What does Heidegger mean "the closedness of earth" in 'the origin of the work of art' aka 'Der Ursprung des Kunstwerkes'?
First of all, this is an awful translation IMHO. It even omits whole sentences and loses a lot of the obviously intended connotations. The German original (as pointed out in Mr. White's answer, can be found in Holzwege (Gesamtausgabe, Band 5), Klostermann 1977, pp. 50-51) reads:
Indem aber eine Welt sich öffnet, kommt die Erde zum Ragen. Sie zeigt sich als das alles Tragende, als das in sein Gesetz Geborgene und ständig Sichverschließende. Welt verlangt ihre Entschiedenheit und ihr Maß und läßt das Seiende in das Offene ihrer Bahnen gelangen. Erde trachtet, tragend-aufragend sich verschlossen zu halten und alles ihrem Gesetz anzuvertrauen. Der Streit ist kein Riß als das Aufreißen einer bloßen Kluft, sondern der Streit ist die Innigkeit des Sichzugehörens der Streitenden. Dieser Riß reißt die Gegenwendigen in die Herkunft ihrer Einheit aus dem einigen Grunde zusammen. Er ist Grundriß. Er ist Auf-riß, der die Grundzüge des Aufgehens der Lichtung des Seienden zeichnet. Dieser Riß läßt die Gegenwendigen nicht auseinanderbersten, er bringt das Gegenwendige von Maß und Grenze in den einigen Umriß.
My translation would be:
But as a world opens itself up, earth starts to tower. It shows itself as that which carries everything, that which shelters all being in its own law and is ever enclosing itself. The world demands its determination and measure and lets that which is into the openness of its ways. Earth strives, carrying and towering, to keep itself closed and entrust everything to its laws. The strife is not a rip [Riss] as the ripping open [Aufreißen] of a mere gap, the strife is the intimacy of a belonging together of contestants. This rip rips those which turn away from another back together, into the source of their unity. It is the primordial rip [Grundriss]. It is a ripping open [Aufriss] that outlines the rise of the illumination of being. This rip does not let those who turn away from one another rip apart, it brings the measure and boundary of them into a unified shape [Umriss].
Other than in the surrounding text, Heidegger makes a more fundamental point here. He speaks generally about the problems of extracting truth from the world as there is a fundamental tension between the world as an object that is the original, self-sustained being and a subject that tries to get a perspective on that all-encompassing being, from the outside, as it were.
Art is, according to Heidegger, about how the world can be formed so that it is depicted (or, more generally, represented) with the very substance that makes it up. That is closely linked to phenomenology, but also about technique of art. This is why he uses word plays like "Riss", which means both sketch (literally the first few lines drawn) and chasm/rift/tear, "Grundriss", which means both the ground plan of an object (the drawing perspective from above) and the basic structure of something, "Aufriss" which means both the frontal view of an object in drawing and the process of tearing apart/opening a rift, and "Umriss" which means the outline in terms of drawing, concepts, and phenomenologically.
Now on to the phenomenological context. In phenomenology, the closedness of reality is the default state. It is generally shut away from us, and only in the phenomenon "opens itself up" to us, shows itself to us in certain ways that depend both on the object and us, the subject. Heidegger again plays with words when he says that the way earth opens itself up is as a closed entity, something that wants "to preserve its closedness" (or, as I put it, "keep itself closed") and thus, in opening itself up, is closed: The world discloses itself as all-encompassing, as that which can have nothing beyond itself, so that everything has to follow its laws since it is a closed system that contains everything.
Therefore, when the artist tries to take a standpoint in relation to the world, a perspective onto the world, they have to tear themself apart from the world, open a rift, try to set themselves apart from it while the world does not allow for it according to the internal logic of it as a phenomenon. As that means a complete severance is impossible, we end up with this "intimacy of the mutual dependence of the contestants" and a work of art that is a result of the being ripped together again, of mutual aspects of both artist and substance.
Hope that helps.