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Banded Iron Formation

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Terra Dinâmica: Um Curso para Educadores

Museu Americano de História NaturalMuseu Americano de História Natural

4.6 (109 classificações) | 9.3K alunos inscritos

The AMNH course The Dynamic Earth: A Course for Educators provides students with an overview of the origin and evolution of the Earth. Informed by the recently released Next Generation Science Standards, this course examines geological time scales, radiometric dating, and how scientists “read the rocks.” We will explore dramatic changes in the Earth over the last 4 billion years, including how the evolution of life on Earth has affected its atmosphere. In addition to looking at geology on a global scale, participants will take to their own backyards to explore and share their local geologic history. Course participants will bring their understanding of the dynamic Earth - along with content resources, discussion questions, and assignments - into their own teaching.

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Avaliações

4.6 (109 classificações)

5 stars
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AK

Sep 06, 2019

it's an amazing course easy to understand and as science teacher it really helped me developing new ways in teaching science and how can i engage student

DF

Sep 01, 2016

Excellent course. Required some corollary reading, but basically supplied all materials required to meet course outline requirements.

Na lição

Evolution of the Atmosphere

You will learn how the evolution of photosynthetic life changed the concentration of oxygen in the oceans and atmosphere, and how this is reflected in the rock record. You will also become familiar with how the Next Generation Science Standards connect to this week’s content. Finally, you will complete a written assignment: an analysis of a local geologic feature.

Evolution of Early Earth's Atmosphere11:06

Banded Iron Formation3:19

Stromatolites2:42

Ministrado por

Edmond Mathez, Ph.D.
Curator and Professor
Ro Kinzler, Ph.D.
Senior Director

Transcrição

We are standing in the Hall of Planet Earth and I'm here before a banded iron formation. And the reason this sample is on display here is that it is the primary evidence that the early atmosphere contained no oxygen, and so that's what I would like to explain to you, the logic behind that. But first let me describe to you what this is. obviously banding refers to the fact that this banded iron formations are layered and the layers consist of this red stuff. which is basically jasper, a form of silica, and the individual, very small grains in jasper are coated with iron oxide, and that's what gives them the red color. And these dark layers are made of the iron oxide mineral magnetite. the formula of which FE304. The question is why is it that this sample provides evidence for no oxygen in the early atmosphere? It turns out that first of all, these samples only exist in the geologic record before about 1.8 billion years ago, or so ago. We don't see them in the relatively, young record. And there's a good reason for that. Present day seawater contains basically no iron. The concentrations are so low, they're almost impossible to measure. and the reason it contains no iron is because as soon as one puts a little bit of iron in ocean water, it immediately reacts with the oxygen in the ocean water to precipitate as, iron oxide minerals. So that's probably what happened in the early earth. This graphic illustrates the process by which we think banded iron formations formed. In the early earth there was hot spring deposits driven by volcanic activity, causing circulation of sea water through the ocean crust. And as that sea water moved through the ocean crust it dissolved some of the minerals in the crust and became enriched in metals, especially iron, and so introduced large amounts of iron into the ocean. So that ocean at that time contained no oxygen. So because it contained no oxygen, it was able to dissolve significant quantities of iron. Eventually, stromatolites appeared on the scene. Organisms that engaged in photosynthesis and thus introduced oxygen into the system. The earliest known stromatolites appear in the geologic record about 3.4 billion years ago. So the stromatolites were producing oxygen, but that oxygen was immediately reacting with the iron in ocean water. To precipitate the iron oxide minerals that form the band and iron formations. The ocean basically ran out of iron and other things that were removing oxygen from the system, that the oxygen content of the ocean and the atmosphere were able to rise. And we know pretty much when that was, because we have means of dating that time and it started, that rise started about 2.45 billion years ago, and it ended about, 2 billion years ago. In that interval, the oxygen content of the atmosphere increased from essentially nothing to perhaps a percent or so, and it didn't reach its present of about 20% until the end of the Proterozoic, era.

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