Research press release


Nature Geoscience

Earth’s earliest tectonics driven by impacts



Craig O’Neillたちは今回、数値モデルを用いて大規模な衝突が地球のテクトニクス進化に及ぼす影響をシミュレーションした。彼らは、衝突事象によりもたらされるエネルギーが地球内部を加熱し、マントル物質を上昇させるプリュームを生じさせることを見いだした。これが初期地球の薄く弱い沈み込むプレートのマントルへの沈み込みを駆動することとなった。テクトニック・プレート境界にある現在の沈み込み帯とは対照的に、シミュレーションされた沈み込み事象は地理的に局所的で短命であった。従って地球は、衝突の規模と頻度により、テクトニクス的に不活発な状態から活発な状態へと交互に変わっていたことになる。

関連する「Sketch-up」でJames Tuttle Keaneは、衝突を考慮したときに初期地球のダイナミクスは、それがない場合と比べてどのように異なって進化するかを描いている(「Sketch-up」は論文についての手描きの情報画像(infographic)である)。

Asteroid impacts may have triggered episodes of crustal recycling on Earth more than four billion years ago, according to a paper published online this week in Nature Geoscience. The study challenges the assumption that early Earth was tectonically stagnant.

Little is known about Earth during the Hadean Eon more than four billion years ago, but the Earth’s interior was thought to have been too hot at this time to sustain plate tectonics, with some evidence suggesting little mixing between crust and mantle. However, there is other evidence - from the oldest surviving geological materials: ancient zircon grains - for crustal recycling similar to that seen at modern subduction zones, where one tectonic plate sinks below another. One possible explanation is that impact cratering, a process that was more prevalent in the early Solar System than it is today, could have kick-started subduction before the onset of plate tectonics as we know it.

Craig O’Neill and colleagues used numerical models to simulate the influence of large impacts on the Earth’s tectonic evolution. They found that the energy delivered by impact events heats the Earth’s interior and induces rising plumes of mantle material. This in turn drives subduction of early Earth’s thin and weak sinking plates into the mantle. In contrast to modern Earth’s subduction zones along tectonic plate boundaries, the simulated subduction events are geographically localized and short lived. Thus, the Earth flips from tectonically stagnant to tectonically active, depending on the size and frequency of impacts.

In an accompanying ‘Sketch-up’, James Tuttle Keane illustrates how the dynamics of early Earth progresses differently when impacts are considered, compared to a scenario without them. (A Sketch-up is a hand-drawn infographic about the paper.)

doi: 10.1038/ngeo3029


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