine the absolute age so geologists use method to find a rock’s relative age. Use the: LAW OF SUPERPOSITION: in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Other Clues to Relative Age Clues from Igneous Rocks: 1. Lava that hardens on the surface is called an Extrusion (Also called a sill. ) (example – an eruption would put a layer of igneous rock on top of sedimentary rocks. A. Rock layers below an extrusion are always older than the extrusion.

The extrusion is in black

Now the extrusion is the youngest layer.

A. Clues from Igneous Rocks: 2. Magma that cools and pushes into bodies of rock and hardens is called an Intrusion An intrusion is always younger than the rock layers around and beneath it.

The intrusion (in red) is now younger than the surrounding rocks. Also called a dike.

Draw this in your notes and label each of the parts listed below. 1. Intrusion 2. Extrusion 3. Oldest rock 4. Youngest rock

Law of Cross-Cutting Relationships • Any feature that cuts across a rock or layer of sediment is younger than the rock or layer it cuts

• Example of Cross. Cutting • Intrusion- igneousrockthatformswhenmagma squeezesbetweenexistingrockand andhardens

• Examples of Cross-Cutting • Example of Cross-cutting • Extrusion - igneous rock that forms when lava • flows on Earth’s surface and hardens

• Example of of Cross-cutting • Inclusion - pieces of older rock trapped within younger rock

• Examples of Cross-Cutting • Faults, joints, tilts, and veins also follow the • law of cross--cutting relationships

B. Clues from Faults: Fault: is a break in the Earth’s crust. • Forces inside the Earth cause movement of the rock on opposite sides of a fault. • Fault is always younger than the rock it cuts through.

Law of Included Fragments • Sediments are older than the cement • and rock they are part of • The pebble is older than the conglomerate rock it is found in.

REVIEW n Geologists use the Relative and Absolute Age of rocks to determine age. n Using the Law of Superposition n Other clues are from Igneous rocks – Extrusion – Intrusion n Clues from Faults

GAPS IN THE GEOLOGIC RECORD - Record of sedimentary rock layers is not always complete - Deposition slowly builds layers upon layer of sedimentary rock, BUT some of these layers may erode away, exposing an older rock surface. Unconformity – is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion.

• Unconformity • Gaps or missing layers in the rock record due to erosion • Angular unconformity

Exceptions to Law of Superposition • Rock layers can be overturned, older layers pushed on top of younger layers • THEREFORE, geologists use these clues to find the original position of rock layers…

Exceptions to Law of Superposition • Graded Bedding - coarse, heavy particles - bottom layer

Exceptions to Law of Superposition • Ripple-Marks - tops of ripple marks point

Exceptions to Law of Superposition • Cross-Beds - curved at bottom layer, cut off at the top

Cross Beds

USING FOSSILS TO DATE ROCKS To date rock layers, geologists first give a relative age to a layer of rock at one location. THEN they can give the same age to matching layers of rock at other locations. Certain fossils, called Index Fossils help geologist match rock layers. INDEX FOSSILS – Fossils of widely distributed organisms that lived during only one short period.

Fossils -the remains of plants and animals that lived in the past - mainly formed in sedimentary rock

Fossils Form in Several Ways • Form in several ways • 1. Original remains - unchanged remains of a plant or animal • Ex. Dinosaur bones/teeth • 2. Replaced remains - soft parts of original animal replaced by minerals • Ex. Petrified wood

Fossils Form in Several Ways • 3. Mold - fossilized shell or bone dissolves and leaves a hollow depression in a rock • Ex. Ferns, leaves, or fish • 4. Cast - new mineral material fills a mold • Ex. Shellfish

Fossils form in Several Ways • 5. Trace fossils - fossil evidence of animal movement • Ex. Trails, footprints, burrows & borings

Evolution • - process of change that produces new life forms over time • - fossils provide evidence of evolution

Natural Selection -theory of evolution -best adapted organisms will survive in large numbers and pass on these adaptations to their offspring

Index Fossils • - special fossils that give the relative age of the rocks that contain them • Index fossils must: • 1. Be recognizable • 2. Widespread • 3. Exist for a short time

Key Bed • - single rock layer that is recognizable, widespread, and exists for a short time

Correlation • - matching rock layers from one area to another • This can be done in several ways: • 1. “Walking the outcrop” • 2. Matching similar rock features • 3. Index fossils

• Correlating or matching index fossils from three outcrops.

• Example of an Index Fossil: • Trilobites (hard shelled animals whose bodies had three distinct parts. • Trilobites evolved in shallow seas more than 500 million years ago. • Over time, many types have appeared. • They became extinct about 245 million years ago. • They have been found in many different places.

To become a Index Fossil … a trilobite must be different in some way from other trilobites. Example – type with large eyes These large-eyed. . bites survived for a time AFTER other bites became extinct. If a geologist finds large-eyed Trilobites in a rock layer, the geologist can infer that those rocks are younger than rocks containing other types of trilobites

• Absolute Ages of Rocks • Absolute age is the age, in years, of a rock or other object. • Geologists determine absolute ages by using properties of the atoms that make up materials.

• Radioactive Decay • Some isotopes are unstable and break down into other isotopes and particles. • Sometimes a lot of energy is given off during this process. • The process of breaking down is called radioactive decay.

• Half-Life • The half-life of an isotope is the time it takes for half of the atoms in the isotope to decay. • After two half-lives, one fourth of the original isotope still remain. • After three half-lives, one eighth of the original isotope still remain.

• Radiometric Ages • As time passes, the amount of parent isotope in a rock decreases as the amount of daughter product increases. • By measuring the ratio of parent isotope to daughter product in a mineral and by knowing the half-life of the parent, in many cases you can calculate the absolute age of a rock. This process is called radiometric dating.

• Radiometric Ages