Earth: From Core to Crust Chapter 2

Reading the Geologic Record The difference between geologist and historians is that historians think of history over a period on thousands of years and geologists over billions of years. The time frames that geologists talk about are referred to as either absolute or relative. –Relative time – refers to an event that happens before or after another event –Absolute time – the exact time

Three Basic Principles to Read the Geologic Record Geologic Record uses relative time to date the features of the earth. James Hutton came up with three principles that lead to the creating of the Geologic Record. Three principles help Geologists figure out what happened and when: 1.Superposition 2.Uniformitarianism 3.Fossil Correlation

1. Superposition New sediments are deposited over old ones. Therefore the lower the layer of undisturbed rock the older it is.

2. Uniformitarianism This principle is based on the idea that natural processes occur in the same way today as they did in the past For example a river would flow the same way and cause the same type of erosion and deposition This is important because when geologists study ancient layer of earth and their appearance they can make assumptions based on processes occurring on earth today.

3. Fossil Correlation Fossils are organic plant and animal matter that has decomposed and been removed and replaced by minerals in solution This principle is based on the idea that different rock deposits that contain the same fossil types must be the same age. By looking at the fossils in different layers of rock geologists can determine when certain animal appeared and disappeared. Approximate climates can also be determined.

3. Fossil Correlation

Layers Beneath our Feet As the earth formed the heavier material settled in the center and the lighter material on top. We cannot drill holes to the center of the earth because of the distance and heat so we rely on seismology to gain information on the layers.

Seismology A seismograph records the earth’s vibrations when there is an earthquake. As the ground shakes different waves are generated. These waves act differently depending on the material that it passes through

Types of Waves There are two types of waves that pass through the earth. –Primary Waves (P waves) – squeeze and stretch the material they pass through. They pass through any type of material and move very quickly. –Secondary Waves (S waves) – vibrate at right angles to P waves and only travel through solids

Types of Waves

Gutenberg Discontinuity An interesting thing happens at about 2,900 Km below the surface. –The P waves slow right down and the S waves stop completely From readings that Beno Gutenberg took (1914) he figured out that this is the depth at which the liquid core begins.

Layers of the Earth

The Core Makes up a third of the total mass of the planet Made up of two parts – the inner and the outer Inner Core –Made mostly of iron –Temperatures range from 4000 C to 6650 C (hotter than the sun) –It is solid because of the immense pressure Outer Core –Not as much pressure so this layer is liquid –Generates our magnetic field –Made up mostly of iron –This layer begins about 5,200 Km below earth’s surface

The Mantel Takes up 80% of earth’s total volume Made up of iron, silicone and magnesium Divided into two parts – Lower and Upper Lower Mantel –Starts at 2,900 km below surface and ends about 500 km below –Moving towards the surface the temperature gradually cools.

The Mantel

Upper Mantel – Asthenosphere This layer of the earth is responsible for many of the processes that cause the crust to look the way it does. Hot spots in this layer create convection currents that move the earth’s tectonic plates When volcanoes erupt the magma that forms this layer of the earth can flow out on to the surface as lava.

Moho Discontinuity Discovered by Andrija Mohorovicic This is a depth where there is a sharp increase in the speed of waves This discontinuity form the boundary between the asthenosphere and the crust

Crust Very thin covering on the surface of the earth Makes up only 0.1% of earth’s total volume Made up of two parts: –sima - beneath the ocean, made up of basaltic rock (iron, magnesium and silica) –sial – makes up the continents, contains many more minerals (silica, potassium, aluminum to name a few), it’s thicker than the sima

SimaSial